linux-kernel - Re: [PATCH 2/3] mmc: sdhci: add quirk to prevent higher speed modes

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Date:   Wed, 4 Jul 2018 12:07:32 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Stefan Agner <stefan@...er.ch>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Haibo Chen <haibo.chen@....com>,
        Aisheng Dong <aisheng.dong@....com>,
        Michael Trimarchi <michael@...rulasolutions.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] mmc: sdhci: add quirk to prevent higher speed modes

On 3 July 2018 at 10:48, Stefan Agner <stefan@...er.ch> wrote:
> On 02.07.2018 16:36, Ulf Hansson wrote:
>> On 28 June 2018 at 10:13, Stefan Agner <stefan@...er.ch> wrote:
>>> Some hosts are capable of running higher speed modes but do not
>>> have the board support for it. Introduce a quirk which prevents
>>> the stack from using modes running at 100MHz or faster.
>>
>> To cap the freq, use the DT property "max-frequency". To enable
>> certain speed modes, use the corresponding speed mode binding. For
>> example "sd-uhs-sdr*" and "mmc-hs200*".
>> Documented in Documentation/devicetree/bindings/mmc/mmc.txt
>
> I had bad experience with max-frequency: Some higher speed modes seem
> not to work reliably if constraint to low frequencies. E.g. we had lots
> of devices fail in practise with HS400@...MHz... So it is doing what it
> should, but it just seems that higher speed modes do not necessarily run
> well with lower frequencies...
>
> So I'd rather prefer to limit speed modes as it is done right now.
>
>>
>> In case the sdhci cap register, doesn't reflect the board support
>> properly, such that you may want to disable some speed modes, then you
>> may benefit from using the DT properties "sdhci-caps*.
>> Documented in Documentation/devicetree/bindings/mmc/sdhci.txt
>
> Hm, yeah I guess something like
>
> sdhci-caps-mask =  /bits/ 64 <((SDHCI_SUPPORT_SDR104 |
> SDHCI_SUPPORT_SDR50 | SDHCI_SUPPORT_DDR50) << 32)>
>
> would come close.
>
> But it does not restrict MMC modes such as HS200/HS400. There seem to be
> no mmc-caps...

Right.

The solution to fix this, should be to *not* set those DT properties,
like "mmc-hs*" for example. That should work, no?

>
>
> My aim is to replace the SDHCI_QUIRK2_NO_1_8_V fix, which does not
> restrict modes correctly. Currently the driver checks whether >=100MHz
> pinctrl settings are available, and if not uses the quirk to restrict
> higher speed modes. Removing that would break device tree backward
> compatibility...

Looks like the problem is not really SDHCI_QUIRK2_NO_1_8_V, but rather
how the pinctrl setting becomes interpreted when setting the quirk.

>
> We probably could do something like this:
> if (!100mhzpinctrl) {
>      if (!sdhci-caps) {
>           /*
>            * If no 100MHz/200MHz pinctrl are available, SDHC caps should
> be used to restrict
>            * modes. Falling back to old behavior...
>            */
>          pr_warn(...)
>          host->quirks2 |= SDHCI_QUIRK2_NO_1_8_V;
>      }
> }
>

I am not sure what makes best sense here. Let me have a look at patch 3 as well.

[...]

Kind regards
Uffe