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

[Shankar, Uma]

> -----Original Message-----
> From: Jani Nikula <jani.nikula at linux.intel.com>
> Sent: Tuesday, September 12, 2023 1:08 PM
> To: Shankar, Uma <uma.shankar at intel.com>; 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 Mon, 11 Sep 2023, "Shankar, Uma" <uma.shankar at intel.com> wrote:
> >> -----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.
> 
> That part is clear; the why isn't. :)

Yes I agree, to add the reasoning will be good 😊. Some justification is added though:
"intel_dsb_commit() in particular needs to be called from the vblank evade
critical section and thus needs to be fast".

> > 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
> 
> --
> Jani Nikula, Intel