Re: [PATCH 2/3] drm: Make drm_mode_vrefresh() a bit more accurate

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Mar 14, 2018 at 02:56:28PM +0100, Daniel Vetter wrote:
> On Tue, Mar 13, 2018 at 05:07:58PM +0200, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > Do the refresh rate calculation with a single division. This gives
> > us slightly more accurate results, especially for interlaced since
> > we don't just double the final truncated result.
> > 
> > We do lose one bit compared to the old way, so with an interlaced
> > mode the new code can only handle ~2GHz instead of the ~4GHz the
> > old code handeled.
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> I kinda want special integers here that Oops on overflow, I thought
> they're coming.

Would be nice indeed.

> That aside:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_modes.c | 19 +++++++++----------
> >  1 file changed, 9 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> > index 4157250140b0..f6b7c0e36a1a 100644
> > --- a/drivers/gpu/drm/drm_modes.c
> > +++ b/drivers/gpu/drm/drm_modes.c
> > @@ -773,24 +773,23 @@ EXPORT_SYMBOL(drm_mode_hsync);
> >  int drm_mode_vrefresh(const struct drm_display_mode *mode)
> >  {
> >  	int refresh = 0;
> > -	unsigned int calc_val;
> >  
> >  	if (mode->vrefresh > 0)
> >  		refresh = mode->vrefresh;
> >  	else if (mode->htotal > 0 && mode->vtotal > 0) {
> > -		int vtotal;
> > -		vtotal = mode->vtotal;
> > -		/* work out vrefresh the value will be x1000 */
> > -		calc_val = (mode->clock * 1000);
> > -		calc_val /= mode->htotal;
> > -		refresh = (calc_val + vtotal / 2) / vtotal;
> > +		unsigned int num, den;
> > +
> > +		num = mode->clock * 1000;
> > +		den = mode->htotal * mode->vtotal;
> >  
> >  		if (mode->flags & DRM_MODE_FLAG_INTERLACE)
> > -			refresh *= 2;
> > +			num *= 2;
> >  		if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
> > -			refresh /= 2;
> > +			den *= 2;
> >  		if (mode->vscan > 1)
> > -			refresh /= mode->vscan;
> > +			den *= mode->vscan;
> > +
> > +		refresh = DIV_ROUND_CLOSEST(num, den);
> >  	}
> >  	return refresh;
> >  }
> > -- 
> > 2.16.1
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Ville Syrjälä
Intel OTC
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