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

Marek Vasut marek.vasut at gmail.com
Tue Jun 11 03:01:45 UTC 2019

On 6/11/19 3:17 AM, Peng Fan wrote:
>> partitions
>> 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'll pick up your patch in this release.


But I would still like to know whether we can access the HW partitions
in HS modes or not. And I would also like to know why we originally
prevented it.

Best regards,
Marek Vasut

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