[Intel-gfx] [PATCH] drm/i915: GMBUS don't need no forcewake

[Chris Wilson]

On Wed, Oct 12, 2016 at 03:39:47PM +0300, Ville Syrjälä wrote:
> On Wed, Oct 12, 2016 at 12:58:34PM +0100, Chris Wilson wrote:
> > On Wed, Oct 12, 2016 at 02:44:47PM +0300, ville.syrjala at linux.intel.com wrote:
> > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > 
> > > GMBUS is part of the display engine, and thus has no need for
> > > forcewake. Let's not bother trying to grab it then.
> > > 
> > > I don't recall if the display engine suffers from system hangs
> > > due to multiple accesses to the same "cacheline" in mmio space.
> > > I hope not since we're no longer protected by the uncore lock
> > > since commit 4e6c2d58ba86 ("drm/i915: Take forcewake once for
> > > the entire GMBUS transaction")
> > 
> > Only applies to concurrent access to the same cacheline, in this case
> > should be serialised by the mutex around the gmbus xfer.
> 
> Hmm. Yeah, I suppose there shouldn't be unrelated stuff nearby. Haven't
> double checked though.
> 
> >  
> > > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: David Weinehall <david.weinehall at linux.intel.com>
> > > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_i2c.c | 5 -----
> > >  1 file changed, 5 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
> > > index 79aab9ad6faa..49c7824a4c29 100644
> > > --- a/drivers/gpu/drm/i915/intel_i2c.c
> > > +++ b/drivers/gpu/drm/i915/intel_i2c.c
> > > @@ -468,13 +468,9 @@ do_gmbus_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs, int num)
> > >  					       struct intel_gmbus,
> > >  					       adapter);
> > >  	struct drm_i915_private *dev_priv = bus->dev_priv;
> > > -	const unsigned int fw =
> > > -		intel_uncore_forcewake_for_reg(dev_priv, GMBUS0,
> > > -					       FW_REG_READ | FW_REG_WRITE);
> > >  	int i = 0, inc, try = 0;
> > >  	int ret = 0;
> > 
> > I915_WARN_ON(intel_uncore_forcewake_for_reg(dev_priv, GMBUS0,
> > 					    FW_REG_READ |
> > 					    FW_REG_WRITE));
> > 
> > ? Would be good to test the fw handling as well.
> 
> Not sure I'd want to sprinkle forcewake testing into modeset code.

You never use registers here? ;)

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre