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

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Date:   Mon, 17 Oct 2016 16:49:05 -0500
From:   Zach Brown <zach.brown@...com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
CC:     Rob Herring <robh+dt@...nel.org>,
        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 Thu, Oct 06, 2016 at 10:03:56AM +0200, Ulf Hansson wrote:
> On 5 October 2016 at 22:03, Rob Herring <robh+dt@...nel.org> 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.
>
> I understand, although the SDHCI capabilities register is broken for
> most SDHCI variants. In principle, we would more or less have to add a
> *-broken binding for each bit in that register. I don't like that!
>
> Maybe a better option is to add a "sdhci-cap-broken" or perhaps
> "sdhci-cap-speed-modes-broken", which tells the driver to not rely on
> the capabilities register and instead find out what *is* supported by
> looking at the other mmc generic DT bindings.
>
> What do you think of that?
>
> Kind regards
> Uffe

"sdhci-cap-broken" seems too aggressive. There might only be one capability
that the register incorrectly advertises.

"sdhci-cap-speed-modes-broken" makes more sense and I will re-create this patch
set using that idea.