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

[Marek Vasut]

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.

If nothing else, this patch should go in for the sake of consistency.

-- 
Best regards,
Marek Vasut