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

[Avri Altman]

> 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)?
Ack on that.

Also, specifically, please refer to Figure 28 — HS200 Selection flow diagram.
You can see that the flow goes though:
BUS_WIDTH [183] -> from 0x06 (DDR 8bit) to 0x02 (SDR 8bit)
HS_TIMING [185] ->  from 0x03(HS400) to 0x02 (HS200)
Host may changes the frequency , but it’s no needed as HS400 and HS200 use same CLK

Thanks,
Avri
> 
> Apologies if I'm off base. But you did CC me ;)
> 
> Regards,
> Brian