Re: [PATCH] drm/edid: Make the detailed timing CEA/HDMI mode fixup accept up to 5kHz clock difference

From: Adam Jackson <ajax@redhat.com>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>,
	dri-devel@lists.freedesktop.org
Subject: Re: [PATCH] drm/edid: Make the detailed timing CEA/HDMI mode fixup accept up to 5kHz clock difference
Date: Mon, 30 Nov 2015 13:02:59 -0500	[thread overview]
Message-ID: <1448906579.4256.57.camel@redhat.com> (raw)
In-Reply-To: <20151126170115.GH4437@intel.com>

On Thu, 2015-11-26 at 19:01 +0200, Ville Syrjälä wrote:
> On Mon, Nov 16, 2015 at 09:05:12PM +0200, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > Rather than using drm_match_cea_mode() to see if the EDID detailed
> > timings are supposed to represent one of the CEA/HDMI modes, add a
> > special version of that function that takes in an explicit clock
> > tolerance value (in kHz). When looking at the detailed timings specify
> > the tolerance as 5kHz due to the 10kHz clock resolution limit inherent
> > in detailed timings.
> > 
> > drm_match_cea_mode() uses the normal KHZ2PICOS() matching of clocks,
> > which only allows smaller errors for lower clocks (eg. for 25200 it
> > won't allow any error) and a bigger error for higher clocks (eg. for
> > 297000 it actually matches 296913-297000). So it doesn't really match
> > what we want for the fixup. Using the explicit +-5kHz is much better
> > for this use case.
> > 
> > Not sure if we should change the normal mode matching to also use
> > something else besides KHZ2PICOS() since it allows a different
> > proportion of error depending on the clock. I believe VESA CVT
> > allows a maximum deviation of .5%, so using that for normal mode
> > matching might be a good idea?
> 
> Ping. Any thoughts from anyone?

Reviewed-by: Adam Jackson <ajax@redhat.com>

- ajax
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