Re: [PATCH] drm/i915: GMBUS don't need no forcewake

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

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@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@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@chris-wilson.co.uk>
> > Cc: David Weinehall <david.weinehall@linux.intel.com>
> > Signed-off-by: Ville Syrjälä <ville.syrjala@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.

-- 
Ville Syrjälä
Intel OTC
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