Question on 640x480 @ 72fps

[abhinavk at codeaurora.org]

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.

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