Re: [PATCH v2] mmc: sdhci-esdhc-imx: allow 1.8V modes without 100/200MHz pinctrl states

[Ulf Hansson]

On 4 July 2018 at 17:18, Stefan Agner <stefan@xxxxxxxx> wrote:
> On 04.07.2018 17:07, Stefan Agner wrote:
>> If pinctrl nodes for 100/200MHz are missing, the controller should
>> not select any mode which need signal frequencies 100MHz or higher.
>> To prevent such speed modes the driver currently uses the quirk flag
>> SDHCI_QUIRK2_NO_1_8_V. This works nicely for SD cards since 1.8V
>> signaling is required for all faster modes and slower modes use 3.3V
>> signaling only.
>>
>> However, there are eMMC modes which use 1.8V signaling and run below
>> 100MHz, e.g. DDR52 at 1.8V. With using SDHCI_QUIRK2_NO_1_8_V this
>> mode is prevented. When using a fixed 1.8V regulator as vqmmc-supply
>> the stack has no valid mode to use. In this tenuous situation the
>> kernel continuously prints voltage switching errors:
>>   mmc1: Switching to 3.3V signalling voltage failed
>>
>> Avoid using SDHCI_QUIRK2_NO_1_8_V and prevent faster modes by
>> altering the SDHCI capability register. With that the stack is able
>> to select 1.8V modes even if no faster pinctrl states are available:
>>   # cat /sys/kernel/debug/mmc1/ios
>>   ...
>>   timing spec:    8 (mmc DDR52)
>>   signal voltage: 1 (1.80 V)
>>   ...
>>
>> Link: http://lkml.kernel.org/r/20180628081331.13051-1-stefan@xxxxxxxx
>> Signed-off-by: Stefan Agner <stefan@xxxxxxxx>
>> ---
>
> Btw, I still get the switching error once during boot-up:
>   mmc1: Switching to 3.3V signalling voltage failed

I guess the this happens then also at system resume?

The core tries first with 3.3 then if it fails, it continues with 1.8V, etc.

>
> This is due to the call from mmc_set_initial_signal_voltage. It is a bit
> unfortunate since this is printed as a warning. Not sure if that could
> be prevented somehow?

Seems like SDHCI_SIGNALING_330 should not be set, unless 3.3V I/O is
supported. That should avoid SDHCI from trying and instead just
returning an error code immediately.

This seems like a generic issues for all SDHCI variant drivers.

[...]

Kind regards
Uffe