Re: [PATCH 6/7] drm/i915/fbc: move from crtc_state->enable_fbc to plane_state->enable_fbc

From: Paulo Zanoni <paulo.r.zanoni@intel.com>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 6/7] drm/i915/fbc: move from crtc_state->enable_fbc to plane_state->enable_fbc
Date: Fri, 11 Nov 2016 18:49:59 -0200	[thread overview]
Message-ID: <1478897399.19391.61.camel@intel.com> (raw)
In-Reply-To: <20161111202453.GU31595@intel.com>

Em Sex, 2016-11-11 às 22:24 +0200, Ville Syrjälä escreveu:
> On Fri, Nov 11, 2016 at 05:57:28PM -0200, Paulo Zanoni wrote:
> > 
> > Em Sex, 2016-11-11 às 21:13 +0200, Ville Syrjälä escreveu:
> > > 
> > > On Fri, Nov 11, 2016 at 05:01:54PM -0200, Paulo Zanoni wrote:
> > > > 
> > > > 
> > > > Em Sex, 2016-11-11 às 20:51 +0200, Ville Syrjälä escreveu:
> > > > > 
> > > > > 
> > > > > On Fri, Nov 11, 2016 at 02:57:40PM -0200, Paulo Zanoni wrote:
> > > > > > 
> > > > > > 
> > > > > > 
> > > > > > Ville pointed out that intel_fbc_choose_crtc() is iterating
> > > > > > over
> > > > > > all
> > > > > > planes instead of just the primary planes. There are no
> > > > > > real
> > > > > > consequences of this problem for HSW+, and for the other
> > > > > > platforms
> > > > > > it
> > > > > > just means that in some obscure multi-screen cases we'll
> > > > > > keep
> > > > > > FBC
> > > > > > disabled when we could have enabled it. Still, iterating
> > > > > > over
> > > > > > all
> > > > > > planes doesn't seem to be the best thing to do.
> > > > > > 
> > > > > > My initial idea was to just add a check for plane->type and
> > > > > > be
> > > > > > done,
> > > > > > but then I realized that in commits not involving the
> > > > > > primary
> > > > > > plane
> > > > > > we
> > > > > > would reset crtc_state->enable_fbc back to false even when
> > > > > > FBC
> > > > > > is
> > > > > > enabled. That also wouldn't result in a bug due to the way
> > > > > > the
> > > > > > enable_fbc variable is checked, but, still, our code can be
> > > > > > better
> > > > > > than this.
> > > > > > 
> > > > > > So I went for the solution that involves tracking
> > > > > > enable_fbc in
> > > > > > the
> > > > > > primary plane state instead of the CRTC state. This way, if
> > > > > > a
> > > > > > commit
> > > > > > doesn't involve the primary plane for the CRTC we won't be
> > > > > > resetting
> > > > > > enable_fbc back to false, so the variable will always
> > > > > > reflect
> > > > > > the
> > > > > > reality. And this change also makes more sense since FBC is
> > > > > > actually
> > > > > > tied to the single plane and not the full pipe. As a bonus,
> > > > > > we
> > > > > > only
> > > > > > iterate over the CRTCs instead of iterating over all
> > > > > > planes.
> > > > > > 
> > > > > > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > > > Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > > > Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
> > > > > > ---
> > > > > >  drivers/gpu/drm/i915/intel_drv.h |  4 ++--
> > > > > >  drivers/gpu/drm/i915/intel_fbc.c | 36 +++++++++++++++++++-
> > > > > > ----
> > > > > > ----
> > > > > > --------
> > > > > >  2 files changed, 21 insertions(+), 19 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/gpu/drm/i915/intel_drv.h
> > > > > > b/drivers/gpu/drm/i915/intel_drv.h
> > > > > > index 003afb8..025cb74 100644
> > > > > > --- a/drivers/gpu/drm/i915/intel_drv.h
> > > > > > +++ b/drivers/gpu/drm/i915/intel_drv.h
> > > > > > @@ -403,6 +403,8 @@ struct intel_plane_state {
> > > > > >  	int scaler_id;
> > > > > >  
> > > > > >  	struct drm_intel_sprite_colorkey ckey;
> > > > > > +
> > > > > > +	bool enable_fbc;
> > > > > >  };
> > > > > >  
> > > > > >  struct intel_initial_plane_config {
> > > > > > @@ -648,8 +650,6 @@ struct intel_crtc_state {
> > > > > >  
> > > > > >  	bool ips_enabled;
> > > > > >  
> > > > > > -	bool enable_fbc;
> > > > > > -
> > > > > >  	bool double_wide;
> > > > > >  
> > > > > >  	bool dp_encoder_is_mst;
> > > > > > diff --git a/drivers/gpu/drm/i915/intel_fbc.c
> > > > > > b/drivers/gpu/drm/i915/intel_fbc.c
> > > > > > index b095175..fc4ac57 100644
> > > > > > --- a/drivers/gpu/drm/i915/intel_fbc.c
> > > > > > +++ b/drivers/gpu/drm/i915/intel_fbc.c
> > > > > > @@ -1055,16 +1055,17 @@ void intel_fbc_choose_crtc(struct
> > > > > > drm_i915_private *dev_priv,
> > > > > >  			   struct drm_atomic_state *state)
> > > > > >  {
> > > > > >  	struct intel_fbc *fbc = &dev_priv->fbc;
> > > > > > -	struct drm_plane *plane;
> > > > > > -	struct drm_plane_state *plane_state;
> > > > > > +	struct drm_crtc *crtc;
> > > > > > +	struct drm_crtc_state *crtc_state;
> > > > > >  	bool crtc_chosen = false;
> > > > > >  	int i;
> > > > > >  
> > > > > >  	mutex_lock(&fbc->lock);
> > > > > >  
> > > > > > -	/* Does this atomic commit involve the CRTC
