Question on 640x480 @ 72fps

[Sean Paul]

On Thu, Aug 30, 2018 at 07:32:58PM -0700, abhinavk at codeaurora.org wrote:
> Hello
> 
> During one of our internal tests, we ran into an issue where the calculated
> refresh rate for the mode using the drm_mode_vrefresh() API doesnt
> match the theoretical value due to rounding.
> 
> 552    { DRM_MODE("640x480", DRM_MODE_TYPE_DRIVER, 31500, 640, 664,
> 553           704,  832, 0, 480, 489, 492, 520, 0,
> 554           DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC) }, /*
> 640x480 at 72Hz */
> 
> 
> int drm_mode_vrefresh(const struct drm_display_mode *mode)
> {
>     int refresh = 0;
> 
>     if (mode->vrefresh > 0)
>         refresh = mode->vrefresh;
>     else if (mode->htotal > 0 && mode->vtotal > 0) {
>         unsigned int num, den;
> 
>         num = mode->clock * 1000;
>         den = mode->htotal * mode->vtotal;
> 
>         if (mode->flags & DRM_MODE_FLAG_INTERLACE)
>             num *= 2;
>         if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
>             den *= 2;
>         if (mode->vscan > 1)
>             den *= mode->vscan;
> 
>         refresh = DIV_ROUND_CLOSEST(num, den);
>     }
>     return refresh;
> }
> EXPORT_SYMBOL(drm_mode_vrefresh);
> 
> As per the math of this API, the vrefresh comes up to 72.8 fps ( 31500 *
> 1000 ) / (832 * 520) .
> 
> Hence this gets rounded to 73fps.
> 
> However as per the spec, this mode should have the vrefresh as 72fps.

I'm not sure where the official spec is, but this random webpage [1] I found with
Google has the same timing values as you have above. The timing information for
the mode doesn't specify the refresh rate, but rather the pclk. The refresh rate
comes from (pclk / (vtotal * htotal)), which comes out to 72.8Hz. We have to
round one way or the other, so DIV_ROUND_CLOSEST is more correct.

Like Ville said, the rounding doesn't really make a difference. Since we'll be
generating the correct pixel clock with the correct pitch, the hardware will be
operating with a 72.8Hz refresh.

Hopefully that makes sense?

Sean

[1] http://martin.hinner.info/vga/timing.html

> 
> So to satisfy that, we must round-down in this function. That might break
> other modes though.
> 
> Do you have any suggestions on how to fix-up this mode ? Shall we just
> directly specify the vrefresh in the edid_est_modes[] and drm_dmt_modes[]
> static array?
> 
> I can submit a PATCH based on the approach we agree on here.
> 
> Thanks
> 
> Abhinav
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-- 
Sean Paul, Software Engineer, Google / Chromium OS