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Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed

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On Wed, Oct 05, 2016 at 03:03:44PM -0500, Rob Herring wrote:
> On Wed, Oct 5, 2016 at 1:33 PM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> > On 23 September 2016 at 22:01, Zach Brown <zach.brown@ni.com> wrote:
> >> Certain board configurations can make highspeed malfunction due to
> >> timing issues. In these cases a way is needed to force the controller
> >> and card into standard speed even if they otherwise appear to be capable
> >> of highspeed.
> >>
> >> The sd-broken-highspeed property will let the sdhci driver know that
> >> highspeed will not work.
> >>
> >> Signed-off-by: Zach Brown <zach.brown@ni.com>
> >> ---
> >>  Documentation/devicetree/bindings/mmc/mmc.txt | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/Documentation/devicetree/bindings/mmc/mmc.txt b/Documentation/devicetree/bindings/mmc/mmc.txt
> >> index 8a37782..59332ea 100644
> >> --- a/Documentation/devicetree/bindings/mmc/mmc.txt
> >> +++ b/Documentation/devicetree/bindings/mmc/mmc.txt
> >> @@ -52,6 +52,8 @@ Optional properties:
> >>  - no-sdio: controller is limited to send sdio cmd during initialization
> >>  - no-sd: controller is limited to send sd cmd during initialization
> >>  - no-mmc: controller is limited to send mmc cmd during initialization
> >> +- sd-broken-highspeed: Highspeed is broken, even if the controller and card
> >> +  themselves claim they support highspeed.
> >
> > Regarding a broken card, that is managed via the card quirks and not in DT.
> >
> > If this is about a controller limitation, we already have the option
> > to describe what it supports, so we don't need an option to tell what
> > it *not* supports.
> >
> > For example "cap-sd-highspeed" tells whether the controller supports
> > SD high-speed, please use that instead.
> 
> If a controller has a capability register and it lies (perhaps the
> board has limitations that the SoC does not), then you may need to
> disable a feature.

That's precisely the case here.  This is a board-level problem, not a
card or controller problem.  As Zach mentioned in the cover letter, the
trace length between controller and card on some of our boards is too
long to meet high-speed timings, even though both card and controller
advertise it.

Thanks,
   Julia

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[RFC 0/2] Add device tree property and quirk for supporting sdhci Zach Brown <zach.brown@ni.com> - 2016-09-23 22:30 +0200
  [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Zach Brown <zach.brown@ni.com> - 2016-09-23 22:30 +0200
    Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Rob Herring <robh@kernel.org> - 2016-10-03 19:40 +0200
    Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Ulf Hansson <ulf.hansson@linaro.org> - 2016-10-05 20:50 +0200
      Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Rob Herring <robh+dt@kernel.org> - 2016-10-05 22:10 +0200
        Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Julia Cartwright <julia@ni.com> - 2016-10-05 23:30 +0200
          Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Shawn Lin <shawn.lin@rock-chips.com> - 2016-10-06 03:40 +0200
            Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Adrian Hunter <adrian.hunter@intel.com> - 2016-10-06 08:20 +0200
        Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed Ulf Hansson <ulf.hansson@linaro.org> - 2016-10-06 10:10 +0200

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