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Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4.

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  Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Rob Herring <robherring2@gmail.com> - 2015-08-24 15:50 +0200
    Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Rob Clark <robdclark@gmail.com> - 2015-08-25 22:50 +0200
      Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Rob Herring <robherring2@gmail.com> - 2015-08-26 01:30 +0200
      Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Daniel Vetter <daniel@ffwll.ch> - 2015-08-26 14:00 +0200
        Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Thierry Reding <thierry.reding@gmail.com> - 2015-08-26 14:20 +0200
          Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Rob Herring <robherring2@gmail.com> - 2015-08-26 16:40 +0200
            Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Dave Airlie <airlied@gmail.com> - 2015-08-26 23:00 +0200
              Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4. Rob Herring <robherring2@gmail.com> - 2015-08-27 02:40 +0200

#1212199 — Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4.

On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
> Stephen Warren <swarren@wwwdotorg.org> writes:
>
>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
>>> Signed-off-by: Eric Anholt <eric@anholt.net>
>>
>> This one definitely needs a patch description, since someone might not
>> know what a VC4 is, and "git log" won't show the text from the binding
>> doc itself. I'd suggest adding the initial paragraph of the binding doc
>> as the patch description, or more.
>>
>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
>
>>> +- hvss:             List of references to HVS video scalers
>>> +- encoders: List of references to output encoders (HDMI, SDTV)
>>
>> Would it make sense to make all those nodes child node of the vc4
>> object. That way, there's no need to have these lists of objects; they
>> can be automatically built up as the DT is enumerated. I know that e.g.
>> the NVIDIA Tegra host1x binding works this way, and I think it may have
>> been inspired by other similar cases.
>
> I've looked at tegra, and the component system used by msm appears to be
> nicer than it.  To follow tegra's model, it looks like I need to build
> this extra bus thing corresponding to host1x that is effectively the
> drivers/base/component.c code, so that I can get at vc4's structure from
> the component drivers.
>
>> Of course, this is only appropriate if the HW modules really are
>> logically children of the VC4 HW module. Perhaps they aren't. If they
>> aren't though, I wonder what this "vc4" module actually represents in HW?
>
> It's the subsystem, same as we use a subsystem node for msm, sti,
> rockchip, imx, and exynos.  This appears to be the common model of how
> the collection of graphics-related components is represented in the DT.

I think most of these bindings are wrong. They are grouped together
because that is what DRM wants not because that reflects the h/w. So
convince me this is one block, not that it is what other people do.

Rob
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#1213330

On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@gmail.com> wrote:
> On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
>> Stephen Warren <swarren@wwwdotorg.org> writes:
>>
>>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
>>>> Signed-off-by: Eric Anholt <eric@anholt.net>
>>>
>>> This one definitely needs a patch description, since someone might not
>>> know what a VC4 is, and "git log" won't show the text from the binding
>>> doc itself. I'd suggest adding the initial paragraph of the binding doc
>>> as the patch description, or more.
>>>
>>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
>>
>>>> +- hvss:             List of references to HVS video scalers
>>>> +- encoders: List of references to output encoders (HDMI, SDTV)
>>>
>>> Would it make sense to make all those nodes child node of the vc4
>>> object. That way, there's no need to have these lists of objects; they
>>> can be automatically built up as the DT is enumerated. I know that e.g.
>>> the NVIDIA Tegra host1x binding works this way, and I think it may have
>>> been inspired by other similar cases.
>>
>> I've looked at tegra, and the component system used by msm appears to be
>> nicer than it.  To follow tegra's model, it looks like I need to build
>> this extra bus thing corresponding to host1x that is effectively the
>> drivers/base/component.c code, so that I can get at vc4's structure from
>> the component drivers.
>>
>>> Of course, this is only appropriate if the HW modules really are
>>> logically children of the VC4 HW module. Perhaps they aren't. If they
>>> aren't though, I wonder what this "vc4" module actually represents in HW?
>>
>> It's the subsystem, same as we use a subsystem node for msm, sti,
>> rockchip, imx, and exynos.  This appears to be the common model of how
>> the collection of graphics-related components is represented in the DT.
>
> I think most of these bindings are wrong. They are grouped together
> because that is what DRM wants not because that reflects the h/w. So
> convince me this is one block, not that it is what other people do.

I think, when it comes to more complex driver subsystems (like drm in
particular) we have a bit of mismatch between how things look from the
"pure hw ignoring sw" perspective, and the "how sw and in particular
userspace expects things" perspective.  Maybe it is less a problem in
other subsystems, where bindings map to things that are only visible
in the kernel, or well defined devices like uart or sata controller.
But when given the choice, I'm going to err on the side of not
confusing userspace and the large software stack that sits above
drm/kms, over dt purity.

Maybe it would be nice to have a sort of dt overlay that adds the bits
needed to tie together hw blocks that should be assembled into a
single logical device for linux and userspace (but maybe not some
other hypothetical operating system).  But so far that doesn't exist.
All we have is a hammer (devicetree), everything looks like a nail.
End result is we end up adding some things in the bindings which
aren't purely about the hw.  Until someone invents a screwdriver, I'm
not sure what else to do.  In the end, other hypothetical OS is free
to ignore those extra fields in the bindings if it doesn't need them.
So meh?

BR,
-R


> Rob
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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#1213434

On Tue, Aug 25, 2015 at 3:42 PM, Rob Clark <robdclark@gmail.com> wrote:
> On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@gmail.com> wrote:
>> On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
>>> Stephen Warren <swarren@wwwdotorg.org> writes:
>>>
>>>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
>>>>> Signed-off-by: Eric Anholt <eric@anholt.net>
>>>>
>>>> This one definitely needs a patch description, since someone might not
>>>> know what a VC4 is, and "git log" won't show the text from the binding
>>>> doc itself. I'd suggest adding the initial paragraph of the binding doc
>>>> as the patch description, or more.
>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
>>>
>>>>> +- hvss:             List of references to HVS video scalers
>>>>> +- encoders: List of references to output encoders (HDMI, SDTV)
>>>>
>>>> Would it make sense to make all those nodes child node of the vc4
>>>> object. That way, there's no need to have these lists of objects; they
>>>> can be automatically built up as the DT is enumerated. I know that e.g.
>>>> the NVIDIA Tegra host1x binding works this way, and I think it may have
>>>> been inspired by other similar cases.
>>>
>>> I've looked at tegra, and the component system used by msm appears to be
>>> nicer than it.  To follow tegra's model, it looks like I need to build
>>> this extra bus thing corresponding to host1x that is effectively the
>>> drivers/base/component.c code, so that I can get at vc4's structure from
>>> the component drivers.
>>>
>>>> Of course, this is only appropriate if the HW modules really are
>>>> logically children of the VC4 HW module. Perhaps they aren't. If they
>>>> aren't though, I wonder what this "vc4" module actually represents in HW?
>>>
>>> It's the subsystem, same as we use a subsystem node for msm, sti,
>>> rockchip, imx, and exynos.  This appears to be the common model of how
>>> the collection of graphics-related components is represented in the DT.
>>
>> I think most of these bindings are wrong. They are grouped together
>> because that is what DRM wants not because that reflects the h/w. So
>> convince me this is one block, not that it is what other people do.
>
> I think, when it comes to more complex driver subsystems (like drm in
> particular) we have a bit of mismatch between how things look from the
> "pure hw ignoring sw" perspective, and the "how sw and in particular
> userspace expects things" perspective.  Maybe it is less a problem in
> other subsystems, where bindings map to things that are only visible
> in the kernel, or well defined devices like uart or sata controller.
> But when given the choice, I'm going to err on the side of not
> confusing userspace and the large software stack that sits above
> drm/kms, over dt purity.

I wasn't implying that this should get exposed to userspace as
components. V4L2 has gone that route with media controller and
sub-devs. Perhaps that is needed for DRM, perhaps not. For the moment,
I definitely agree the kernel should hide most/all of those details,
but I don't think that means DT has to hide the details or know what
components are handled by a single driver. My point was that on the DT
side we have a mixture of OF graph usage, parent-child nodes or custom
phandles (this case) to describe the relationships between h/w
components. That's not necessarily wrong, but we should have some
rules around how certain relationships are described. Then in the
drivers we have a mixture of deferred probe, component API, and custom
inter-module APIs to control init order. We then have a mixture of all
those which leads to very few if any drivers having the same overall
structure that could be shared. Should we mandate using the component
API for h/w that is discrete blocks? Should we throw out the component
API for something else? Can we tie the graph parsing and component API
together with common code?


> Maybe it would be nice to have a sort of dt overlay that adds the bits
> needed to tie together hw blocks that should be assembled into a
> single logical device for linux and userspace (but maybe not some
> other hypothetical operating system).  But so far that doesn't exist.

OF graph is supposed to do this. OF graph is a double edged sword. It
is very flexible, but then each platform can do something different.
We need to have some level of requirements around how the OF graph is
used. As an example, any system with an HDMI connector should have an
"hdmi-connector" compatible node or encoder/bridge chips/blocks must
have certain ports defined.


> All we have is a hammer (devicetree), everything looks like a nail.
> End result is we end up adding some things in the bindings which
> aren't purely about the hw.  Until someone invents a screwdriver, I'm
> not sure what else to do.  In the end, other hypothetical OS is free
> to ignore those extra fields in the bindings if it doesn't need them.
> So meh?

We really want to err on the side of fewer bindings, not more as once
used they are an ABI.

Rob
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#1213766

On Tue, Aug 25, 2015 at 04:42:18PM -0400, Rob Clark wrote:
> On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@gmail.com> wrote:
> > On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
> >> Stephen Warren <swarren@wwwdotorg.org> writes:
> >>
> >>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
> >>>> Signed-off-by: Eric Anholt <eric@anholt.net>
> >>>
> >>> This one definitely needs a patch description, since someone might not
> >>> know what a VC4 is, and "git log" won't show the text from the binding
> >>> doc itself. I'd suggest adding the initial paragraph of the binding doc
> >>> as the patch description, or more.
> >>>
> >>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
> >>
> >>>> +- hvss:             List of references to HVS video scalers
> >>>> +- encoders: List of references to output encoders (HDMI, SDTV)
> >>>
> >>> Would it make sense to make all those nodes child node of the vc4
> >>> object. That way, there's no need to have these lists of objects; they
> >>> can be automatically built up as the DT is enumerated. I know that e.g.
> >>> the NVIDIA Tegra host1x binding works this way, and I think it may have
> >>> been inspired by other similar cases.
> >>
> >> I've looked at tegra, and the component system used by msm appears to be
> >> nicer than it.  To follow tegra's model, it looks like I need to build
> >> this extra bus thing corresponding to host1x that is effectively the
> >> drivers/base/component.c code, so that I can get at vc4's structure from
> >> the component drivers.
> >>
> >>> Of course, this is only appropriate if the HW modules really are
> >>> logically children of the VC4 HW module. Perhaps they aren't. If they
> >>> aren't though, I wonder what this "vc4" module actually represents in HW?
> >>
> >> It's the subsystem, same as we use a subsystem node for msm, sti,
> >> rockchip, imx, and exynos.  This appears to be the common model of how
> >> the collection of graphics-related components is represented in the DT.
> >
> > I think most of these bindings are wrong. They are grouped together
> > because that is what DRM wants not because that reflects the h/w. So
> > convince me this is one block, not that it is what other people do.
> 
> I think, when it comes to more complex driver subsystems (like drm in
> particular) we have a bit of mismatch between how things look from the
> "pure hw ignoring sw" perspective, and the "how sw and in particular
> userspace expects things" perspective.  Maybe it is less a problem in
> other subsystems, where bindings map to things that are only visible
> in the kernel, or well defined devices like uart or sata controller.
> But when given the choice, I'm going to err on the side of not
> confusing userspace and the large software stack that sits above
> drm/kms, over dt purity.
> 
> Maybe it would be nice to have a sort of dt overlay that adds the bits
> needed to tie together hw blocks that should be assembled into a
> single logical device for linux and userspace (but maybe not some
> other hypothetical operating system).  But so far that doesn't exist.
> All we have is a hammer (devicetree), everything looks like a nail.
> End result is we end up adding some things in the bindings which
> aren't purely about the hw.  Until someone invents a screwdriver, I'm
> not sure what else to do.  In the end, other hypothetical OS is free
> to ignore those extra fields in the bindings if it doesn't need them.
> So meh?

I thought we agreed a while back that these kind of "pull everything for
the logical device together" dt nodes which just have piles of phandles
are totally accepted? At least that's the point behind the component
helpers, and Eric even suggested to create dt-specific component helpers
to cut down a bit on the usual boilerplate. dt maintainers are also fine
with this approach afaik. From my understanding tegra with the host1x bus
really is the odd one out and not the norm.

Given that and with the hope that we'll eventually see a dt-enabled
component functions to standardize this even more the overall concept is

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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#1213779

[Multipart message — attachments visible in raw view] — view raw

On Wed, Aug 26, 2015 at 01:52:29PM +0200, Daniel Vetter wrote:
> On Tue, Aug 25, 2015 at 04:42:18PM -0400, Rob Clark wrote:
> > On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@gmail.com> wrote:
> > > On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
> > >> Stephen Warren <swarren@wwwdotorg.org> writes:
> > >>
> > >>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
> > >>>> Signed-off-by: Eric Anholt <eric@anholt.net>
> > >>>
> > >>> This one definitely needs a patch description, since someone might not
> > >>> know what a VC4 is, and "git log" won't show the text from the binding
> > >>> doc itself. I'd suggest adding the initial paragraph of the binding doc
> > >>> as the patch description, or more.
> > >>>
> > >>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
> > >>
> > >>>> +- hvss:             List of references to HVS video scalers
> > >>>> +- encoders: List of references to output encoders (HDMI, SDTV)
> > >>>
> > >>> Would it make sense to make all those nodes child node of the vc4
> > >>> object. That way, there's no need to have these lists of objects; they
> > >>> can be automatically built up as the DT is enumerated. I know that e.g.
> > >>> the NVIDIA Tegra host1x binding works this way, and I think it may have
> > >>> been inspired by other similar cases.
> > >>
> > >> I've looked at tegra, and the component system used by msm appears to be
> > >> nicer than it.  To follow tegra's model, it looks like I need to build
> > >> this extra bus thing corresponding to host1x that is effectively the
> > >> drivers/base/component.c code, so that I can get at vc4's structure from
> > >> the component drivers.
> > >>
> > >>> Of course, this is only appropriate if the HW modules really are
> > >>> logically children of the VC4 HW module. Perhaps they aren't. If they
> > >>> aren't though, I wonder what this "vc4" module actually represents in HW?
> > >>
> > >> It's the subsystem, same as we use a subsystem node for msm, sti,
> > >> rockchip, imx, and exynos.  This appears to be the common model of how
> > >> the collection of graphics-related components is represented in the DT.
> > >
> > > I think most of these bindings are wrong. They are grouped together
> > > because that is what DRM wants not because that reflects the h/w. So
> > > convince me this is one block, not that it is what other people do.
> > 
> > I think, when it comes to more complex driver subsystems (like drm in
> > particular) we have a bit of mismatch between how things look from the
> > "pure hw ignoring sw" perspective, and the "how sw and in particular
> > userspace expects things" perspective.  Maybe it is less a problem in
> > other subsystems, where bindings map to things that are only visible
> > in the kernel, or well defined devices like uart or sata controller.
> > But when given the choice, I'm going to err on the side of not
> > confusing userspace and the large software stack that sits above
> > drm/kms, over dt purity.
> > 
> > Maybe it would be nice to have a sort of dt overlay that adds the bits
> > needed to tie together hw blocks that should be assembled into a
> > single logical device for linux and userspace (but maybe not some
> > other hypothetical operating system).  But so far that doesn't exist.
> > All we have is a hammer (devicetree), everything looks like a nail.
> > End result is we end up adding some things in the bindings which
> > aren't purely about the hw.  Until someone invents a screwdriver, I'm
> > not sure what else to do.  In the end, other hypothetical OS is free
> > to ignore those extra fields in the bindings if it doesn't need them.
> > So meh?
> 
> I thought we agreed a while back that these kind of "pull everything for
> the logical device together" dt nodes which just have piles of phandles
> are totally accepted? At least that's the point behind the component
> helpers, and Eric even suggested to create dt-specific component helpers
> to cut down a bit on the usual boilerplate. dt maintainers are also fine
> with this approach afaik. From my understanding tegra with the host1x bus
> really is the odd one out and not the norm.

