drm/msm/dsi: hs_zero timing

[hali at codeaurora.org]

>
>
> On 08/27/2015 07:12 AM, Werner Johansson wrote:
>> On Aug 26, 2015 11:31 AM, "Rob Clark" <robdclark at gmail.com
>> <mailto:robdclark at gmail.com>> wrote:
>>  >
>>  > 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.
>>
>> Yes, the code definitely calculates different timing (as can be seen
>> with the calculations for this particular Panasonic panel as well). We
>> need more of the spreadsheet magic in the code it seems. This hs_zero
>> issue seems to be a bug of sorts inside the MSM SoC itself though, not
>> an issue with the panel (as exactly the same issue occurred with all
>> three panels I tried. The "every-eighth value fails" failure mode does
>> not seem to be timing related as I was able to fuzz the timing way
>> outside of the specified ranges and still have perfectly good display,
>> as long as I stayed out of the "every-eighth" value for hs_zero. The
>> panels are typically not crystal controlled so their frequency
>> tolerances are wide.
>>
>> The display mode seems a bit over-specified, can we derive clock from
>> htotal * vtotal * refreshrate instead and get the resolution needed (for
>> DSI that should always result in the correct clock, right)?
>
> There are certain modes (generally for HDMI/DVI) where the refresh rate
> isn't an integer. It can be something like 59.94 Hz, or 60.04Hz. The
> above calculation may not work well with such modes.
>
> For example, a 720p at 59.94Hz mode requires 74.175 Mhz. This value can
> be expressed relatively accurately in KHz if stored beforehand in
> mode->clock. If we try to calculate the pixel clock here ourselves,
> we'll need to round off the vrefresh to 60Hz, which gives us a less
> accurate 74.25 Mhz.
>
> We have platforms where the DSI output is connected to HDMI bridge
> chips. So the issue I mentioned holds true for msm/dsi too.
>
> Thanks,
> Archit
>

For this specific dphy timing issue, the KHz pixel clock precision is good
enough, but I agree to change it to Hz for long term.
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