[Intel-gfx] [PATCH xf86-video-intel 2/2] sna: Eliminate sna_mode_wants_tear_free()

[Chris Wilson]

Quoting Ville Syrjala (2019-12-09 15:01:37)
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> The modparam checks performed by sna_mode_wants_tear_free() don't
> generally work when the server is running as a regular user. Hence
> we can't rely on them to indicate whether FBC/PSR/etc is enabled.
> A lso the "Panel Self-Refresh" connector property doesn't actually
> exist so we can nuke that part as well. Let's just nuke the whole
> thing and assume we want dirtyfb always when tearfree is not enabled.
> 
> I'll anyway want to enable FBC by default across the board soonish
> so the check wouldn't really buy us much (would just exclude i830
> and a few old desktop chipsets which don't have FBC hardware).
> 
> Additionally if we don't have working dirtyfb we really should
> enable tearfree by default because otherwise we're going to
> get horrible lag due to missing frontbuffer flushes.

But we also want to enable TearFree anyway in most cases, and here we
are defaulting to off in cases where it was already on.

I still don't know on what grounds the cut-off should be based, the
primary question is can we afford to keep an extra framebuffer plus any
gubbins memory? The worry about perf are now larger moot, so it boils
down to available memory -- in quite a few cases TearFree is a big
improvement on power management, but that I guess is currently snb+
(although we can fix ilk render powerstandby).

How about GTT > mappable aperture, based on the idea that we have room
to spare that can't be used for scanout? That would only disable gen2 by
default.
-Chris