Re: [Intel-gfx] [PATCH 3/3] drm/i915: Disable PMSI sleep messages on all rings around context switches

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Dec 11, 2014 at 08:17:01AM +0000, Chris Wilson wrote:
> There exists a current workaround to prevent a hang on context switch
> should the ring go to sleep in the middle of the restore,
> WaProgramMiArbOnOffAroundMiSetContext (applicable to all gen7+). In
> spite of disabling arbitration (which prevents the ring from powering
> down during the critical section) we were still hitting hangs that had
> the hallmarks of the known erratum. That is we are still seeing hangs
> "on the last instruction in the context restore". By comparing -nightly
> (broken) with requests (working), we were able to deduce that it was the
> semaphore LRI cross-talk that reproduced the original failure.
> Explicitly disabling PMSI sleep on the RCS ring was insufficient, all
> the rings had to be awake to prevent the hangs. Fortunately, we can
> reduce the wakelock to the MI_SET_CONTEXT operation itself, and so
> should be able to limit the extra power implications.
> 
> Since the MI_ARB_ON_OFF workaround is listed for all gen7 and above
> products, we should apply this extra hammer for all of the same
> platforms despite so far that we have only been able to reproduce the
> hang on certain ivb and hsw models. The last question is whether we want
> to always use the extra hammer or only when we know semaphores are in
> operation. At the moment, we only use LRI on non-RCS rings for
> semaphores, but that may change in the future with the possibility of
> reintroducing this bug under subtle conditions.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80660
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83677
> Cc: Simon Farnsworth <simon@farnz.org.uk>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: stable@vger.kernel.org

Oh dear, but awesome piece of debugging!

First question: How does this work on vlv with the split forcewake domain?
Any bad side effects like longer ctx switch times?

> ---
>  drivers/gpu/drm/i915/i915_gem_context.c | 39 ++++++++++++++++++++++++---------
>  drivers/gpu/drm/i915/i915_reg.h         |  2 ++
>  2 files changed, 31 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index 2acf5803cf32..724ccecea06a 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -484,7 +484,9 @@ mi_set_context(struct intel_engine_cs *ring,
>  	       u32 hw_flags)
>  {
>  	u32 flags = hw_flags | MI_MM_SPACE_GTT;
> -	int ret;
> +	struct intel_engine_cs *engine;
> +	const int num_rings = i915_semaphore_is_enabled(ring->dev) ? hweight32(INTEL_INFO(ring->dev)->ring_mask) : 0;
> +	int len, i, ret;
>  
>  	/* w/a: If Flush TLB Invalidation Mode is enabled, driver must do a TLB
>  	 * invalidation prior to MI_SET_CONTEXT. On GEN6 we don't set the value
> @@ -501,15 +503,26 @@ mi_set_context(struct intel_engine_cs *ring,
>  	if (!IS_HASWELL(ring->dev) && INTEL_INFO(ring->dev)->gen < 8)
>  		flags |= (MI_SAVE_EXT_STATE_EN | MI_RESTORE_EXT_STATE_EN);
>  
> -	ret = intel_ring_begin(ring, 6);
> +
> +	len = 4;
> +	if (INTEL_INFO(ring->dev)->gen >= 7)
> +		len += 4*num_rings + 2;
> +
> +	ret = intel_ring_begin(ring, len);
>  	if (ret)
>  		return ret;
>  
>  	/* WaProgramMiArbOnOffAroundMiSetContext:ivb,vlv,hsw,bdw,chv */
> -	if (INTEL_INFO(ring->dev)->gen >= 7)
> -		intel_ring_emit(ring, MI_ARB_ON_OFF | MI_ARB_DISABLE);
> -	else
> -		intel_ring_emit(ring, MI_NOOP);
> +	if (INTEL_INFO(ring->dev)->gen >= 7) {
> +		if (num_rings) {
> +			intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(num_rings));
> +			for_each_ring(engine, to_i915(ring->dev), i) {
> +				intel_ring_emit(ring, RING_PSMI_CTL(engine->mmio_base));
> +				intel_ring_emit(ring, _MASKED_BIT_ENABLE(GEN6_PSMI_SLEEP_MSG_DISABLE));
> +			}
> +		} else
> +			intel_ring_emit(ring, MI_ARB_ON_OFF | MI_ARB_DISABLE);

For added paranoia, shouldn't we keep this one here? Or is the only thing
this does frob the idle notifications like PSMI? I kinda prefer we just
keep it if there's no harmful effects ...

Otherwise lgtm.
-Daniel

> +	}
>  
>  	intel_ring_emit(ring, MI_NOOP);
>  	intel_ring_emit(ring, MI_SET_CONTEXT);
> @@ -521,10 +534,16 @@ mi_set_context(struct intel_engine_cs *ring,
>  	 */
>  	intel_ring_emit(ring, MI_NOOP);
>  
> -	if (INTEL_INFO(ring->dev)->gen >= 7)
> -		intel_ring_emit(ring, MI_ARB_ON_OFF | MI_ARB_ENABLE);
> -	else
> -		intel_ring_emit(ring, MI_NOOP);
> +	if (INTEL_INFO(ring->dev)->gen >= 7) {
> +		if (num_rings) {
> +			intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(num_rings));
> +			for_each_ring(engine, to_i915(ring->dev), i) {
> +				intel_ring_emit(ring, RING_PSMI_CTL(engine->mmio_base));
> +				intel_ring_emit(ring, _MASKED_BIT_DISABLE(GEN6_PSMI_SLEEP_MSG_DISABLE));
> +			}
> +		} else
> +			intel_ring_emit(ring, MI_ARB_ON_OFF | MI_ARB_ENABLE);
> +	}
>  
>  	intel_ring_advance(ring);
>  
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 0ddef7256d02..1ce303cb8852 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -1134,6 +1134,7 @@ enum punit_power_well {
>  #define GEN6_VERSYNC	(RING_SYNC_1(VEBOX_RING_BASE))
>  #define GEN6_VEVSYNC	(RING_SYNC_2(VEBOX_RING_BASE))
>  #define GEN6_NOSYNC 0
> +#define RING_PSMI_CTL(base)	((base)+0x50)
>  #define RING_MAX_IDLE(base)	((base)+0x54)
>  #define RING_HWS_PGA(base)	((base)+0x80)
>  #define RING_HWS_PGA_GEN6(base)	((base)+0x2080)
> @@ -1464,6 +1465,7 @@ enum punit_power_well {
>  #define   GEN6_BLITTER_FBC_NOTIFY			(1<<3)
>  
>  #define GEN6_RC_SLEEP_PSMI_CONTROL	0x2050
> +#define   GEN6_PSMI_SLEEP_MSG_DISABLE	(1 << 0)
>  #define   GEN8_RC_SEMA_IDLE_MSG_DISABLE	(1 << 12)
>  #define   GEN8_FF_DOP_CLOCK_GATE_DISABLE	(1<<10)
>  
> -- 
> 2.1.3
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch