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

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Message-ID: <20161005212223.GK10625@jcartwri.amer.corp.natinst.com>
Date:   Wed, 5 Oct 2016 16:22:23 -0500
From:   Julia Cartwright <julia@...com>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Zach Brown <zach.brown@...com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/2] sdhci: Add device tree property sd-broken-highspeed

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@...aro.org> wrote:
> > On 23 September 2016 at 22:01, Zach Brown <zach.brown@...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@...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