Re: [PATCH] mmc: mmc: Fix HS setting in mmc_hs400_to_hs200()

[Ulf Hansson]

On Thu, 31 Jan 2019 at 08:53, Chaotian Jing <chaotian.jing@xxxxxxxxxxxx> wrote:
>
> mmc_hs400_to_hs200() begins with the card and host in HS400 mode.
> Therefore, any commands sent to the card should use HS400 timing.
> It is incorrect to reduce frequency to 50Mhz before sending the switch
> command, in this case, only reduce clock frequency to 50Mhz but without
> host timming change, host is still in hs400 mode but clock changed from
> 200Mhz to 50Mhz, which makes the tuning result unsuitable and cause
> the switch command gets response CRC error.

According the eMMC spec there is no violation by decreasing the clock
frequency like this. We can use whatever value <=200MHz.

However, perhaps in practice this becomes an issue, due to the tuning
for HS400 has been done on the "current" frequency.

As as start, I think you need to clarify this in the changelog.

>
> this patch refers to mmc_select_hs400(), make the reduce clock frequency
> after card timing change.
>
> Signed-off-by: Chaotian Jing <chaotian.jing@xxxxxxxxxxxx>
> ---
>  drivers/mmc/core/mmc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mmc/core/mmc.c b/drivers/mmc/core/mmc.c
> index da892a5..21b811e 100644
> --- a/drivers/mmc/core/mmc.c
> +++ b/drivers/mmc/core/mmc.c
> @@ -1239,10 +1239,6 @@ int mmc_hs400_to_hs200(struct mmc_card *card)
>         int err;
>         u8 val;
>
> -       /* Reduce frequency to HS */
> -       max_dtr = card->ext_csd.hs_max_dtr;
> -       mmc_set_clock(host, max_dtr);
> -

As far as I can tell, the reason to why we change the clock frequency
*before* the call to __mmc_switch() below, is probably to try to be on
the safe side and conform to the spec.

However, I think you have a point, as the call to __mmc_switch(),
passes the "send_status" parameter as false, no other command than the
CMD6 is sent to the card.

>         /* Switch HS400 to HS DDR */
>         val = EXT_CSD_TIMING_HS;
>         err = __mmc_switch(card, EXT_CSD_CMD_SET_NORMAL, EXT_CSD_HS_TIMING,
> @@ -1253,6 +1249,10 @@ int mmc_hs400_to_hs200(struct mmc_card *card)
>
>         mmc_set_timing(host, MMC_TIMING_MMC_DDR52);
>
> +       /* Reduce frequency to HS */
> +       max_dtr = card->ext_csd.hs_max_dtr;
> +       mmc_set_clock(host, max_dtr);
> +

Perhaps it's even more correct to change the clock frequency before
the call to mmc_set_timing(host, MMC_TIMING_MMC_DDR52). Otherwise you
will be using the DDR52 timing in the controller, but with a too high
frequency.

>         err = mmc_switch_status(card);
>         if (err)
>                 goto out_err;
> --
> 1.8.1.1.dirty
>

Finally, it sounds like you are trying to fix a real problem, can you
please provide some more information what is happening when the
problem occurs at your side?

Kind regards
Uffe