Re: [PATCH 11/14] drm/i915/dsb: Allow intel_dsb_chain() to use DSB_WAIT_FOR_VBLANK

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

On Wed, Aug 21, 2024 at 02:58:20PM +0000, Manna, Animesh wrote:
> 
> 
> > -----Original Message-----
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Sent: Friday, July 5, 2024 11:09 PM
> > To: Manna, Animesh <animesh.manna@intel.com>
> > Cc: intel-gfx@lists.freedesktop.org
> > Subject: Re: [PATCH 11/14] drm/i915/dsb: Allow intel_dsb_chain() to use
> > DSB_WAIT_FOR_VBLANK
> > 
> > On Fri, Jul 05, 2024 at 03:58:32PM +0000, Manna, Animesh wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Intel-gfx <intel-gfx-bounces@lists.freedesktop.org> On Behalf
> > > > Of Ville Syrjala
> > > > Sent: Tuesday, June 25, 2024 12:40 AM
> > > > To: intel-gfx@lists.freedesktop.org
> > > > Subject: [PATCH 11/14] drm/i915/dsb: Allow intel_dsb_chain() to use
> > > > DSB_WAIT_FOR_VBLANK
> > > >
> > > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > >
> > > > Allow intel_dsb_chain() to start the chained DSB at start of the
> > > > undelaye vblank. This is slightly more involved than simply setting
> > > > the bit as we must use the DEwake mechanism to eliminate pkgC
> > > > latency.
> > > >
> > > > And DSB_ENABLE_DEWAKE itself is problematic in that it allows us to
> > > > configure just a single scanline, and if the current scanline is
> > > > already past that DSB_ENABLE_DEWAKE won't do anything, rendering the
> > > > whole thing moot.
> > > >
> > > > The current workaround involves checking the pipe's current scanline
> > > > with the CPU, and if it looks like we're about to miss the
> > > > configured DEwake scanline we set DSB_FORCE_DEWAKE to immediately
> > > > assert DEwake. This is somewhat racy since the hardware is making
> > > > progress all the while we're checking it on the CPU.
> > > >
> > > > We can make things less racy by chaining two DSBs and handling the
> > > > DSB_FORCE_DEWAKE stuff entirely without CPU involvement:
> > > > 1. CPU starts the first DSB immediately 2. First DSB configures the
> > > > second DSB, including its dewake_scanline 3. First DSB starts the
> > > > second w/ DSB_WAIT_FOR_VBLANK 4. First DSB asserts
> > DSB_FORCE_DEWAKE
> > > > 5. First DSB waits until we're outside the dewake_scanline-vblank_start
> > > >    window
> > > > 6. First DSB deasserts DSB_FORCE_DEWAKE
> > > >
> > > > That will guarantee that the we are fully awake when the second DSB
> > > > starts to actually execute.
> > > >
> > > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/display/intel_dsb.c | 43
> > > > +++++++++++++++++++++---  drivers/gpu/drm/i915/display/intel_dsb.h |
> > > > 3 +-
> > > >  2 files changed, 40 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > b/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > index 4c0519c41f16..cf710f0bf430 100644
> > > > --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > @@ -130,8 +130,8 @@ static int dsb_vtotal(struct intel_atomic_state
> > *state,
> > > >  		return intel_mode_vtotal(&crtc_state->hw.adjusted_mode);
> > > >  }
> > > >
> > > > -static int dsb_dewake_scanline(struct intel_atomic_state *state,
> > > > -			       struct intel_crtc *crtc)
> > > > +static int dsb_dewake_scanline_start(struct intel_atomic_state *state,
> > > > +				     struct intel_crtc *crtc)
> > > >  {
> > > >  	const struct intel_crtc_state *crtc_state =
> > > > pre_commit_crtc_state(state, crtc);
> > > >  	struct drm_i915_private *i915 = to_i915(state->base.dev); @@
> > > > -141,6 +141,14 @@ static int dsb_dewake_scanline(struct
> > > > intel_atomic_state *state,
> > > >  		intel_usecs_to_scanlines(&crtc_state->hw.adjusted_mode,
> > > > latency);
> > > >  }
> > > >
> > > > +static int dsb_dewake_scanline_end(struct intel_atomic_state *state,
