Question on 640x480 @ 72fps

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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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