Re: [PATCH V1] mmc: core: Select HS mode in device first and then in the host

[Brian Norris]

Hi,

On Tue, Apr 26, 2022 at 2:16 AM Srinivasarao Pathipati
<quic_spathi@xxxxxxxxxxx> wrote:
>
> From: Sayali Lokhande <sayalil@xxxxxxxxxxxxxx>
>
> While switching from hs400 to hs200 mode, high speed mode
> timing should be selected in the device before changing the
> clock frequency in the host. But current implementation,
> (mmc_hs400_to_hs200) first updates the frequency in the host
> and then updates mode in the device. This is a spec violation.
> Hence update the sequence to comply with the spec.

I'm a bit new to interpreting eMMC specs, but are you sure this is a
violation? In JESD84-B51, I see:

"The bus frequency can be changed at any time (under the restrictions
of maximum data transfer frequency, defined by the Device, and the
identification frequency defined by the standard document)."

I think that suggests we can lower the host clock first, and then
lower the device timing. And (according to my limited knowledge) that
makes sense too: the device timing is a "maximum" (to some extent) and
we're free to run the host bus somewhat slower.

And on the flip side: it sounds like you may be _introducing_ a spec
violation (that we'll be running the host faster than the device
timing, briefly)?

Apologies if I'm off base. But you did CC me ;)

Regards,
Brian