> > > > +				   struct intel_crtc *crtc)
> > > > +{
> > > > +	const struct intel_crtc_state *crtc_state =
> > > > pre_commit_crtc_state(state, crtc);
> > > > +
> > > > +	return intel_mode_vdisplay(&crtc_state->hw.adjusted_mode);
> > > > +}
> > > > +
> > > >  static int dsb_scanline_to_hw(struct intel_atomic_state *state,
> > > >  			      struct intel_crtc *crtc, int scanline)  { @@ -529,19
> > > > +537,44 @@ static void _intel_dsb_chain(struct intel_atomic_state
> > > > *state,
> > > >  			    dsb_error_int_status(display) |
> > DSB_PROG_INT_STATUS |
> > > >  			    dsb_error_int_en(display));
> > > >
> > > > +	if (ctrl & DSB_WAIT_FOR_VBLANK) {
> > > > +		int dewake_scanline = dsb_dewake_scanline_start(state,
> > > > crtc);
> > > > +		int hw_dewake_scanline = dsb_scanline_to_hw(state, crtc,
> > > > dewake_scanline);
> > > > +
> > > > +		intel_dsb_reg_write(dsb, DSB_PMCTRL(pipe, chained_dsb-
> > > > >id),
> > > > +				    DSB_ENABLE_DEWAKE |
> > > > +
> > > > DSB_SCANLINE_FOR_DEWAKE(hw_dewake_scanline));
> > > > +	}
> > > > +
> > > >  	intel_dsb_reg_write(dsb, DSB_HEAD(pipe, chained_dsb->id),
> > > >  			    intel_dsb_buffer_ggtt_offset(&chained_dsb-
> > > > >dsb_buf));
> > > >
> > > >  	intel_dsb_reg_write(dsb, DSB_TAIL(pipe, chained_dsb->id),
> > > >  			    intel_dsb_buffer_ggtt_offset(&chained_dsb-
> > > > >dsb_buf) + tail);
> > > > +
> > > > +	if (ctrl & DSB_WAIT_FOR_VBLANK) {
> > > > +		/*
> > > > +		 * Keep DEwake alive via the first DSB, in
> > > > +		 * case we're already past dewake_scanline,
> > > > +		 * and thus DSB_ENABLE_DEWAKE on the second
> > > > +		 * DSB won't do its job.
> > > > +		 */
> > > > +		intel_dsb_reg_write_masked(dsb, DSB_PMCTRL_2(pipe, dsb-
> > > > >id),
> > > > +					   DSB_FORCE_DEWAKE,
> > > > DSB_FORCE_DEWAKE);
> > > > +
> > > > +		intel_dsb_wait_scanline_out(state, dsb,
> > > > +					    dsb_dewake_scanline_start(state,
> > > > crtc),
> > > > +					    dsb_dewake_scanline_end(state,
> > > > crtc));
> > > > +	}
> > > >  }
> > > >
> > > >  void intel_dsb_chain(struct intel_atomic_state *state,
> > > >  		     struct intel_dsb *dsb,
> > > > -		     struct intel_dsb *chained_dsb)
> > > > +		     struct intel_dsb *chained_dsb,
> > > > +		     bool wait_for_vblank)
> > > >  {
> > > >  	_intel_dsb_chain(state, dsb, chained_dsb,
> > > > -			 0);
> > > > +			 wait_for_vblank ? DSB_WAIT_FOR_VBLANK : 0);
> > >
> > > As per commit description and current implementation always need
> > DSB_WAIT_FOR_VBLANK. Just wondering is there any scenario where will
> > pass false through wait_for_vblank flag to  intel_dsb_chain()? If no can we
> > drop the wait_for_vblank flag?
> > 
> > Shrug. For now I wanted to model it on intel_dsb_commit().
> > I'm actually thinking of removing the wait_for_vblank stuff from
> > intel_dsb_commit() after this lands. We could think about making the
> > wait_for_vblank unconditional for intel_dsb_chain() at that point, assuming
> > no one comes up with a use case for the immediate start version.
> 
> As I understood the whole concept of intel_dsb_chain() is based on DSB_WAIT_FOR_VBLANK which should be unconditional and always true.

You can chain without the wait for vblank just fine. Currently we have
no actual need to do that, but I do have selftests that check both
behaviours.

> Not sure is it really needed to add in this patch series and later again removing it. So, I was waiting to see your next patch series related to plane update using DSB.
> For now, with the modification to make it unconditional the other changes look good to me. Good to understand your plan on this.

The fact that it matches the current intel_dsb_commit() is
all I really care about at this point.

-- 
Ville Syrjälä
Intel