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.
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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