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

[Ulf Hansson]

On Mon, 4 Feb 2019 at 10:58, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> On 1/02/19 10:10 AM, Ulf Hansson wrote:
> > On Fri, 1 Feb 2019 at 02:38, Chaotian Jing <chaotian.jing@xxxxxxxxxxxx> wrote:
> >>
> >> On Thu, 2019-01-31 at 16:58 +0100, Ulf Hansson wrote:
> >>> 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.
> >>>
> >> Yes, reduce clock frequency to 50Mhz is no Spec violation, but it may
> >> cause __mmc_switch() gets response CRC error, decreasing the clock but
> >> without HOST mode change, on the host side, host driver do not know
> >> what's operation the core layer want to do and can only set current bus
> >> clock to 50Mhz, without tuning parameter change, it has a chance lead to
> >> response CRC error. even lower clock frequency, but with the wrong
> >> tuning parameter setting(the setting is of hs400 tuning @200Mhz).
> >
> > Right, makes sense.
> >
> >>>>
> >>>> 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.
> >>>
> >> Agree, it Must be more safe with lower clock frequency, but the
> >> precondition is to make the host side recognize current timing is not
> >> HS400 mode. it has no method to find a safe setting to ensure no
> >> response CRC error when reduce clock from 200Mhz to 50Mhz.
> >>> 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.
> >>>
> >> yes, the send status command was sent only after __mmc_switch() done.
> >>>>         /* 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.
> >>>
> >> for Our host, it has no impact to change the clock before or after
> >> change timing, as the mmc_set_timing() is only for host side, not
> >> related to MMC card side and no commands sent do card before the
> >> timing/clock change completed.
> >
> > Alright. After a second thought, it actually looks more consistent
> > with mmc_select_hs400() to do it after, as what you propose in
> > $subject patch.
> >
> > So, let's keep it as is.
> >
> >>>>         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?
> >>>
> >> Yes, I got a problem with new kernel version. with
> >> commit:57da0c042f4af52614f4bd1a148155a299ae5cd8, this commit makes
> >> re-tuning every time when access RPMB partition.
> >
> > Okay, could you please add this as fixes tag for the next version of the patch.
> >
> >>
> >> in fact, our host tuning result of hs400 is very stable and almost never
> >> get response CRC error with clock frequency at 200Mhz. but cannot ensure
> >> this tuning result also suitable when running at HS400 mode @50Mhz. as I
> >> mentioned before, the host side does not know the reason of reduce clock
> >> frequency to 50Mhz at HS400 mode, so what's the host side can do is only
> >> reduce the bus clock to 50Mhz, even it can just only set the tuning
> >> setting to default when clock frequency lower than 50Mhz, but both card
> >> & host side are still at HS400 mode, still cannot ensure this setting is
> >> suitable.
> >
> > Right, thanks for clarifying.
> >
> > So I am expecting a new version with a fixes tag and some
> > clarification of the changelog, then I am ready to apply this to give
> > it some test.
>
> The switch from HS400 mode is done for tuning at times when CRC errors are a
> possibility e.g. after a CRC error during transfer.  So if the frequency is
> not to be reduced, then some mitigation is needed for the possibility that
> the CMD6 response itself will have a CRC error.

That's a good point!

However, how can we know that a CMD6 command is successfully
completed, if there is CRC errors detected during the transmission? I
guess we can't!?

Kind regards
Uffe