IMX8MM SD UHS support

[ZHIZHIKIN Andrey]

Hello Tim,

> -----Original Message-----
> From: Tim Harvey <tharvey at gateworks.com>
> Sent: Tuesday, January 19, 2021 6:32 PM
> To: ZHIZHIKIN Andrey <andrey.zhizhikin at leica-geosystems.com>
> Cc: haibo.chen at nxp.com; Adam Ford <aford173 at gmail.com>; Fabio Estevam
> <festevam at gmail.com>; Peng Fan <Peng.Fan at nxp.com>; u-boot <u-
> boot at lists.denx.de>; Stefano Babic <sbabic at denx.de>; Jaehoon Chung
> <jh80.chung at samsung.com>
> Subject: Re: IMX8MM SD UHS support
> 
> On Mon, Jan 18, 2021 at 11:38 AM ZHIZHIKIN Andrey
> <andrey.zhizhikin at leica-geosystems.com> wrote:
> >
> > Hello Tim,
> >
> > Sorry it took me quite some time to get this sorted out, but I believe I was able
> to identify an offending commit that is preventing the USDHC to switch to higher
> speed modes.
> >
> > It is in fact b5874b552f ("mmc: fsl_esdhc_imx: add wait_dat0() support"),
> reverting it makes SD Card to properly report capabilities and switch to high
> speed modes.
> >
> > Can you try to revert this on your end to see if the SD Card would start to
> operate in high speed mode?
> >
> > I'm still investigating why this addition of wait_dat0() caused this, I believe this
> is due to the fact that the same wait is already performed while voltage switch to
> handle the errata, thus this addition wait might erroneously timeout.
> >
> > ++ Haibo Chen <haibo.chen at nxp.com>
> >
> > Haibo,
> >
> > Can you please explain the purpose of adding wait_dat0() Introduced with
> commit b5874b552f? It is not clear from the commit message what was the
> purpose of adding it. Have you tested the USDHC switch to higher modes with
> this change?
> 
> Andrey,
> 
> Reverting b5874b552f ("mmc: fsl_esdhc_imx: add wait_dat0() support")
> does not resolve the issue. That function waits for a specified
> timeout for the card to assert DAT[0] either high or low depending on
> arg and is used to check for command busy or failure. The patch in
> question adds that function so that the common mmc code can utilize
> it. If the function does not exist in the host controller driver any
> call to mmc_wait_dat0 returns -ENOSYS so it must be there to support
> UHS anyway.

Perhaps, this is due to the fact that the same wait cycle is already
executed during the invocation of mmc_send_cmd above the
mmc_wait_dat0() in drivers/mmc/mmc.c.

mmc_send_cmd calls esdhc_send_cmd_common in
drivers/mmc/fsl_esdhc_imx.c, which already has a wait loop
implemented to cover the "Workaround for ESDHC errata ENGcm03648"
(as seen from the comment), which might explain why consecutive
wait fails to return within timeout value.

> 
> What is not clear to me is why the card would hold DAT[0] low on the
> voltage switch indicating a failure as it does not in the Linux driver
> which appears to me to be doing the same thing.

This behavior might be explained by the implementation I traced above.

> Again, if we ignore the return code UHS works fine and I'm not at all clear why you
> wouldn't run into this issue:
> diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
> index a6394bc..5ea3cd2 100644
> --- a/drivers/mmc/mmc.c
> +++ b/drivers/mmc/mmc.c
> @@ -579,10 +579,12 @@ static int mmc_switch_voltage(struct mmc *mmc,
> int signal_voltage)
>          * dat[0:3] low. Wait for at least 1 ms according to spec
>          */
>         err = mmc_wait_dat0(mmc, 1, 1000);
> +#if 0 // hack: not clear why card always holds DAT[0] high here on
> fsl_esdhc_imx
>         if (err == -ENOSYS)
>                 udelay(1000);
>         else if (err)
>                 return -ETIMEDOUT;
> +#endif
> 
>         return 0;
>  }

This is expected. When the wait_dat0 callback is undefined in dm_mmc_ops
structure - it defaults to return -ENOSYS, which effectively just inhibit a delay
in mmc_switch_voltage and returns 0 indicating that call was successful. This
all can be seen from the code snippet you've posted above and had commented
out under #if0 block.

Looking at the change you've posted, it seems to me that you haven't attempted
to revert the patch I mentioned, as by reverting it - the "if (err == -ENOSYS)"
path would've been taken and the desired return 0; would've been called.

> 
> Honestly I don't have the time to keep delving into this. I don't have
> any reason to believe that UHS has ever worked with fsl_esdhc_imx and
> because my boards boot from eMMC (and HS200/HS400 works) I'm not
> missing out on much by having a slow microSD.

I still believe (and witnessed it myself) that the original implementation was
indeed capable of successfully switching the USDHC to high speed modes.

At this point in time it might not be relevant for your implementation,
but could help those who has a severe impact from the microSD RW
performance in U-Boot.

Anyways, thanks a lot for writing back on the findings you have!

As for me, it would still be beneficial if the patch author (Haibo) could
comment on the intention of its introduction, because I clearly see
that reverting it on the current master branch does improve the behavior.

> 
> Best regards,
> 
> Tim

Cheers,
Andrey