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

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Date:   Thu, 6 Oct 2016 10:03:56 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     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 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