Re: [PATCH 1/3] arm: dts: qcom: apq8064: Use 27MHz PXO clock as DSI PLL reference

[Marijn Suijten]

On Mon, Aug 30, 2021 at 05:18:37PM +0300, Dmitry Baryshkov wrote:
> On Mon, 30 Aug 2021 at 17:14, Marijn Suijten
> <marijn.suijten@xxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Aug 30, 2021 at 04:24:58PM +0300, Dmitry Baryshkov wrote:
> > > On Mon, 30 Aug 2021 at 11:28, Marijn Suijten
> > > <marijn.suijten@xxxxxxxxxxxxxx> wrote:
> > > >
> > > > Hi Dmitry,
> > > >
> > > > On 8/30/21 3:18 AM, Dmitry Baryshkov wrote:
> > > > > On Sun, 29 Aug 2021 at 23:30, Marijn Suijten
> > > > > <marijn.suijten@xxxxxxxxxxxxxx> wrote:
> > > > >>
> > > > >> The 28NM DSI PLL driver for msm8960 calculates with a 27MHz reference
> > > > >> clock and should hence use PXO, not CXO which runs at 19.2MHz.
> > > > >>
> > > > >> Note that none of the DSI PHY/PLL drivers currently use this "ref"
> > > > >> clock; they all rely on (sometimes inexistant) global clock names and
> > > > >> usually function normally without a parent clock.  This discrepancy will
> > > > >> be corrected in a future patch, for which this change needs to be in
> > > > >> place first.
> > > > >>
> > > > >> Cc: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> > > > >> Signed-off-by: Marijn Suijten <marijn.suijten@xxxxxxxxxxxxxx>
> > > > >
> > > > > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> > > > >
> > > > > Checked the downstream driver, it always uses 27 MHz clock in calculations.
> > > >
> > > >
> > > > Given our concerns for msm8974 not updating DT in parallel with the
> > > > kernel (hence the need for a global-name fallback because "ref" is
> > > > missing from the DT), should we worry about the same for apq8064?  That
> > > > is, is there a chance that the kernel but not the firmware is upgraded
> > > > leading to the wrong parent clock being used?  The msm8960 variant of
> > > > the 28nm PLL driver uses parent_rate in a few places and might read
> > > > cxo's 19.2MHz erroneously instead of using pxo's 27MHz.
> > >
> > > Checked the code. It uses .parent_names =  "pxo", so changing ref
> > > clock should not matter. We'd need to fix ref clocks and after that we
> > > can switch parent names to fw_name.
> >
> > Correct, hence why this patch is ordered before the switch to .fw_name.
> > These patches can't go in the same series if apq8064 doesn't update its
> > firmware in parallel with the kernel just like msm8974.  Do you know if
> > this is the case?  If so, how much time do you think should be between
> > the DT fix (this patch) and migrating the drivers?
> 
> You can have parent_data with .fw_name and .name in it.  .name will be
> used as a fallback if .fw_name doesn't match.

The problem is that it will always find the "ref" clock which references
&cxo_board until the DT is updated with this patch to use &pxo_board
instead.  Question is, will the kernel and DT usually/always be updated
in parallel?

- Marijn