> > > > > > currently
> > > > > > tied
> > > > > > to FBC? */
> > > > > > +	/* Does this atomic commit involve the plane
> > > > > > currently
> > > > > > tied to FBC? */
> > > > > >  	if (fbc->crtc &&
> > > > > > -	    !drm_atomic_get_existing_crtc_state(state,
> > > > > > &fbc-
> > > > > > > 
> > > > > > > crtc-
> > > > > > > 
> > > > > > > base))
> > > > > > +	    !drm_atomic_get_existing_plane_state(state,
> > > > > > +						 fbc-
> > > > > > >crtc-
> > > > > > > 
> > > > > > > 
> > > > > > > base.primary))
> > > > > >  		goto out;
> > > > > >  
> > > > > >  	if (!intel_fbc_can_enable(dev_priv))
> > > > > > @@ -1074,25 +1075,26 @@ void intel_fbc_choose_crtc(struct
> > > > > > drm_i915_private *dev_priv,
> > > > > >  	 * plane. We could go for fancier schemes such as
> > > > > > checking
> > > > > > the plane
> > > > > >  	 * size, but this would just affect the few
> > > > > > platforms
> > > > > > that
> > > > > > don't tie FBC
> > > > > >  	 * to pipe or plane A. */
> > > > > > -	for_each_plane_in_state(state, plane, plane_state,
> > > > > > i)
> > > > > > {
> > > > > > -		struct intel_plane_state
> > > > > > *intel_plane_state =
> > > > > > -			to_intel_plane_state(plane_state);
> > > > > > -		struct intel_crtc_state *intel_crtc_state;
> > > > > > -		struct intel_crtc *crtc =
> > > > > > to_intel_crtc(plane_state->crtc);
> > > > > > +	for_each_crtc_in_state(state, crtc, crtc_state, i)
> > > > > > {
> > > > > > +		struct intel_plane_state *plane_state =
> > > > > > to_intel_plane_state(
> > > > > > +			drm_atomic_get_existing_plane_stat
> > > > > > e(st
> > > > > > ate,
> > > > > > +							  
> > > > > >   cr
> > > > > > tc-
> > > > > > > 
> > > > > > > 
> > > > > > > primary));
> > > > > > +		struct intel_crtc *intel_crtc =
> > > > > > to_intel_crtc(crtc);
> > > > > >  
> > > > > > -		if (!intel_plane_state->base.visible)
> > > > > > +		if (!plane_state)
> > > > > >  			continue;
> > > > > >  
> > > > > > -		if (fbc_on_pipe_a_only(dev_priv) && crtc-
> > > > > > >pipe 
> > > > > > !=
> > > > > > PIPE_A)
> > > > > > +		if (!plane_state->base.visible)
> > > > > >  			continue;
> > > > > >  
> > > > > > -		if (fbc_on_plane_a_only(dev_priv) && crtc-
> > > > > > > 
> > > > > > > plane
> > > > > > != PLANE_A)
> > > > > > +		if (fbc_on_pipe_a_only(dev_priv) &&
> > > > > > intel_crtc-
> > > > > > > 
> > > > > > > 
> > > > > > > pipe != PIPE_A)
> > > > > >  			continue;
> > > > > >  
> > > > > > -		intel_crtc_state = to_intel_crtc_state(
> > > > > > -			drm_atomic_get_existing_crtc_state
> > > > > > (sta
> > > > > > te,
> > > > > > &crtc->base));
> > > > > > +		if (fbc_on_plane_a_only(dev_priv) &&
> > > > > > +		    intel_crtc->plane != PLANE_A)
> > > > > > +			continue;
> > > > > >  
> > > > > > -		intel_crtc_state->enable_fbc = true;
> > > > > > +		plane_state->enable_fbc = true;
> > > > > 
> > > > > So looking at this whole thing, I can't see anything that
> > > > > would
> > > > > prevent
> > > > > enable_fbc being true for multiple primary planes at the same
> > > > > time
> > > > > Well, apart from the whole "we enable it only for platforms
> > > > > that
> > > > > can
> > > > > only do
> > > > > pipe A" thing.
> > > > > 
> > > > > So what happens in that case? FBC just ends up getting
> > > > > enabling
> > > > > on
> > > > > one of the pipes based on the order intel_fbc_enable() gets
> > > > > called,
> > > > > or something?
> > > > 
> > > > The first check of intel_fbc_choose_crtc() is supposed to
> > > > prevent
> > > > this
> > > > case: if fbc->crtc->primary is not included in the commit we
> > > > just
> > > > return without selecting any plane.
> > > 
> > > The fbc->crtc thing only works if intel_fbc_enable() was already
> > > called
> > > for some crtc. But what it it wasn't?
> > > 
> > > > 
> > > > 
> > > > Otherwise, we only pick one CRTC
> > > > due to the "break;" statement after setting plane_state-
> > > > >enable_fbc 
> > > > to
> > > > true.
> > > 
> > > Only one per atomic operation. But what if there are several
> > > happening
> > > in parallel on different crtcs?
> > 
> > I see your point now. Yeah, we'll end up with
> > plane_state.enable_fbc=true for two different planes. Later, the
> > first
> > one to call intel_fbc_enable() will win, and the others will be
> > ignored, so we'll indeed end up with plane states having
> > enable_fbc=true but FBC not enabled by them. Not a real bug, I
> > would
> > still like to avoid this confusion.
> > 
> > The simplest solution I can think would be to just
> > s/plane_state.enable_fbc/plane_state.can_enable_fbc/ and just let
> > the
> > first one to call intel_fbc_enable() win... And then, if we ever
> > decide
> > to enable FBC on the older platforms, we can choose to maybe
> > implement
> > a better method
> 
> Maybe something like "fbc_score"? ;)

