Validating modes should be more fuzzy
ajax at nwnk.net
Thu Aug 23 07:07:54 PDT 2007
On Wed, 2007-08-22 at 15:14 +0200, Matthias Hopf wrote:
> While testing the current Xserver + drivers it occurred to us that a
> mode wasn't accepted though it looked perfectly sane. It turned out,
> that its refresh rate (60.01fps) was *just* above the nominal maximum
> refresh rate of the monitor (60fps).
> As PLLs cannot be programmed exactly to any given value, there should be
> a certain fuzziness in mode validation, like the attached patch will do
> (in this case +/- 0.5Hz for vrefresh, +/- 0.5kHz for hrefresh, +/-
> 0.5MHz for clock).
> I chose these values arbitrarily, and it should be discussed what to use
> here (absolute/relative? How much?). I would suggest going for 0.5
> absolute like in this patch (because people tend to specify integer
> values for limits), or 0.1% to 1% relative.
> If nobody cares, I will commit it the way it is.
The classic mode selection code has some tolerance already. In
#define SYNC_TOLERANCE 0.01 /* 1 percent */
Which seems to have served us well. So I'd prefer to either have the
new mode code use the same fuzz, or else change it in both places.
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