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

From: Adrian Hunter
Date: Tue Feb 12 2019 - 03:05:38 EST

On 12/02/19 4:04 AM, Chaotian Jing wrote:
> On Tue, 2019-02-05 at 15:42 +0200, Adrian Hunter wrote:
>> On 5/02/19 3:06 PM, Ulf Hansson wrote:
>>> On Mon, 4 Feb 2019 at 14:42, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>>>> On 4/02/19 12:54 PM, Ulf Hansson wrote:
>>>>> 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;
>>>>>>>>>> --
>>>>>>>>> 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.
> Ok, sorry for late reply due to Chinese New Year.
>>>>>>>> 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@xxxxxxxxx>
>>>> 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.
> Yes, but should we add a new argument of __mmc_switch(), like "bool
> ignore_crc" ?? for now, there are too many argument of __mmc_switch().

One solution for too many arguments is to make a structure to contain them. e.g.

struct mmc_switch_args {
u8 set;
u8 index;
u8 value;
unsigned int timeout_ms;
unsigned char timing;
bool use_busy_signal;
bool send_status;
bool retry_crc_err;

int __mmc_switch(struct mmc_card *card, struct mmc_switch_args *args)

>>>> 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?
>> Not at present, but some drivers seem to be adjusting their settings for
>> HS400 based on the frequency e.g. sdhci_msm_hs400()
> It's hard to find a suitable setting for all cards when running at HS400
> mode @50Mhz.