> -----Original Message-----
> From: Manna, Animesh
> Sent: Friday, July 5, 2024 9:29 PM
> To: Ville Syrjala <ville.syrjala@xxxxxxxxxxxxxxx>; intel-
> gfx@xxxxxxxxxxxxxxxxxxxxx
> Subject: RE: [PATCH 11/14] drm/i915/dsb: Allow intel_dsb_chain() to use
> DSB_WAIT_FOR_VBLANK
>
>
>
> > -----Original Message-----
> > From: Intel-gfx <intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
> Ville
> > Syrjala
> > Sent: Tuesday, June 25, 2024 12:40 AM
> > To: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> > Subject: [PATCH 11/14] drm/i915/dsb: Allow intel_dsb_chain() to use
> > DSB_WAIT_FOR_VBLANK
> >
> > From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx>
> > ---
> > 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));
One quick check: As per bspec DSB_PMCTRL can be updated only before the DSB_CTRL register is programmed to enable the DSB engine. Here programming is done later, not sure if it causes any negative impact.
Regards,
Animesh
> > + }
> > +
> > 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?
>
> Regards,
> Animesh
> > }
> >
> > static void _intel_dsb_commit(struct intel_dsb *dsb, u32 ctrl,
> > @@ -699,7 +732,7 @@ struct intel_dsb *intel_dsb_prepare(struct
> > intel_atomic_state *state,
> >
> > dsb->chicken = dsb_chicken(state, crtc);
> > dsb->hw_dewake_scanline =
> > - dsb_scanline_to_hw(state, crtc, dsb_dewake_scanline(state,
> > crtc));
> > + dsb_scanline_to_hw(state, crtc,
> > dsb_dewake_scanline_start(state, crtc));
> >
> > return dsb;
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dsb.h
> > b/drivers/gpu/drm/i915/display/intel_dsb.h
> > index e59fd7da0fc0..c352c12aa59f 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dsb.h
> > +++ b/drivers/gpu/drm/i915/display/intel_dsb.h
> > @@ -47,7 +47,8 @@ void intel_dsb_wait_scanline_out(struct
> > intel_atomic_state *state,
> > int lower, int upper);
> > 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);
> >
> > void intel_dsb_commit(struct intel_dsb *dsb,
> > bool wait_for_vblank);
> > --
> > 2.44.2
