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

[Peng Fan <peng.fan@nxp.com>]

> 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.

Regards,
Peng.

> 
> Thanks,
> Faiz
>