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

[Faiz Abbas]

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?

Thanks,
Faiz