[Intel-gfx] [PATCH v3 3/4] drm/i915: Skip CURBASE write when nothing changed

[Chris Wilson]

On Mon, Sep 15, 2014 at 04:36:38PM +0300, Ville Syrjälä wrote:
> On Sat, Sep 13, 2014 at 05:20:10PM +0100, Chris Wilson wrote:
> > On Fri, Sep 12, 2014 at 08:53:34PM +0300, ville.syrjala at linux.intel.com wrote:
> > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > 
> > > Only write CURBASE when something about the cursor changed. Also
> > > eliminate the unnecessary posting read after writing CURCNTR.
> > > 
> > > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_display.c | 10 ++++++----
> > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > > index 82c0ad1..60c1aa4 100644
> > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > @@ -8322,16 +8322,18 @@ static void i9xx_update_cursor(struct drm_crtc *crtc, u32 base)
> > >  			cntl |= CURSOR_PIPE_CSC_ENABLE;
> > >  	}
> > >  
> > > -	if (intel_crtc->cursor_cntl != cntl) {
> > > +	if (intel_crtc->cursor_cntl != cntl)
> > >  		I915_WRITE(CURCNTR(pipe), cntl);
> > > -		POSTING_READ(CURCNTR(pipe));
> > > -		intel_crtc->cursor_cntl = cntl;
> > > -	}
> > > +
> > > +	if (intel_crtc->cursor_cntl == cntl &&
> > > +	    intel_crtc->cursor_base == base)
> > > +		return;
> > 
> > I'd vote for doing this first and then
> > 
> > I915_WRITE(CURCNTR(pipe), cntl);
> > 
> > unconditionally along with the CURBASE flush.
> 
> So you want to write all the cursor registers unconditionally when
> anything changes? Would work for CURCNTR, but it would also make
> CUR_FBC_CTL stand out like a sore thumb since it can't be written
> uncoditionally.

Alternatively, something like

	bool dirty = intel_crtc->cursor_base != base;

	if (intel_crtc->cursor_cntl != cntl) {
		I915_WRITE(CURCNTR(pipe), cntl);
		dirty = true;
	}

	/* ... */

	if (!dirty)
		return;

	I915_WRITE(CURBASE(pipe), base);
	POSTING_READ(CURBASE(pipe));

	intel_crtc->cursor_cntl = cntl;
	intel_crtc->cursor_base = base;

I think would have clearer control flow.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre