Re: [PATCH 2/4] mmc: mmc: Add change to set controller to HS400 frequency in enhanced strobe

Ritesh Harjani
2016-12-20T12:08:39Z

Hi Shawn,

On 12/20/2016 4:33 PM, Shawn Lin wrote:
> On 2016/12/20 18:33, Ritesh Harjani wrote:
>> Hi Shawn,
>>
>> On 12/20/2016 12:59 PM, Shawn Lin wrote:
>>> On 2016/12/20 13:53, Ritesh Harjani wrote:
>>>> Currently mmc_select_hs400es is not setting the desired frequency
>>>> before sending switch command. This makes CMD13 to timeout on
>>>> some controllers.
>>>> Thus add a change to set the desired HS400 frequency
>>>> in mmc_select_hs400es when the timing is switched to HS400.
>>>
>>> What is the desired HS400 freq? I guess it is 200MHz, right?
>>>
>>> Well, per the spec, it just say "host *may* changes frequency to
>>> <= 200Mhz". So before setting timing to MMC_TIMING_MMC_HS400, we
>>> already reach ext_csd.hs_max_dtr. So it should meet the spec. If
>>> the fact that some controllers would see CMD13 timeout here, I guess
>>> you wouldn't let folks add max-frequency to the DT as if the max freq
>>> is set to just lower than your *desired HS400 freq* , it the same
>>> failure result even you add this patch, right?
>>>
>>> the root cause the controllers see failure for CMD13 is it didn't
>>> configure the right delay phase or something similar to fit the timing
>>> under the freq of hs_max_dtr, I guess.
>> Actually this may be limitation of sdhc msm controller that it cannot
>> work at lower frequency while in HS400 mode. I would have to see more on
>> this to confirm on why this limitation is there (mostly it was with some
>> DLL limitation).
>> In that case maybe I should not generalize this for other controllers.
>>
>>
>> But still the below code change to set the bus_speed after setting the
>> timing should be fine right ? Since host controller may as well change
>> the frequency to 200MHz.
>
> maybe not, I was guessing what would happend when folks add
> max-frequency = <100000000> as well as mmc-hs400-enhanced-strobe in
> the DT? It won't work for your case. So the question is how you would
> make that case work?
>
>>
>> Do you think the below change is fine in this sense? I can change the
>> commit message to just -
>> "set both timing mode and frequency before sending CMD13 in
>> mmc_select_hs400es."
>>
>> Or do you see any other approach to this ?
>>
>
> I haven't had clear thought about how to deal with it but
> I guess you are not alone for this case maybe. Now there
> will be more and more mmc controllers using PHY or DLL and
> some more special limitions is ineluctable, so we need to
> discuss a more general/legit way to make it work.
>
> Your patch is a good begin to open this topic. :)
> I would like to hear some input from Ulf and Adian.
I see there is a miss from my side here. Although there is still some
limitation on msm host controller which I cannot recollect. But for this
case we can get it done by configuring the DLL properly.

I missed that particular check of clock >= 100MHz in
sdhci_msm_set_uhs_signaling which configures the DLL.

I guess if I could use the host->mmc_host_ops.hs400_enhanced_strobe ops
and provide a callback to configure the DLL without any restriction on
clock frequency, then this can be worked out.

I will check more on this and get back. Thanks for your inputs.

Regards
Ritesh

>
>>
>> Regards
>> Ritesh
>>
>>>
>>>>
>>>> Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
>>>> ---
>>>> drivers/mmc/core/mmc.c | 1 +
>>>> 1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/mmc/core/mmc.c b/drivers/mmc/core/mmc.c
>>>> index b61b52f9..eb69497 100644
>>>> --- a/drivers/mmc/core/mmc.c
>>>> +++ b/drivers/mmc/core/mmc.c
>>>> @@ -1329,6 +1329,7 @@ static int mmc_select_hs400es(struct mmc_card
>>>> *card)
>>>>
>>>> /* Set host controller to HS400 timing and frequency */
>>>> mmc_set_timing(host, MMC_TIMING_MMC_HS400);
>>>> + mmc_set_bus_speed(card);
>>>>
>>>> /* Controller enable enhanced strobe function */
>>>> host->ios.enhanced_strobe = true;
>>>>
>>>
>>>
>>
>
>

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