I agree that in many aspects Tegra is somewhat special. But the same
principles that the host1x infrastructure uses could be implemented in a
similar way for other DRM drivers. You can easily collect information
about subdevices by walking the device tree and matching on known
compatible strings. And you can also instantiate the top-level device
from driver code rather than have it in DT. It should still be possible
to make this work without an artificial device node in DT. The component
and master infrastructure is largely orthogonal to that, and as far as I
remember the only blocker is the need for a top-level device. I wonder
if perhaps this could be made to work by binding the master to the top-
level SoC device.

Obviously adding the node in DT is easier, but to my knowledge easy has
never been a good excuse for mangling DT. Perhaps that's different these
days...

Thierry

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#1213923

On Wed, Aug 26, 2015 at 7:09 AM, Thierry Reding
<thierry.reding@gmail.com> wrote:
> On Wed, Aug 26, 2015 at 01:52:29PM +0200, Daniel Vetter wrote:
>> On Tue, Aug 25, 2015 at 04:42:18PM -0400, Rob Clark wrote:
>> > On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@gmail.com> wrote:
>> > > On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
>> > >> Stephen Warren <swarren@wwwdotorg.org> writes:
>> > >>
>> > >>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
>> > >>>> Signed-off-by: Eric Anholt <eric@anholt.net>
>> > >>>
>> > >>> This one definitely needs a patch description, since someone might not
>> > >>> know what a VC4 is, and "git log" won't show the text from the binding
>> > >>> doc itself. I'd suggest adding the initial paragraph of the binding doc
>> > >>> as the patch description, or more.
>> > >>>
>> > >>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
>> > >>
>> > >>>> +- hvss:             List of references to HVS video scalers
>> > >>>> +- encoders: List of references to output encoders (HDMI, SDTV)
>> > >>>
>> > >>> Would it make sense to make all those nodes child node of the vc4
>> > >>> object. That way, there's no need to have these lists of objects; they
>> > >>> can be automatically built up as the DT is enumerated. I know that e.g.
>> > >>> the NVIDIA Tegra host1x binding works this way, and I think it may have
>> > >>> been inspired by other similar cases.
>> > >>
>> > >> I've looked at tegra, and the component system used by msm appears to be
>> > >> nicer than it.  To follow tegra's model, it looks like I need to build
>> > >> this extra bus thing corresponding to host1x that is effectively the
>> > >> drivers/base/component.c code, so that I can get at vc4's structure from
>> > >> the component drivers.
>> > >>
>> > >>> Of course, this is only appropriate if the HW modules really are
>> > >>> logically children of the VC4 HW module. Perhaps they aren't. If they
>> > >>> aren't though, I wonder what this "vc4" module actually represents in HW?
>> > >>
>> > >> It's the subsystem, same as we use a subsystem node for msm, sti,
>> > >> rockchip, imx, and exynos.  This appears to be the common model of how
>> > >> the collection of graphics-related components is represented in the DT.
>> > >
>> > > I think most of these bindings are wrong. They are grouped together
>> > > because that is what DRM wants not because that reflects the h/w. So
>> > > convince me this is one block, not that it is what other people do.
>> >
>> > I think, when it comes to more complex driver subsystems (like drm in
>> > particular) we have a bit of mismatch between how things look from the
>> > "pure hw ignoring sw" perspective, and the "how sw and in particular
>> > userspace expects things" perspective.  Maybe it is less a problem in
>> > other subsystems, where bindings map to things that are only visible
>> > in the kernel, or well defined devices like uart or sata controller.
>> > But when given the choice, I'm going to err on the side of not
>> > confusing userspace and the large software stack that sits above
>> > drm/kms, over dt purity.
>> >
>> > Maybe it would be nice to have a sort of dt overlay that adds the bits
>> > needed to tie together hw blocks that should be assembled into a
>> > single logical device for linux and userspace (but maybe not some
>> > other hypothetical operating system).  But so far that doesn't exist.
>> > All we have is a hammer (devicetree), everything looks like a nail.
>> > End result is we end up adding some things in the bindings which
>> > aren't purely about the hw.  Until someone invents a screwdriver, I'm
>> > not sure what else to do.  In the end, other hypothetical OS is free
>> > to ignore those extra fields in the bindings if it doesn't need them.
>> > So meh?
>>
>> I thought we agreed a while back that these kind of "pull everything for
>> the logical device together" dt nodes which just have piles of phandles
>> are totally accepted? At least that's the point behind the component
>> helpers, and Eric even suggested to create dt-specific component helpers
>> to cut down a bit on the usual boilerplate. dt maintainers are also fine
>> with this approach afaik. From my understanding tegra with the host1x bus
>> really is the odd one out and not the norm.
>
> I agree that in many aspects Tegra is somewhat special. But the same
> principles that the host1x infrastructure uses could be implemented in a
> similar way for other DRM drivers. You can easily collect information
> about subdevices by walking the device tree and matching on known
> compatible strings. And you can also instantiate the top-level device
> from driver code rather than have it in DT. It should still be possible
> to make this work without an artificial device node in DT. The component
> and master infrastructure is largely orthogonal to that, and as far as I
> remember the only blocker is the need for a top-level device. I wonder
> if perhaps this could be made to work by binding the master to the top-
> level SoC device.
>
> Obviously adding the node in DT is easier, but to my knowledge easy has
> never been a good excuse for mangling DT. Perhaps that's different these
> days...

