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

[Ulf Hansson]

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.

Kind regards
Uffe