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

[Marijn Suijten]

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.

- Marijn