I agree we should avoid the virtual node if possible. It is certainly
possible as I started out with one and removed it. At least in my
case, it essentially required the drm_device and crtc to be a single
driver rather than 2 components. However, I'm more concerned that we
are consistent from platform to platform where it makes sense than
whether we have a somewhat questionable node or not.

Rob
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#1214191

On 27 August 2015 at 00:30, Rob Herring <robherring2@gmail.com> wrote:
> On Wed, Aug 26, 2015 at 7:09 AM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
>> On Wed, Aug 26, 2015 at 01:52:29PM +0200, Daniel Vetter wrote:
>>> On Tue, Aug 25, 2015 at 04:42:18PM -0400, Rob Clark wrote:
>>> > On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@gmail.com> wrote:
>>> > > On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
>>> > >> Stephen Warren <swarren@wwwdotorg.org> writes:
>>> > >>
>>> > >>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
>>> > >>>> Signed-off-by: Eric Anholt <eric@anholt.net>
>>> > >>>
>>> > >>> This one definitely needs a patch description, since someone might not
>>> > >>> know what a VC4 is, and "git log" won't show the text from the binding
>>> > >>> doc itself. I'd suggest adding the initial paragraph of the binding doc
>>> > >>> as the patch description, or more.
>>> > >>>
>>> > >>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
>>> > >>
>>> > >>>> +- hvss:             List of references to HVS video scalers
>>> > >>>> +- encoders: List of references to output encoders (HDMI, SDTV)
>>> > >>>
>>> > >>> Would it make sense to make all those nodes child node of the vc4
>>> > >>> object. That way, there's no need to have these lists of objects; they
>>> > >>> can be automatically built up as the DT is enumerated. I know that e.g.
>>> > >>> the NVIDIA Tegra host1x binding works this way, and I think it may have
>>> > >>> been inspired by other similar cases.
>>> > >>
>>> > >> I've looked at tegra, and the component system used by msm appears to be
>>> > >> nicer than it.  To follow tegra's model, it looks like I need to build
>>> > >> this extra bus thing corresponding to host1x that is effectively the
>>> > >> drivers/base/component.c code, so that I can get at vc4's structure from
>>> > >> the component drivers.
>>> > >>
>>> > >>> Of course, this is only appropriate if the HW modules really are
>>> > >>> logically children of the VC4 HW module. Perhaps they aren't. If they
>>> > >>> aren't though, I wonder what this "vc4" module actually represents in HW?
>>> > >>
>>> > >> It's the subsystem, same as we use a subsystem node for msm, sti,
>>> > >> rockchip, imx, and exynos.  This appears to be the common model of how
>>> > >> the collection of graphics-related components is represented in the DT.
>>> > >
>>> > > I think most of these bindings are wrong. They are grouped together
>>> > > because that is what DRM wants not because that reflects the h/w. So
>>> > > convince me this is one block, not that it is what other people do.
>>> >
>>> > I think, when it comes to more complex driver subsystems (like drm in
>>> > particular) we have a bit of mismatch between how things look from the
