[Intel-gfx] [PATCH v2 02/19] drm/i915/dsb: Use non-locked register access

[Shankar, Uma]

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Jani
> Nikula
> Sent: Thursday, June 8, 2023 5:16 PM
> To: Ville Syrjala <ville.syrjala at linux.intel.com>; intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH v2 02/19] drm/i915/dsb: Use non-locked register
> access
> 
> On Tue, 06 Jun 2023, Ville Syrjala <ville.syrjala at linux.intel.com> wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> >
> > Avoid the locking overhead for DSB registers. We don't need the locks
> > and intel_dsb_commit() in particular needs to be called from the
> > vblank evade critical section and thus needs to be fast.
> 
> Mmmh, I always find it slightly puzzling to encounter _fw calls in code, wondering
> what the rationale was, and why we can use the _fw variants.
> 
> Should we start adding comments explaining why?

I believe it’s a light weight write without any locks and forcewake.
Maybe a comment to explain the rationale would be good.

With that added, this is:
Reviewed-by: Uma Shankar <uma.shankar at intel.com>

> BR,
> Jani.
> 
> 
> >
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dsb.c | 18 +++++++++---------
> >  1 file changed, 9 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c
> > b/drivers/gpu/drm/i915/display/intel_dsb.c
> > index bed058d2c3ac..97e593d9f100 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> > @@ -96,7 +96,7 @@ static bool assert_dsb_has_room(struct intel_dsb
> > *dsb)  static bool is_dsb_busy(struct drm_i915_private *i915, enum pipe pipe,
> >  			enum dsb_id id)
> >  {
> > -	return intel_de_read(i915, DSB_CTRL(pipe, id)) & DSB_STATUS_BUSY;
> > +	return intel_de_read_fw(i915, DSB_CTRL(pipe, id)) & DSB_STATUS_BUSY;
> >  }
> >
> >  static void intel_dsb_emit(struct intel_dsb *dsb, u32 ldw, u32 udw)
> > @@ -243,13 +243,13 @@ void intel_dsb_commit(struct intel_dsb *dsb, bool
> wait_for_vblank)
> >  		return;
> >  	}
> >
> > -	intel_de_write(dev_priv, DSB_CTRL(pipe, dsb->id),
> > -		       (wait_for_vblank ? DSB_WAIT_FOR_VBLANK : 0) |
> > -		       DSB_ENABLE);
> > -	intel_de_write(dev_priv, DSB_HEAD(pipe, dsb->id),
> > -		       i915_ggtt_offset(dsb->vma));
> > -	intel_de_write(dev_priv, DSB_TAIL(pipe, dsb->id),
> > -		       i915_ggtt_offset(dsb->vma) + tail);
> > +	intel_de_write_fw(dev_priv, DSB_CTRL(pipe, dsb->id),
> > +			  (wait_for_vblank ? DSB_WAIT_FOR_VBLANK : 0) |
> > +			  DSB_ENABLE);
> > +	intel_de_write_fw(dev_priv, DSB_HEAD(pipe, dsb->id),
> > +			  i915_ggtt_offset(dsb->vma));
> > +	intel_de_write_fw(dev_priv, DSB_TAIL(pipe, dsb->id),
> > +			  i915_ggtt_offset(dsb->vma) + tail);
> >  }
> >
> >  void intel_dsb_wait(struct intel_dsb *dsb) @@ -266,7 +266,7 @@ void
> > intel_dsb_wait(struct intel_dsb *dsb)
> >  	/* Attempt to reset it */
> >  	dsb->free_pos = 0;
> >  	dsb->ins_start_offset = 0;
> > -	intel_de_write(dev_priv, DSB_CTRL(pipe, dsb->id), 0);
> > +	intel_de_write_fw(dev_priv, DSB_CTRL(pipe, dsb->id), 0);
> >  }
> >
> >  /**
> 
> --
> Jani Nikula, Intel Open Source Graphics Center