Re: [Intel-gfx] [PATCH CI 1/2] drm/i915/display/skl+: Drop frontbuffer rendering support

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

On Wed, Sep 15, 2021 at 04:48:49PM +0300, Ville Syrjälä wrote:
> On Mon, Sep 13, 2021 at 10:54:14PM +0000, Souza, Jose wrote:
> > On Thu, 2021-09-09 at 23:28 +0300, Ville Syrjälä wrote:
> > > On Thu, Sep 09, 2021 at 08:23:20PM +0000, Souza, Jose wrote:
> > > > On Thu, 2021-09-09 at 23:20 +0300, Ville Syrjälä wrote:
> > > > > On Thu, Sep 09, 2021 at 12:49:16PM -0700, José Roberto de Souza wrote:
> > > > > > By now all the userspace applications should have migrated to atomic
> > > > > > or at least be calling DRM_IOCTL_MODE_DIRTYFB.
> > > > > > 
> > > > > > With that we can kill frontbuffer rendering support in i915 for
> > > > > > modern platforms.
> > > > > > 
> > > > > > So here converting legacy APIs into atomic commits so it can be
> > > > > > properly handled by driver i915.
> > > > > > 
> > > > > > Several IGT tests will fail with this changes, because some tests
> > > > > > were stressing those frontbuffer rendering scenarios that no userspace
> > > > > > should be using by now, fixes to IGT should be sent soon.
> > > > > > 
> > > > > 
> > > > > I just gave this a try here and it's unusable. glxgears went from
> > > > > 9000 to 120 fps (was expecting 60fps tbh, not sure why I get
> > > > > double), everything lags like mad, if I drag a window around
> > > > > glxgears/other windows stop updating entirely, etc. NAK
> > > > 
> > > > Can you share your setup? What GPU? Desktop environment? Mesa version? resolutions of sinks?
> > > > Will try it in my end.
> > > 
> > > Doesn't really matter as long as you don't have a compositor making a
> > > mess of things. This machine is a cfl running mate w/ compositor off,
> > > and some 1920x1200 display.
> > > 
> > 
> > Making drm_atomic_helper_dirtyfb() do a non-blocking atomic commit makes user experience pretty similar to the one with compositing enabled:
> > drm_atomic_commit() + compositor off: https://youtu.be/NBt6smXs99U
> > drm_atomic_nonblocking_commit() + compositor off: https://youtu.be/QiMhkeGX_L8
> > drm_atomic_nonblocking_commit() + compositor on: https://youtu.be/KdpJyJ5k6sQ
> > 
> > 
> > I do not completly agree with the comment in drm_atomic_helper_dirtyfb() about why it uses a blocking implementation.
> > With frontbuffer rendering the registers are programmed but the content could only show up for user a whole frame later.
> > 
> > Perhaps if this solutions is accetable we could have a non-blocking version of drm_atomic_helper_dirtyfb() so the drivers current using it don't have
> > their behavior changed.
> 
> Non-blocking update would make sense to me, whereas a blocking
> update makes no sense given how this is used by actual userspace.

Actually neither maybe makes total sense since userspace probably
isn't expecting -EBUSY from dirtyfb. So we might end up with stale
junk on the screen if no further updates come in after an -EBUSY.

The current frontbuffer stuff works much more like a mailbox style
update so we don't lose stuff and neither do we block.

-- 
Ville Syrjälä
Intel