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

[Adrian Hunter]

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.