Question on 640x480 @ 72fps

[Ville Syrjälä]

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.

Why should anyone care if it gets rounded differently?

> 
> 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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-- 
Ville Syrjälä
Intel