Re: [PATCH] mmc: mmci_sdmmc: fix power on issue due to pwr_reg initialization

[Ulf Hansson]

On Wed, 22 Apr 2020 at 15:40, Ludovic BARRE <ludovic.barre@xxxxxx> wrote:
>
> hi Ulf
>
> Le 4/21/20 Ã 11:38 AM, Ulf Hansson a Ãcrit :
> > On Tue, 21 Apr 2020 at 11:25, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
> >>
> >> On Mon, 20 Apr 2020 at 18:18, Ludovic Barre <ludovic.barre@xxxxxx> wrote:
> >>>
> >>> This patch fix a power-on issue, and avoid to retry the power sequence.
> >>>
> >>> In power off sequence: sdmmc must set pwr_reg in "power-cycle" state
> >>> (value 0x2), to prevent the card from being supplied through the signal
> >>> lines (all the lines are driven low).
> >>>
> >>> In power on sequence: when the power is stable, sdmmc must set pwr_reg
> >>> in "power-off" state (value 0x0) to drive all signal to high before to
> >>> set "power-on".
> >>
> >> Just a question to gain further understanding.
> >>
> >> Let's assume that the controller is a power-on state, because it's
> >> been initialized by the boot loader. When the mmc core then starts the
> >> power-on sequence (not doing a power-off first), would $subject patch
> >> then cause the
> >> MMCIPOWER to remain as is, or is it going to be overwritten?
>
> On sdmmc controller, the PWRCTRL[1:0] field of MMCIPOWER register allow
> to manage sd lines and has a specific bahavior.
>
> PWRCTRL value:
>   - 0x0: After reset, Reset: the SDMMC is disabled and the clock to the
>          Card is stopped, SDMMC_D[7:0], and SDMMC_CMD are HiZ and
>          SDMMC_CK is driven low.
>          When written 00, power-off: the SDMMC is disabled and the clock
>          to the card is stopped, SDMMC_D[7:0], SDMMC_CMD and SDMMC_CK
>          are driven high.
>
>   - 0x2: Power-cycle, the SDMMC is disabled and the clock to the card is
>          stopped, SDMMC_D[7:0], SDMMC_CMD and SDMMC_CK are driven low.
>
>   - 0x3: Power-on: the card is clocked, The first 74 SDMMC_CK cycles the
>          SDMMC is still disabled. After the 74 cycles the SDMMC is
>          enabled and the SDMMC_D[7:0], SDMMC_CMD and SDMMC_CK are
>          controlled according the SDMMC operation.
>          **Any further write will be ignored, PWRCTRL value
>          will keep 0x3**. when the SDMMC is ON (0x3) only a reset could
>          change pwrctrl value and the state of sdmmc lines.
>
> So if the lines are already "ON", the power-on sequence (decribed in
> commit message) not overwrite the pwctrl field and not disturb the sdmmc
> lines.

Thanks for the detailed information, much appreciated!

>
> >>
> >> I am a little worried that we may start to rely on boot loader
> >> conditions, which isn't really what we want either...
> >>
>
> We not depend of boot loader conditions.
>
> This patch simply allows to drive high the sd lines before to set
> "power-on" value (no effect if already power ON).

Yep, thanks!

>
> >>>
> >>> To avoid writing the same value to the power register several times, this
> >>> register is cached by the pwr_reg variable. At probe pwr_reg is initialized
> >>> to 0 by kzalloc of mmc_alloc_host.
> >>>
> >>> Like pwr_reg value is 0 at probing, the power on sequence fail because
> >>> the "power-off" state is not writes (value 0x0) and the lines
> >>> remain drive to low.
> >>>
> >>> This patch initializes "pwr_reg" variable with power register value.
> >>> This it done in sdmmc variant init to not disturb default mmci behavior.
> >>>
> >>> Signed-off-by: Ludovic Barre <ludovic.barre@xxxxxx>
> >>
> >> Besides the comment, the code and the approach seems reasonable to me.
> >
> > Another related question. I just realized why you probably haven't set
> > .pwrreg_nopower for the variant_stm32_sdmmc and variant_stm32_sdmmcv2.
> >
> > I guess it's because you need a slightly different way to restore the
> > context of MMCIPOWER register at ->runtime_resume(), rather than just
> > re-writing it with the saved register values. Is this something that
> > you are looking into as well?
>
> Yes exactly, the sequence is slightly different. I can't write 0 on
> mmci_runtime_suspend, and can't just re-writing the saved register.

So, it seems like you need to use the ->set_ios() callback, to
re-configure the controller correctly.

Just tell if you need more help to make that work, otherwise I am here
to review your patches.

In regards to $subject patch, I have applied it for next, thanks!

Kind regards
Uffe