The design of the current function was supposed to allow Ville to
implement his fbc_score in case he wanted. But this certainly didn't
take into account multiple parallel commits: it would only work if
multiple CRTCs were included in the same commit (as you just pointed
today).

But then: if we're having separate parallel commits, when would we be
able to loop through the scores to only actually enable FBC on the best
score? 

For example, if we do two parallel atomic_check()s and end with
plane_a_score=1 and plane_b_score=2, then later we do A's commit() and
call intel_fbc_enable() for it, how do we conclude that we shouldn't
enable FBC for plane A? We're not even sure if plane B is going to
actually commit the plane state it calculated (maybe it was
check_only).

And then, if we decide to only compute everything during commit()
instead of check(), we'll just also end up enabling FBC for plane A
since A's commit() will run first and we'll have no idea that B's
commit is incoming.

The only option would be to disable FBC for plane A and enable for
plane B during B's commit. But I'm not looking forward to implement
this right now.

So we kinda have 3 options:

1 - Keep the current approach (with the renamed variable) that at least
allows us to pick the best CRTC in case a single commit includes
multiple CRTCs.

2 - Go the simplest route and not even bother picking the best CRTC
among a single commit due to the fact that parallel commits won't
guarantee we always have the best FBC plane enabled, so why bother.

3 - Wait for someone to implement the takeover approach.

Thanks for the review,
Paulo

> > 
> > (and fix all the possible FBC problems for those
> > platforms).
> 
> I think I fixed all the platforms specific issues back in the day.
> All
> the actual problems were in the common code. Although I guess some of
> my patches might not have made it in, or I just missed some things.
> 
> > 
> > Is this solution worth a R-B from you?
> > 
> > If you have any better idea here, feel free to suggest it. I'm not
> > sure
> > it's possible to come up with something much better due to all the
> > parallelism involved. Making one CRTC "take over" FBC from the
> > other is
> > something I'd really love to avoid.
> 
> Yeah, it looked safe enough to me to have the thing set to true on
> multiple planes, and if you came to the same conclusion I think we
> should be good.
> 
> With the rename of the flag to avoid confusion:
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> > 
> > 
> > Thanks,
> > Paulo
> > 
> > > 
> > > 
> > > > 
> > > > 
> > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > > 
> > > > > > 
> > > > > > 
> > > > > >  		crtc_chosen = true;
> > > > > >  		break;
> > > > > >  	}
> > > > > > @@ -1130,13 +1132,13 @@ void intel_fbc_enable(struct
> > > > > > intel_crtc
> > > > > > *crtc,
> > > > > >  	if (fbc->enabled) {
> > > > > >  		WARN_ON(fbc->crtc == NULL);
> > > > > >  		if (fbc->crtc == crtc) {
> > > > > > -			WARN_ON(!crtc_state->enable_fbc);
> > > > > > +			WARN_ON(!plane_state->enable_fbc);
> > > > > >  			WARN_ON(fbc->active);
> > > > > >  		}
> > > > > >  		goto out;
> > > > > >  	}
> > > > > >  
> > > > > > -	if (!crtc_state->enable_fbc)
> > > > > > +	if (!plane_state->enable_fbc)
> > > > > >  		goto out;
> > > > > >  
> > > > > >  	WARN_ON(fbc->active);
> > > > > > -- 
> > > > > > 2.7.4
> > > > > 
> > > 
> 
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/intel-gfx