Re: [PATCH] drm/i915/hsw: Flush RING_IMR changes before changing the global GT IMR (vecs)

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Haswell also requires the RING_IMR flush for its unique vebox setup to
> avoid losing interrupts, as per 476af9c26063 ("drm/i915/gen6: Flush
> RING_IMR changes before changing the global GT IMR"):
>
> On Baytail, notably, we can still detect missed interrupt syndrome
> (where we never spot a completed request). In this case, it can be
> alleviated by always keeping the interrupt unmasked, implying that the
> interrupt is being lost in the window after modifying the IMR. (This is
> the reason we still have the posting reads on enable_irq, if we remove
> them we miss interrupts!) Having narrowed the issue down to the IMR,
> rather than keeping it always enabled, applying the usual posting
> read/flush of the RING_IMR before unmasking the GT IMR also seems to
> prevent the missed interrupt. So be it.
>
> References: 476af9c26063 ("drm/i915/gen6: Flush RING_IMR changes before changing the global GT IMR")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 3d5d6b908148..6e2661e86d18 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -996,6 +996,10 @@ hsw_vebox_irq_enable(struct intel_engine_cs *engine)
>  	struct drm_i915_private *dev_priv = engine->i915;
>  
>  	I915_WRITE_IMR(engine, ~engine->irq_enable_mask);
> +
> +	/* Flush/delay to ensure the RING_IMR is active before the GT IMR */
> +	POSTING_READ_FW(RING_IMR(engine->mmio_base));
> +
>  	gen6_unmask_pm_irq(dev_priv, engine->irq_enable_mask);
>  }
>  
> -- 
> 2.20.1
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