>>> > "pure hw ignoring sw" perspective, and the "how sw and in particular
>>> > userspace expects things" perspective.  Maybe it is less a problem in
>>> > other subsystems, where bindings map to things that are only visible
>>> > in the kernel, or well defined devices like uart or sata controller.
>>> > But when given the choice, I'm going to err on the side of not
>>> > confusing userspace and the large software stack that sits above
>>> > drm/kms, over dt purity.
>>> >
>>> > Maybe it would be nice to have a sort of dt overlay that adds the bits
>>> > needed to tie together hw blocks that should be assembled into a
>>> > single logical device for linux and userspace (but maybe not some
>>> > other hypothetical operating system).  But so far that doesn't exist.
>>> > All we have is a hammer (devicetree), everything looks like a nail.
>>> > End result is we end up adding some things in the bindings which
>>> > aren't purely about the hw.  Until someone invents a screwdriver, I'm
>>> > not sure what else to do.  In the end, other hypothetical OS is free
>>> > to ignore those extra fields in the bindings if it doesn't need them.
>>> > So meh?
>>>
>>> I thought we agreed a while back that these kind of "pull everything for
>>> the logical device together" dt nodes which just have piles of phandles
>>> are totally accepted? At least that's the point behind the component
>>> helpers, and Eric even suggested to create dt-specific component helpers
>>> to cut down a bit on the usual boilerplate. dt maintainers are also fine
>>> with this approach afaik. From my understanding tegra with the host1x bus
>>> really is the odd one out and not the norm.
>>
>> I agree that in many aspects Tegra is somewhat special. But the same
>> principles that the host1x infrastructure uses could be implemented in a
>> similar way for other DRM drivers. You can easily collect information
>> about subdevices by walking the device tree and matching on known
>> compatible strings. And you can also instantiate the top-level device
>> from driver code rather than have it in DT. It should still be possible
>> to make this work without an artificial device node in DT. The component
>> and master infrastructure is largely orthogonal to that, and as far as I
>> remember the only blocker is the need for a top-level device. I wonder
>> if perhaps this could be made to work by binding the master to the top-
>> level SoC device.
>>
>> Obviously adding the node in DT is easier, but to my knowledge easy has
>> never been a good excuse for mangling DT. Perhaps that's different these
>> days...
>
> I agree we should avoid the virtual node if possible. It is certainly
> possible as I started out with one and removed it. At least in my
> case, it essentially required the drm_device and crtc to be a single
> driver rather than 2 components. However, I'm more concerned that we
> are consistent from platform to platform where it makes sense than
> whether we have a somewhat questionable node or not.
>
http://lists.freedesktop.org/archives/dri-devel/2013-July/041159.html

So can we at least have some continuity of decision making,
bikeshedding this every time we submit a driver isn't giving me any
hope going forward.

