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

[Peng Fan]

> 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.

Regards,
Peng.

> 
> I would also like to hear from Jean why he originally introduced this for HS200
> mode.
> 
> --
> Best regards,
> Marek Vasut