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

From: Ulf Hansson <ulf.hansson@linaro.org>
To: Stefan Agner <stefan@agner.ch>
Cc: Adrian Hunter <adrian.hunter@intel.com>,
	Fabio Estevam <fabio.estevam@nxp.com>,
	Haibo Chen <haibo.chen@nxp.com>,
	Aisheng Dong <aisheng.dong@nxp.com>,
	Michael Trimarchi <michael@amarulasolutions.com>,
	Russell King <rmk+kernel@armlinux.org.uk>,
	"linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] mmc: sdhci: add quirk to prevent higher speed modes
Date: Wed, 4 Jul 2018 13:16:22 +0200	[thread overview]
Message-ID: <CAPDyKFrDMmMu1kuJdPDyyGKu6b0dR2MF2U4Rnb4hoHAcwNYtuw@mail.gmail.com> (raw)
In-Reply-To: <ffea342896629cf57b78661b04f60eda@agner.ch>

On 4 July 2018 at 12:55, Stefan Agner <stefan@agner.ch> wrote:
> On 04.07.2018 12:07, Ulf Hansson wrote:
>> On 3 July 2018 at 10:48, Stefan Agner <stefan@agner.ch> wrote:
>>> On 02.07.2018 16:36, Ulf Hansson wrote:
>>>> On 28 June 2018 at 10:13, Stefan Agner <stefan@agner.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@100MHz... 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?
>>
>
> The controller does not make use of the dt modes so far, so I can't not
> set those properties...

Then where are the corresponding caps for the eMMC speed modes being set?

Can't you just avoid setting them?

>
>>>
>>>
>>> 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.
>>
>
> Yes, sorry for the confusion. SDHCI_QUIRK2_NO_1_8_V is fine, it is just
> not the quirk this driver needs.
>
> I argue that commit ad93220de7da ("mmc: sdhci-esdhc-imx: change pinctrl
> state according to uhs mode") chose the wrong quirk from the
> beginning...

That's seems reasonable! But it's been there since 3.13, so I guess we
have to think about backwards compatibility issues, as you stated.

>
> Afaict, the quirk needed here does not exist.

[...]

Kind regards
Uffe