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

[Faiz Abbas]

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.

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.

Thanks,
Faiz