[Intel-gfx] [PATCH 6/6] drm/i915: New drm crtc property for varying the Crtc size

[Ville Syrjälä]

On Thu, Aug 14, 2014 at 08:33:23PM +0200, Daniel Vetter wrote:
> On Thu, Aug 14, 2014 at 7:58 PM, Ville Syrjälä
> <ville.syrjala at linux.intel.com> wrote:
> > Sure but the user can supply any mode, doesn't have to be on any list.
> > And the only sane rule for the frobbing would be that you can (slightly)
> > reduce hdisp/vdisp but never expand them so that there will never be any
> > extra garbage exposed (and the FB might not be big enough anyway). But
> > even reducing hdisp/vdisp by one pixel can be enough to anger the
> > hardware if a plane then extends one pixel into the blanking.
> >
> > This isn't much of a problem for i915 though. The hardware is generally
> > good enough to not need it. Double wide and (s)dvo/lvds gang mode are
> > the only exception that comes to mind. Even there we just need to make
> > pipe src width even, but still that's something we have to account
> > when clipping planes.
> >
> > On older hardware there were generally more restrictions eg. some
> > legacy baggage from VGA days which required horizontal timings to
> > be multiples of 8. I also "fondly" remember much more magic timing
> > restrictions in certain pieces hardware which were something close
> > to "if (foo*bar % this == that) frob else don't". IMO these kinds of
> > restrictions are too magic to make rejecting the mode an option,
> > so frobbing is the lesser of two evils.
> 
> Imo the mode list we provide should be reasonable for everyone, and if
> you start to add your own modes then I expect the user to do that
> adjusting for us. Nowadays there should be very few cases where we
> don't provide decent mode lists and where it's not a super-special
> embedded thing where you need to configure everything yourself anyway.
> So I don't think we should ever adjust the input region for a crtc.

That's fine for decent hardware. But if/when I write a driver for
old junk I'm probably going to frob no matter what anyone says :)

-- 
Ville Syrjälä
Intel OTC