[PATCH] drm/i915: stop using GMBUS IRQs on Gen4 chips (was Re: [3.9-rc1] irq 16: nobody cared (was [3.9-rc1] very poor interrupt responses))

[Chris Wilson]

On Tue, Mar 19, 2013 at 09:56:57AM +0100, Jiri Kosina wrote:
> On Mon, 18 Mar 2013, Chris Wilson wrote:
> 
> > > +#define HAS_GMBUS_IRQ(dev) (INTEL_INFO(dev)->gen >= 5)
> > >  void
> > >  intel_i2c_reset(struct drm_device *dev)
> > >  {
> > >  	struct drm_i915_private *dev_priv = dev->dev_private;
> > >  	I915_WRITE(dev_priv->gpio_mmio_base + GMBUS0, 0);
> > > -	I915_WRITE(dev_priv->gpio_mmio_base + GMBUS4, 0);
> > > +	if (HAS_GMBUS_IRQ(dev))
> > > +		I915_WRITE(dev_priv->gpio_mmio_base + GMBUS4, 0);
> > 
> > There should not be any harm in always clearing GMBUS4, it exists on all
> > platforms.
> > 
> > >  }
> > >  
> > >  static void intel_i2c_quirk_set(struct drm_i915_private *dev_priv, bool enable)
> > > @@ -203,7 +205,6 @@ intel_gpio_setup(struct intel_gmbus *bus, u32 pin)
> > >  	algo->data = bus;
> > >  }
> > >  
> > > -#define HAS_GMBUS_IRQ(dev) (INTEL_INFO(dev)->gen >= 4)
> > >  static int
> > >  gmbus_wait_hw_status(struct drm_i915_private *dev_priv,
> > >  		     u32 gmbus2_status,
> > > @@ -214,6 +215,13 @@ gmbus_wait_hw_status(struct drm_i915_private *dev_priv,
> > >  	u32 gmbus2 = 0;
> > >  	DEFINE_WAIT(wait);
> > >  
> > > +	if (!HAS_GMBUS_IRQ(dev_priv->dev)) {
> > > +		int ret;
> > > +		ret = wait_for((gmbus2 = I915_READ(GMBUS2 + reg_offset)) &
> > > +				(GMBUS_SATOER | gmbus2_status),
> > > +				50);
> > 
> > This should couple up to the normal return code that chooses the
> > appropriate return value based on gmbus2.
> > 
> > How about just using:
> >   if (!HAS_GMBUS_IRQ(dev_priv->dev)) gmbus4_irq_en = 0;
> > and the existing wait loop?
> 
> I explicitly wanted to avoid touching GMBUS4 register, as the real cause 
> of the failure is not clear.
> 
> But, as Yinghai Lu points out, the problem is most likely caused by 
> interrupt disabling not working properly (see his very good point 
> regarding DisINTx+ and INTx+ discrepancy), so zeroing the register out 
> should work .... and it indeed does in my case, hence the (tested) patch 
> below.
> 
> I think it's a 3.9-rc material, and I am all open to debug this further 
> for 3.10 so that the race is closed and gmbus irqs can be used on Gen4 
> platform properly.

Agreed. Using the IRQ for GMBUS is just a performance feature that can
be deferred until after we determine the root cause - and hope that the
failure is somehow peculiar to GMBUS.

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

-- 
Chris Wilson, Intel Open Source Technology Centre