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

Ulf Hansson
2019-02-05T13:06:13Z

On Mon, 4 Feb 2019 at 14:42, Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 4/02/19 12:54 PM, Ulf Hansson wrote:
> > On Mon, 4 Feb 2019 at 10:58, Adrian Hunter <adrian.hunter@intel.com> wrote:
> >>
> >> On 1/02/19 10:10 AM, Ulf Hansson wrote:
> >>> On Fri, 1 Feb 2019 at 02:38, Chaotian Jing <chaotian.jing@mediatek.com> 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@mediatek.com> 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@mediatek.com>
> >>>>>> ---
> >>>>>> 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!?
>
> Yes, in that case, the only option is to assume the CMD6 was successful,
> like in
>
> commit ef3d232245ab7a1bf361c52449e612e4c8b7c5ab
> Author: Adrian Hunter <adrian.hunter@intel.com>
> Date: Fri Dec 2 13:16:35 2016 +0200
>
> mmc: mmc: Relax checking for switch errors after HS200 switch

Well, relaxing the check for switch errors, is to me a different
thing. This means we are first doing the CMD6, then allowing the
following status command (CMD13) to have CRC errors. Actually, even
the spec mention this as a case to consider. I guess it's because the
card internally have switched to a new speed mode timing.

Allowing CRC errors for the actual CMD6 sound more fragile to me. Of
course, we can always try and see what happens.

Chaotian, can you give it a go? Somehow, change the call to
__mmc_switch() in mmc_hs400_to_hs200(), so the CMD6 doesn't have the
CRC flag set.

>
> If we are going to do that, then we could stick with lowering the frequency
> first.

Let's see what Chaotian's test may show.

>
> Also I wonder if the mediatek driver could change to fixed sampling in
> ->set_ios() when the frequency drops for HS400 mode?

Well, this sounds like a generic problem so if this is a possible
generic solution that would be great.

Is this what sdhci is doing already?

Kind regards
Uffe