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

[Dmitry Baryshkov]

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.

>
> - Marijn



-- 
With best wishes
Dmitry