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

[Jean-Jacques Hiblot]

+ Jaehoon


Hi Marek,

On 10/06/2019 13:33, Marek Vasut wrote:
> 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.

Well I didn't look into the specs when working on this part.

I introduced it because of Jaehoon's remark on this thread: 
https://lists.denx.de/pipermail/u-boot/2017-January/278672.html

Maybe Jaehoon can help us here understand why it is needed.

JJ

>