[PATCH 8/9] drm/i915: Perform vblank evasion around legacy cursor updates

[Shankar, Uma]

> -----Original Message-----
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Sent: Wednesday, December 20, 2023 8:19 PM
> To: Shankar, Uma <uma.shankar at intel.com>
> Cc: intel-gfx at lists.freedesktop.org
> Subject: Re: [PATCH 8/9] drm/i915: Perform vblank evasion around legacy cursor
> updates
> 
> On Wed, Dec 20, 2023 at 11:45:44AM +0000, Shankar, Uma wrote:
> >
> >
> > > -----Original Message-----
> > > From: Shankar, Uma
> > > Sent: Wednesday, December 20, 2023 5:11 PM
> > > To: Ville Syrjala <ville.syrjala at linux.intel.com>;
> > > intel-gfx at lists.freedesktop.org
> > > Subject: RE: [PATCH 8/9] drm/i915: Perform vblank evasion around
> > > legacy cursor updates
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On
> > > > Behalf Of Ville Syrjala
> > > > Sent: Wednesday, December 13, 2023 3:55 PM
> > > > To: intel-gfx at lists.freedesktop.org
> > > > Subject: [PATCH 8/9] drm/i915: Perform vblank evasion around
> > > > legacy cursor updates
> > > >
> > > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > >
> > > > Our legacy cursor updates are actually mailbox updates.
> > > > Ie. the hardware latches things once per frame on start of vblank,
> > > > but we issue an number of updates per frame, withough any attempt
> > > > to synchronize against the vblank in software. So in theory only
> > > > the last update issued during the frame will latch, and the previous ones are
> discarded.
> > > >
> > > > However this can lead to problems with maintaining the ggtt/iommu
> > > > mappings as we have no idea which updates will actually latch.
> > > >
> > > > The problem is exacerbated by the hardware's annoying disarming
> > > > behaviour; any non-arming register write will disarm an already
> > > > armed update, only to be rearmed later by the arming register
> > > > (CURBASE in case of cursors). If a disarming write happens just
> > > > before the start of vblank, and the arming write happens after
> > > > start of vblank we have effectively prevented the hardware from
> > > > latching anything. And if we manage to straddle multiple
> > > > sequential vblank starts in this manner we effectively prevent the
> > > > hardware from latching any new registers for an arbitrary amount
> > > > of time. This provides more time for the (potentially still in
> > > use by the hardware) gtt/iommu mappings to be torn down.
> > > >
> > > > A partial solution, of course, is to use vblank evasion to avoid
> > > > the register writes from spreading on both sides of the start of vblank.
> > > >
> > > > I've previously highlighted this problem as a general issue
> > > > affecting mailbox updates. I even added some notes to the
> > > > {i9xx,skl}_crtc_planes_update_arm() to remind us that the noarm
> > > > and arm phases both need to pulled into the vblank evasion
> > > > critical section if we actually decided to implement mailbox
> > > > updates in general. But as I never impelemented the
> > > > noarm+arm split for cursors we don't have to worry about that for the
> moment.
> > > >
> > > > We've been lucky enough so far that this hasn't really caused
> > > > problems. One thing that does help is that Xorg generally sticks
> > > > to the same cursor BO. But igt seems pretty good at hitting this
> > > > on MTL now, so apparently we have to start thinking about this.
> > >
> > > Was not aware that a disarming update will disarm an armed update
> > > and make the vblank sync irrelevant. Thanks for a good writeup
> > > highlighting the issue, really helps.
> > >
> > > Yeah, this should help maintain consistency with cursor updates and
> > > ensure the sync at vblank. Change looks Good to me.
> > > Reviewed-by: Uma Shankar <uma.shankar at intel.com>
> > >
> > > > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/display/intel_cursor.c | 16 ++++++++++------
> > > >  1 file changed, 10 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/display/intel_cursor.c
> > > > b/drivers/gpu/drm/i915/display/intel_cursor.c
> > > > index 926e2de00eb5..77531838001f 100644
> > > > --- a/drivers/gpu/drm/i915/display/intel_cursor.c
> > > > +++ b/drivers/gpu/drm/i915/display/intel_cursor.c
> > > > @@ -22,6 +22,7 @@
> > > >  #include "intel_frontbuffer.h"
> > > >  #include "intel_psr.h"
> > > >  #include "intel_psr_regs.h"
> > > > +#include "intel_vblank.h"
> > > >  #include "skl_watermark.h"
> > > >
> > > >  #include "gem/i915_gem_object.h"
> > > > @@ -647,12 +648,14 @@ intel_legacy_cursor_update(struct drm_plane
> > > > *_plane, {
> > > >  	struct intel_plane *plane = to_intel_plane(_plane);
> > > >  	struct intel_crtc *crtc = to_intel_crtc(_crtc);
> > > > +	struct drm_i915_private *i915 = to_i915(plane->base.dev);
> > > >  	struct intel_plane_state *old_plane_state =
> > > >  		to_intel_plane_state(plane->base.state);
> > > >  	struct intel_plane_state *new_plane_state;
> > > >  	struct intel_crtc_state *crtc_state =
> > > >  		to_intel_crtc_state(crtc->base.state);
> > > >  	struct intel_crtc_state *new_crtc_state;
> > > > +	struct intel_vblank_evade_ctx evade;
> > > >  	int ret;
> > > >
> > > >  	/*
> > > > @@ -745,14 +748,15 @@ intel_legacy_cursor_update(struct drm_plane
> > > *_plane,
> > > >  	 */
> > > >  	crtc_state->active_planes = new_crtc_state->active_planes;
> > > >
> > > > -	/*
> > > > -	 * Technically we should do a vblank evasion here to make
> > > > -	 * sure all the cursor registers update on the same frame.
> > > > -	 * For now just make sure the register writes happen as
> > > > -	 * quickly as possible to minimize the race window.
> > > > -	 */
> > > > +	intel_vblank_evade_init(crtc_state, crtc_state, &evade);
> >
> > Missed to update:
> > Should the 2nd argument not be new_crtc_state ?
> 
> We'll discard 'new_crtc_state' at the end and we just update 'crtc_state' in place
> (only active_planes actually). So essentially 'crtc_state' here is the new state
> already. That should be perfectly fine for the purposes of vblank evasion since
> that only cares about the old state when modesets are involved (which is never
> the case here).

Got it, thanks for clarifying Ville.

Regards,
Uma Shankar

> >
> > > >  	local_irq_disable();
> > > >
> > > > +	if (!drm_WARN_ON(&i915->drm, drm_crtc_vblank_get(&crtc->base))) {
> > > > +		intel_vblank_evade(&evade);
> > > > +		drm_crtc_vblank_put(&crtc->base);
> > > > +	}
> > > > +
> > > >  	if (new_plane_state->uapi.visible) {
> > > >  		intel_plane_update_noarm(plane, crtc_state, new_plane_state);
> > > >  		intel_plane_update_arm(plane, crtc_state, new_plane_state);
> > > > --
> > > > 2.41.0
> >
> 
> --
> Ville Syrjälä
> Intel