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Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

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  Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Mark Brown <broonie@kernel.org> - 2016-05-30 13:30 +0200
    Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Geert Uytterhoeven <geert@linux-m68k.org> - 2016-05-30 14:20 +0200
      Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Mark Brown <broonie@kernel.org> - 2016-05-30 17:10 +0200
        Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Michal Suchanek <hramrach@gmail.com> - 2016-05-30 17:30 +0200
          Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Mark Brown <broonie@kernel.org> - 2016-05-30 18:00 +0200
            Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Michal Suchanek <hramrach@gmail.com> - 2016-05-31 12:50 +0200
              Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Mark Brown <broonie@kernel.org> - 2016-05-31 15:30 +0200
                Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Michal Suchanek <hramrach@gmail.com> - 2016-05-31 16:30 +0200
                  Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Maxime Ripard <maxime.ripard@free-electrons.com> - 2016-06-02 10:20 +0200
            Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Maxime Ripard <maxime.ripard@free-electrons.com> - 2016-06-01 20:10 +0200
              Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Priit Laes <plaes@plaes.org> - 2016-06-02 06:50 +0200
                Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Mark Brown <broonie@kernel.org> - 2016-06-02 11:20 +0200
                Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Michal Suchanek <hramrach@gmail.com> - 2016-06-02 14:20 +0200
                  Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Mark Brown <broonie@kernel.org> - 2016-06-02 16:30 +0200
                    Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Michal Suchanek <hramrach@gmail.com> - 2016-06-05 13:30 +0200
                      Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support Mark Brown <broonie@kernel.org> - 2016-06-06 13:40 +0200

#1408994 — Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

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On Thu, May 26, 2016 at 07:25:25PM -0000, Michal Suchanek wrote:

>  - fallback to previous behaviour when DMA initialization fails
> 
>    + this has the problem that when the driver happens to load before the dma
>      driver it will not use dma - can be addressed with a module parameter

No, you should pay attention to the error you are getting and let probe
deferral happen if that's the error you get.

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#1409037

Hi Mark,

On Mon, May 30, 2016 at 1:26 PM, Mark Brown <broonie@kernel.org> wrote:
> On Thu, May 26, 2016 at 07:25:25PM -0000, Michal Suchanek wrote:
>>  - fallback to previous behaviour when DMA initialization fails
>>
>>    + this has the problem that when the driver happens to load before the dma
>>      driver it will not use dma - can be addressed with a module parameter
>
> No, you should pay attention to the error you are getting and let probe
> deferral happen if that's the error you get.

Unfortunately DMA is an optional feature.

There's no way to distinguish between -EPROBE_DEFER due to the SPI master
driver being probed before the DMA engine driver, and -EPROBE_DEFER due to
support for the DMA engine not having been compiled in.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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#1409205

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On Mon, May 30, 2016 at 02:11:51PM +0200, Geert Uytterhoeven wrote:
> On Mon, May 30, 2016 at 1:26 PM, Mark Brown <broonie@kernel.org> wrote:
> > On Thu, May 26, 2016 at 07:25:25PM -0000, Michal Suchanek wrote:
> >>  - fallback to previous behaviour when DMA initialization fails
> >>
> >>    + this has the problem that when the driver happens to load before the dma
> >>      driver it will not use dma - can be addressed with a module parameter

> > No, you should pay attention to the error you are getting and let probe
> > deferral happen if that's the error you get.

> Unfortunately DMA is an optional feature.

> There's no way to distinguish between -EPROBE_DEFER due to the SPI master
> driver being probed before the DMA engine driver, and -EPROBE_DEFER due to
> support for the DMA engine not having been compiled in.

I really don't think it's worth caring too much about cases where the
DMA driver hasn't been compiled in, it's not like SPI is the only thing
that's going to be using it.  I really think it's better to defer the
problem - not getting DMA (or worse, only getting DMA on some boots) is
not great and if people are optimising on that level my feeling is that
they're probably going to be OK with customizing DT to match.  Ideally
we would have something a bit nicer than deferred probe but right now
this seems the more helpful option.

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#1409228

Hello,

On 30 May 2016 at 17:03, Mark Brown <broonie@kernel.org> wrote:
> On Mon, May 30, 2016 at 02:11:51PM +0200, Geert Uytterhoeven wrote:
>> On Mon, May 30, 2016 at 1:26 PM, Mark Brown <broonie@kernel.org> wrote:
>> > On Thu, May 26, 2016 at 07:25:25PM -0000, Michal Suchanek wrote:
>> >>  - fallback to previous behaviour when DMA initialization fails
>> >>
>> >>    + this has the problem that when the driver happens to load before the dma
>> >>      driver it will not use dma - can be addressed with a module parameter
>
>> > No, you should pay attention to the error you are getting and let probe
>> > deferral happen if that's the error you get.
>
>> Unfortunately DMA is an optional feature.
>
>> There's no way to distinguish between -EPROBE_DEFER due to the SPI master
>> driver being probed before the DMA engine driver, and -EPROBE_DEFER due to
>> support for the DMA engine not having been compiled in.
>
> I really don't think it's worth caring too much about cases where the
> DMA driver hasn't been compiled in, it's not like SPI is the only thing
> that's going to be using it.  I really think it's better to defer the
> problem - not getting DMA (or worse, only getting DMA on some boots) is
> not great and if people are optimising on that level my feeling is that
> they're probably going to be OK with customizing DT to match.  Ideally
> we would have something a bit nicer than deferred probe but right now
> this seems the more helpful option.

It's what the driver did to start with and it was requested to fall
back to non-DMA in the case DMA is not available.

We get to the much discussed problem that it's never possible to tell
if a driver is available or not.

It's possible to add a parameter like require_dma which could be used
to load the driver without dma if unset. If it was set by default then
driver ordering is not important so long as dma driver is loaded
eventually. Also an informative print that such parameter exists when
probing the driver is deferred would be helpful. It would probably
create quite a bit of log spam, however. The driver can be deferred
several times during boot.

Thanks

Michal

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#1409248

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On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
> On 30 May 2016 at 17:03, Mark Brown <broonie@kernel.org> wrote:

> > I really don't think it's worth caring too much about cases where the
> > DMA driver hasn't been compiled in, it's not like SPI is the only thing

> It's what the driver did to start with and it was requested to fall
> back to non-DMA in the case DMA is not available.

Why?  I really can't see any sensible use case for this that doesn't
have a better solution available.

> It's possible to add a parameter like require_dma which could be used
> to load the driver without dma if unset. If it was set by default then
> driver ordering is not important so long as dma driver is loaded
> eventually. Also an informative print that such parameter exists when
> probing the driver is deferred would be helpful. It would probably
> create quite a bit of log spam, however. The driver can be deferred
> several times during boot.

That seems fairly hacky, if we were going to do anything like that it
should be the other way around so that we default to trying to use
resources and even then it seems like something that should be handled
at a framework level rather than having random options in individual
drivers to ignore things.  Having things behave inconsistently between
different drivers is going to lead to a worse user experience and if
this is a good idea for one driver it seems like it'd be a good idea for
all of them.

But really 

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#1410134

On 30 May 2016 at 17:50, Mark Brown <broonie@kernel.org> wrote:
> On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
>> On 30 May 2016 at 17:03, Mark Brown <broonie@kernel.org> wrote:
>
>> > I really don't think it's worth caring too much about cases where the
>> > DMA driver hasn't been compiled in, it's not like SPI is the only thing
>
>> It's what the driver did to start with and it was requested to fall
>> back to non-DMA in the case DMA is not available.
>
> Why?  I really can't see any sensible use case for this that doesn't
> have a better solution available.

Of course, the solution is to compile in the DMA driver.

It's been argued that some drivers which use only short transfers will
just work.

>
>> It's possible to add a parameter like require_dma which could be used
>> to load the driver without dma if unset. If it was set by default then
>> driver ordering is not important so long as dma driver is loaded
>> eventually. Also an informative print that such parameter exists when
>> probing the driver is deferred would be helpful. It would probably
>> create quite a bit of log spam, however. The driver can be deferred
>> several times during boot.
>
> That seems fairly hacky, if we were going to do anything like that it
> should be the other way around so that we default to trying to use
> resources and even then it seems like something that should be handled
> at a framework level rather than having random options in individual
> drivers to ignore things.  Having things behave inconsistently between
> different drivers is going to lead to a worse user experience and if
> this is a good idea for one driver it seems like it'd be a good idea for
> all of them.

Hacky but doable if desirable. It's awesome for testing SPI transfer
fragmentation with different drivers ;-)

The previous discussion of this driver is here
http://thread.gmane.org/gmane.linux.kernel/2162327

Thanks

Michal

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#1410309

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On Tue, May 31, 2016 at 12:44:54PM +0200, Michal Suchanek wrote:
> On 30 May 2016 at 17:50, Mark Brown <broonie@kernel.org> wrote:
> > On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:

> >> It's what the driver did to start with and it was requested to fall
> >> back to non-DMA in the case DMA is not available.

> > Why?  I really can't see any sensible use case for this that doesn't
> > have a better solution available.

> Of course, the solution is to compile in the DMA driver.

> It's been argued that some drivers which use only short transfers will
> just work.

With nothing else in the system that needs DMA?  It's making the
performance of the system less reliable for the benefit of a very narrow
use case.

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#1410346

On 31 May 2016 at 15:27, Mark Brown <broonie@kernel.org> wrote:
> On Tue, May 31, 2016 at 12:44:54PM +0200, Michal Suchanek wrote:
>> On 30 May 2016 at 17:50, Mark Brown <broonie@kernel.org> wrote:
>> > On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
>
>> >> It's what the driver did to start with and it was requested to fall
>> >> back to non-DMA in the case DMA is not available.
>
>> > Why?  I really can't see any sensible use case for this that doesn't
>> > have a better solution available.
>
>> Of course, the solution is to compile in the DMA driver.
>
>> It's been argued that some drivers which use only short transfers will
>> just work.
>
> With nothing else in the system that needs DMA?  It's making the
> performance of the system less reliable for the benefit of a very narrow
> use case.

Some of the platform devices have dedicated DMA *controller* built
into the device IP so the DMA engine really is optional on many sunxi
devices. Besides SPI you definitely need the DMA engine for audio. You
probably don't need it for storage and graphics. I don't have any idea
if it's used for USB and Ethernet.

Thanks

Michal

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#1411983

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On Tue, May 31, 2016 at 04:19:28PM +0200, Michal Suchanek wrote:
> On 31 May 2016 at 15:27, Mark Brown <broonie@kernel.org> wrote:
> > On Tue, May 31, 2016 at 12:44:54PM +0200, Michal Suchanek wrote:
> >> On 30 May 2016 at 17:50, Mark Brown <broonie@kernel.org> wrote:
> >> > On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
> >
> >> >> It's what the driver did to start with and it was requested to fall
> >> >> back to non-DMA in the case DMA is not available.
> >
> >> > Why?  I really can't see any sensible use case for this that doesn't
> >> > have a better solution available.
> >
> >> Of course, the solution is to compile in the DMA driver.
> >
> >> It's been argued that some drivers which use only short transfers will
> >> just work.
> >
> > With nothing else in the system that needs DMA?  It's making the
> > performance of the system less reliable for the benefit of a very narrow
> > use case.
> 
> Some of the platform devices have dedicated DMA *controller* built
> into the device IP so the DMA engine really is optional on many sunxi
> devices. Besides SPI you definitely need the DMA engine for audio. You
> probably don't need it for storage and graphics. I don't have any idea
> if it's used for USB and Ethernet.

USB and Ethernet have their own dedicated DMA engines. So currently,
the only driver that requires it is the audio codec.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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#1411443

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Hi,

On Mon, May 30, 2016 at 04:50:16PM +0100, Mark Brown wrote:
> On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
> > On 30 May 2016 at 17:03, Mark Brown <broonie@kernel.org> wrote:
> 
> > > I really don't think it's worth caring too much about cases where the
> > > DMA driver hasn't been compiled in, it's not like SPI is the only thing
> 
> > It's what the driver did to start with and it was requested to fall
> > back to non-DMA in the case DMA is not available.
> 
> Why?  I really can't see any sensible use case for this that doesn't
> have a better solution available.

SPI works just fine without DMA, which might just be considered an
(optional) optimisation.

We've been using it without DMA for years now, and it was working just
fine, and it will work even better with the other patches in this
serie. There's no reason to add a hard dependency on something that we
don't really need.

