drm/msm/dsi: hs_zero timing

[Rob Clark]

On Wed, Aug 26, 2015 at 2:22 PM, Werner Johansson
<werner.johansson at gmail.com> wrote:
>
> On Aug 26, 2015 10:46, "Rob Clark" <robdclark at gmail.com> wrote:
>>
>> btw, w/ some of these clk rounding issues, I suspect we need 'struct
>> drm_display_mode' to be able to represent mode clock with greater
>> accuracy than Khz..
>
> Interesting point! However, in this case I had to adjust the clock hundreds
> of kHz to make it tick over one step of hs_zero, so it might not be
> absolutely necessary here. Do we need better than 10ppm accuracy for display
> timing (assuming 100MHz pixel clock, 1kHz step size and that I did the math
> correctly)? We don't even have kHz accuracy with the PLLs in the QC
> platforms we currently use..
>
> I think the rounding error happens with the smaller numbers / intermediate
> results but maybe clock should be represented in Hz internally anyway? Not
> sure if it's worth changing the external-facing representation though?

I'm not completely sure.. I did observe that we calculated slightly
different settings w/ the auo novatek panel on z3, compared to what
downstream had hard-coded in dts (which presumably came from
magic-spreadsheet), because (I think) of rounding mode->clock to
integer KHz.  Although in the case of the z3 panel, it didn't seem to
matter.  What I am unsure about is whether other panels might be more
sensitive to different settings.

BR,
-R