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

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Date:   Wed, 5 Oct 2016 15:03:44 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.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 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.

Rob