[U-Boot] [PATCH] mmc: Avoid HS400 mode when accessing boot partitions

[Marek Vasut]

On 6/7/19 9:53 AM, Faiz Abbas wrote:
> Hi,
> 
> + Kernel MMC maintainer
> 
> On 04/06/19 7:08 PM, Marek Vasut wrote:
>> On 6/4/19 3:34 PM, Faiz Abbas wrote:
>>> Hi Marek,
>>>
>>> On 04/06/19 6:56 PM, Marek Vasut wrote:
>>>> On 6/4/19 1:22 PM, Faiz Abbas wrote:
>>>>> Hi Marek, Peng,
>>>>
>>>> Hi,
>>>>
>>>>> On 03/06/19 12:04 PM, Peng Fan wrote:
>>>>>>
>>>>>>> Subject: [PATCH] mmc: Avoid HS400 mode when accessing boot partitions
>>>>>>>
>>>>>>> According to JEDEC JESD84-B51.pdf section 6.3.3 Boot operation ,
>>>>>>> HS200 & HS400 mode is not supported during boot operation. The U-Boot
>>>>>>> code currently only applies this restriction to HS200 mode, extend this to
>>>>>>> HS400 mode as well.
>>>>> The spec in section 6.3.3 (according to my understanding) is talking
>>>>> about "boot operation" which is a way of getting data from the the eMMC
>>>>> without going through the Device identification mode (Section 6.4.4)
>>>>> i.e. without sending any commands. All the host has to do is hold the
>>>>> command line low in Pre-Idle mode to automatically receive data at the
>>>>> preconfigured frequency and bus width.
>>>>>
>>>>> When U-boot is accessing the partition, it has already gone through the
>>>>> Device identification mode and is in data transfer mode (i.e. it needs
>>>>> to send commands for read/write to happen). In this case, we need to
>>>>> switch the partition in Extended CSD to access the boot partition
>>>>> (Section 6.2.5). The spec doesn't say anything about HS200 and HS400 not
>>>>> being supported here.
>>>>>
>>>>> Also, I don't see linux kernel switching down speed when trying to
>>>>> access a boot partition (unless its being very sneaky about it). So if
>>>>> you are seeing issues with accessing boot partitions at HS200/HS400 then
>>>>> you should probably look at how linux code is working instead.
>>>>
>>>> Did you practically verify this ? In my case, the boot partition access
>>>> fails in HS200/HS400 mode (samsung and sandisk emmc, but I'd have to
>>>> check the exact part number).
>>>>
>>>> commit 01298da31d92ecc46cf9130d8cff68bc51698197
>>>>     mmc: Change mode when switching to a boot partition
>>>> seems to confirm that too.
>>>>
>>>
>>> I had tried to raise these concerns for that patch as well
>>> (https://www.spinics.net/lists/linux-mmc/msg50432.html) but it was
>>> missed at that time.
>>>
>>> I did verify that kernel is not switching speeds on a dra7xx board. JJ
>>> and I have seen failures similar to yours in u-boot (which led to this
>>> patch) but not in kernel which makes me think that the fix was wrong.
>>> Have you verified with your setup in kernel?
>>
>> No, but I'll add it to the list of things to look into.
>>
>> In the meantime, this should go in anyway, to make the code consistent.
>>
> 
> If we keep covering up the issue then it will never be solved. We need
> to get more data points and more expert opinions on this and take the
> pains to fix it.

The release is weeks away, this fix is trivial. If we want to do more
intrusive changes, that's for next release anyway.

> @Ulf we are seeing some failures when trying to access eMMC boot
> partitions at HS200/HS400 speed modes.
> 
> Do you know if there are limitations on accessing eMMC boot partitions
> at HS200/HS400 speed modes in kernel? Do you agree with my reading of
> the spec above?
> 
> Thanks,
> Faiz
> 


-- 
Best regards,
Marek Vasut