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

[Marek Vasut]

On 6/10/19 7:59 AM, Peng Fan wrote:
>> Subject: Re: [U-Boot] [PATCH] mmc: Avoid HS400 mode when accessing boot
>> partitions
>>
>> Hi Marek, Peng,
>>
>> 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.
> 
> Yes, the spec does not mention this. It only mentions HS200/400 not supported
> during boot operation.
> 
>>
>> 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.
> 
> There might be bug in U-Boot code.

So are we gonna leave this inconsistency in for current release or
what's it gonna be ? Like I said, we're in rc3, it's fine to do bigger
changes in next release, but we should at least fix this in current release.

I would also like to hear from Jean why he originally introduced this
for HS200 mode.

-- 
Best regards,
Marek Vasut