On Thu, Aug 30, 2018 at 07:32:58PM -0700, abhinavk@xxxxxxxxxxxxxx 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@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
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