Maxime


-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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#1411820 — Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

On Wed, 2016-06-01 at 20:00 +0200, Maxime Ripard wrote:
> Hi,
> 
> On Mon, May 30, 2016 at 04:50:16PM +0100, Mark Brown wrote:
> > 
> > On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
> > > 
> > > On 30 May 2016 at 17:03, Mark Brown <broonie@kernel.org> wrote:
> > > 
> > > > 
> > > > I really don't think it's worth caring too much about cases
> > > > where the
> > > > DMA driver hasn't been compiled in, it's not like SPI is the
> > > > only thing
> > > 
> > > It's what the driver did to start with and it was requested to
> > > fall
> > > back to non-DMA in the case DMA is not available.
> > Why?  I really can't see any sensible use case for this that
> > doesn't
> > have a better solution available.
> SPI works just fine without DMA, which might just be considered an
> (optional) optimisation.
> 
> We've been using it without DMA for years now, and it was working
> just
> fine, and it will work even better with the other patches in this
> serie. There's no reason to add a hard dependency on something that
> we
> don't really need.
> 

Actually it non-DMA case works fine if you don't need SPI transfers
larger than SUN4I_FIFO_DEPTH - 1, which is 63 bytes.

This was addressed by this patch, but was never applied:
http://permalink.gmane.org/gmane.linux.kernel.spi.devel/18950

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#1412018 — Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

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On Thu, Jun 02, 2016 at 07:42:26AM +0300, Priit Laes wrote:

> This was addressed by this patch, but was never applied:
> http://permalink.gmane.org/gmane.linux.kernel.spi.devel/18950

I've never seen that patch before, if it was CCed to me there's a good
chance it'd have been deleted unread because you put ARM: at the start
of the subject line which says it's a patch for arch/arm.  You'll need
to resubmit.

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#1412176 — Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

On 2 June 2016 at 06:42, Priit Laes <plaes@plaes.org> wrote:
> On Wed, 2016-06-01 at 20:00 +0200, Maxime Ripard wrote:
>> Hi,
>>
>> On Mon, May 30, 2016 at 04:50:16PM +0100, Mark Brown wrote:
>> >
>> > On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
>> > >
>> > > On 30 May 2016 at 17:03, Mark Brown <broonie@kernel.org> wrote:
>> > >
>> > > >
>> > > > I really don't think it's worth caring too much about cases
>> > > > where the
>> > > > DMA driver hasn't been compiled in, it's not like SPI is the
>> > > > only thing
>> > >
>> > > It's what the driver did to start with and it was requested to
>> > > fall
>> > > back to non-DMA in the case DMA is not available.
>> > Why?  I really can't see any sensible use case for this that
>> > doesn't
>> > have a better solution available.
>> SPI works just fine without DMA, which might just be considered an
>> (optional) optimisation.
>>
>> We've been using it without DMA for years now, and it was working
>> just
>> fine, and it will work even better with the other patches in this
>> serie. There's no reason to add a hard dependency on something that
>> we
>> don't really need.
>>
>
> Actually it non-DMA case works fine if you don't need SPI transfers
> larger than SUN4I_FIFO_DEPTH - 1, which is 63 bytes.
>
> This was addressed by this patch, but was never applied:
> http://permalink.gmane.org/gmane.linux.kernel.spi.devel/18950

And the code added in that patch will never run unless you

1) use long spi transfers
2) compile in/load SPI without DMA support

There is no reason for doing 2) since we have do DMA support for sunxi.

So that's another code path that needs maintenance and testing and
likely will not get it.

Thanks

Michal

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#1412285 — Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

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On Thu, Jun 02, 2016 at 02:14:26PM +0200, Michal Suchanek wrote:
> On 2 June 2016 at 06:42, Priit Laes <plaes@plaes.org> wrote:
> > On Wed, 2016-06-01 at 20:00 +0200, Maxime Ripard wrote:

> > Actually it non-DMA case works fine if you don't need SPI transfers
> > larger than SUN4I_FIFO_DEPTH - 1, which is 63 bytes.

> > This was addressed by this patch, but was never applied:
> > http://permalink.gmane.org/gmane.linux.kernel.spi.devel/18950

> And the code added in that patch will never run unless you

> 1) use long spi transfers
> 2) compile in/load SPI without DMA support

> There is no reason for doing 2) since we have do DMA support for sunxi.

Well, presumably such code exists and is being worked on?

> So that's another code path that needs maintenance and testing and
> likely will not get it.

Oh, come on.  You might not want to use it yourself but the chances are
that someone will want to use it just like the situation with all the
other SPI drivers.  It's a perfectly reasonable and sensible feature to
support upstream.

I really do not understand why there is such a strong desire to have
these devices be a special snowflake here, the worst that's likely to
happen here is that you're going to end up having to either remove the
DMA controller from the DT or load the driver for it neither of which
seem like the end of the world.

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#1414107 — Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

On 2 June 2016 at 16:26, Mark Brown <broonie@kernel.org> wrote:
> On Thu, Jun 02, 2016 at 02:14:26PM +0200, Michal Suchanek wrote:
>> On 2 June 2016 at 06:42, Priit Laes <plaes@plaes.org> wrote:
>> > On Wed, 2016-06-01 at 20:00 +0200, Maxime Ripard wrote:
>
>> > Actually it non-DMA case works fine if you don't need SPI transfers
>> > larger than SUN4I_FIFO_DEPTH - 1, which is 63 bytes.
>
>> > This was addressed by this patch, but was never applied:
>> > http://permalink.gmane.org/gmane.linux.kernel.spi.devel/18950
>
>> And the code added in that patch will never run unless you
>
>> 1) use long spi transfers
>> 2) compile in/load SPI without DMA support
>
>> There is no reason for doing 2) since we have do DMA support for sunxi.
>
> Well, presumably such code exists and is being worked on?

Which code are you referring to?

This is a reply to patch which adds DMA support to the SPI driver so
that it can work for arbitrarily long transfers.

So there is code for fully working driver.

>
>> So that's another code path that needs maintenance and testing and
>> likely will not get it.
>
> Oh, come on.  You might not want to use it yourself but the chances are
> that someone will want to use it just like the situation with all the
> other SPI drivers.  It's a perfectly reasonable and sensible feature to
> support upstream.

Is it?

Once we have *one* driver that works for arbitrarily long transfers
and it works out of the box with the board defconfig 99% of people who
will use the SPI driver for anything will use this driver. Any other
variant will go untested.

And for the driver to also work without DMA you have to *tell* it to
probe without DMA because it cannot know you are not going to load a
DMA driver later.

>
> I really do not understand why there is such a strong desire to have
> these devices be a special snowflake here, the worst that's likely to
> happen here is that you're going to end up having to either remove the
> DMA controller from the DT or load the driver for it neither of which
> seem like the end of the world.

Why would you do that?

Thanks

Michal

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#1414933 — Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

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On Sun, Jun 05, 2016 at 01:27:11PM +0200, Michal Suchanek wrote:
> On 2 June 2016 at 16:26, Mark Brown <broonie@kernel.org> wrote:
> > On Thu, Jun 02, 2016 at 02:14:26PM +0200, Michal Suchanek wrote:

> >> And the code added in that patch will never run unless you

> >> 1) use long spi transfers
> >> 2) compile in/load SPI without DMA support

> >> There is no reason for doing 2) since we have do DMA support for sunxi.

> > Well, presumably such code exists and is being worked on?

> Which code are you referring to?

> This is a reply to patch which adds DMA support to the SPI driver so
> that it can work for arbitrarily long transfers.

It sounded like there might be a missing DMA driver?  Though I think I
was misreading the above.

> > Oh, come on.  You might not want to use it yourself but the chances are
> > that someone will want to use it just like the situation with all the
> > other SPI drivers.  It's a perfectly reasonable and sensible feature to
> > support upstream.

> Is it?

> Once we have *one* driver that works for arbitrarily long transfers
> and it works out of the box with the board defconfig 99% of people who
> will use the SPI driver for anything will use this driver. Any other
> variant will go untested.

We don't want multiple drivers for this and general maintainability
reasons, I'm not sure where the idea that we might want multiple drivers
came from?

> And for the driver to also work without DMA you have to *tell* it to
> probe without DMA because it cannot know you are not going to load a
> DMA driver later.

Yes.  This puts the cost on the special snowflake systems that
absolutely cannot have any DMA support in their systems for some reason
while keeping the driver featureful by default, and keeps that cost
fairly small.

> > I really do not understand why there is such a strong desire to have
> > these devices be a special snowflake here, the worst that's likely to
> > happen here is that you're going to end up having to either remove the
> > DMA controller from the DT or load the driver for it neither of which
> > seem like the end of the world.

> Why would you do that?

Like I say I really don't understand why this is.  I'm just saying there
are ways to do this fairly simply if people really want it on their
systems.

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