Dave.
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#1214258

On Wed, Aug 26, 2015 at 3:59 PM, Dave Airlie <airlied@gmail.com> wrote:
> On 27 August 2015 at 00:30, Rob Herring <robherring2@gmail.com> wrote:
>> On Wed, Aug 26, 2015 at 7:09 AM, Thierry Reding
>> <thierry.reding@gmail.com> wrote:
>>> On Wed, Aug 26, 2015 at 01:52:29PM +0200, Daniel Vetter wrote:
>>>> On Tue, Aug 25, 2015 at 04:42:18PM -0400, Rob Clark wrote:
>>>> > On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@gmail.com> wrote:
>>>> > > On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@anholt.net> wrote:
>>>> > >> Stephen Warren <swarren@wwwdotorg.org> writes:
>>>> > >>
>>>> > >>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
>>>> > >>>> Signed-off-by: Eric Anholt <eric@anholt.net>
[...]
>>>> > >>>> +- hvss:             List of references to HVS video scalers
>>>> > >>>> +- encoders: List of references to output encoders (HDMI, SDTV)
[...]
>>>> > >>> Of course, this is only appropriate if the HW modules really are
>>>> > >>> logically children of the VC4 HW module. Perhaps they aren't. If they
>>>> > >>> aren't though, I wonder what this "vc4" module actually represents in HW?
>>>> > >>
>>>> > >> It's the subsystem, same as we use a subsystem node for msm, sti,
>>>> > >> rockchip, imx, and exynos.  This appears to be the common model of how
>>>> > >> the collection of graphics-related components is represented in the DT.
>>>> > >
>>>> > > I think most of these bindings are wrong. They are grouped together
>>>> > > because that is what DRM wants not because that reflects the h/w. So
>>>> > > convince me this is one block, not that it is what other people do.
>>>> >
>>>> > I think, when it comes to more complex driver subsystems (like drm in
>>>> > particular) we have a bit of mismatch between how things look from the
>>>> > "pure hw ignoring sw" perspective, and the "how sw and in particular
>>>> > userspace expects things" perspective.  Maybe it is less a problem in
>>>> > other subsystems, where bindings map to things that are only visible
>>>> > in the kernel, or well defined devices like uart or sata controller.
>>>> > But when given the choice, I'm going to err on the side of not
>>>> > confusing userspace and the large software stack that sits above
>>>> > drm/kms, over dt purity.
>>>> >
>>>> > Maybe it would be nice to have a sort of dt overlay that adds the bits
>>>> > needed to tie together hw blocks that should be assembled into a
>>>> > single logical device for linux and userspace (but maybe not some
>>>> > other hypothetical operating system).  But so far that doesn't exist.
>>>> > All we have is a hammer (devicetree), everything looks like a nail.
>>>> > End result is we end up adding some things in the bindings which
>>>> > aren't purely about the hw.  Until someone invents a screwdriver, I'm
>>>> > not sure what else to do.  In the end, other hypothetical OS is free
>>>> > to ignore those extra fields in the bindings if it doesn't need them.
>>>> > So meh?
>>>>
>>>> I thought we agreed a while back that these kind of "pull everything for
>>>> the logical device together" dt nodes which just have piles of phandles
>>>> are totally accepted? At least that's the point behind the component
>>>> helpers, and Eric even suggested to create dt-specific component helpers
>>>> to cut down a bit on the usual boilerplate. dt maintainers are also fine
>>>> with this approach afaik. From my understanding tegra with the host1x bus
>>>> really is the odd one out and not the norm.
>>>
>>> I agree that in many aspects Tegra is somewhat special. But the same
>>> principles that the host1x infrastructure uses could be implemented in a
>>> similar way for other DRM drivers. You can easily collect information
>>> about subdevices by walking the device tree and matching on known
>>> compatible strings. And you can also instantiate the top-level device
>>> from driver code rather than have it in DT. It should still be possible
>>> to make this work without an artificial device node in DT. The component
>>> and master infrastructure is largely orthogonal to that, and as far as I
>>> remember the only blocker is the need for a top-level device. I wonder
>>> if perhaps this could be made to work by binding the master to the top-
>>> level SoC device.
>>>
>>> Obviously adding the node in DT is easier, but to my knowledge easy has
>>> never been a good excuse for mangling DT. Perhaps that's different these
>>> days...
>>
>> I agree we should avoid the virtual node if possible. It is certainly
>> possible as I started out with one and removed it. At least in my
>> case, it essentially required the drm_device and crtc to be a single
>> driver rather than 2 components. However, I'm more concerned that we
>> are consistent from platform to platform where it makes sense than
>> whether we have a somewhat questionable node or not.
>>
> http://lists.freedesktop.org/archives/dri-devel/2013-July/041159.html
>
> So can we at least have some continuity of decision making,
> bikeshedding this every time we submit a driver isn't giving me any
> hope going forward.

As I said, whether we have a virtual node or not is a minor part of
having some consistency in bindings and was the only part Grant
discussed. We have though diverged from the specific problems with
this binding which is that it invents yet another way to describe the
relationship between components. The use of DRM component names in the
binding properties is the first clue. As to what is the "right" way,
well if that is known or documented I've seen no evidence. Defining
what that is and having the infrastructure in place to support it is
what I'm interested in.

Rob
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