Re: [Intel-gfx] [PATCH] drm/i915/gt: Retry RING_HEAD reset until it sticks

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 23/12/2022 12:18, Andi Shyti wrote:
> From: Chris Wilson <chris@chris-wilson.co.uk>
> 
> On Haswell, in particular, we see an issue where resets fails because
> the engine resumes from an incorrect RING_HEAD. Since the RING_HEAD
> doesn't point to the remaining requests to re-run, but may instead point
> into the uninitialised portion of the ring, the GPU may be then fed
> invalid instructions from a privileged context, often pushing the GPU
> into an unrecoverable hang.
> 
> If at first the write doesn't succeed, try, try again.
> 
> References: https://gitlab.freedesktop.org/drm/intel/-/issues/5432
> References: https://gitlab.freedesktop.org/drm/intel/-/issues/3303
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Andrzej Hajda <andrzej.hajda@intel.com>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
> ---
>   .../gpu/drm/i915/gt/intel_ring_submission.c   | 44 +++++++++++++------
>   drivers/gpu/drm/i915/i915_utils.h             |  8 ++++
>   2 files changed, 38 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_ring_submission.c b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> index 827adb0cfaea6..cdf283f5b1427 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> @@ -192,6 +192,7 @@ static bool stop_ring(struct intel_engine_cs *engine)
>   static int xcs_resume(struct intel_engine_cs *engine)
>   {
>   	struct intel_ring *ring = engine->legacy.ring;
> +	ktime_t kt;
>   
>   	ENGINE_TRACE(engine, "ring:{HEAD:%04x, TAIL:%04x}\n",
>   		     ring->head, ring->tail);
> @@ -230,9 +231,20 @@ static int xcs_resume(struct intel_engine_cs *engine)
>   	set_pp_dir(engine);
>   
>   	/* First wake the ring up to an empty/idle ring */
> -	ENGINE_WRITE_FW(engine, RING_HEAD, ring->head);
> +	until_timeout_ns(kt, 2 * NSEC_PER_MSEC) {
> +		ENGINE_WRITE_FW(engine, RING_HEAD, ring->head);
> +		if (ENGINE_READ_FW(engine, RING_HEAD) == ring->head)
> +			break;
> +	}

2ms?! Shudder..

#define done \
({ \
	ENGINE_WRITE_FW(engine, RING_HEAD, ring->head); \
	ENGINE_READ_FW(engine, RING_HEAD) == ring->head; \
})
_wait_for_atomic(done, 2 * USEC_PER_MSEC, needs_to_be_atomic_or_not?);
#undef done

Should work and avoid the need to add yet another helper, please 
double-check. Not as pretty, but accumulating generic sounding helpers 
in i915_utils.h is a bit frowned upon.

Regards,

Tvrtko

> +
>   	ENGINE_WRITE_FW(engine, RING_TAIL, ring->head);
> -	ENGINE_POSTING_READ(engine, RING_TAIL);
> +	if (ENGINE_READ_FW(engine, RING_HEAD) != ENGINE_READ_FW(engine, RING_TAIL)) {
> +		ENGINE_TRACE(engine, "failed to reset empty ring: [%x, %x]: %x\n",
> +			     ENGINE_READ_FW(engine, RING_HEAD),
> +			     ENGINE_READ_FW(engine, RING_TAIL),
> +			     ring->head);
> +		goto err;
> +	}
>   
>   	ENGINE_WRITE_FW(engine, RING_CTL,
>   			RING_CTL_SIZE(ring->size) | RING_VALID);
> @@ -241,12 +253,16 @@ static int xcs_resume(struct intel_engine_cs *engine)
>   	if (__intel_wait_for_register_fw(engine->uncore,
>   					 RING_CTL(engine->mmio_base),
>   					 RING_VALID, RING_VALID,
> -					 5000, 0, NULL))
> +					 5000, 0, NULL)) {
> +		ENGINE_TRACE(engine, "failed to restart\n");
>   		goto err;
> +	}
>   
> -	if (GRAPHICS_VER(engine->i915) > 2)
> +	if (GRAPHICS_VER(engine->i915) > 2) {
>   		ENGINE_WRITE_FW(engine,
>   				RING_MI_MODE, _MASKED_BIT_DISABLE(STOP_RING));
> +		ENGINE_POSTING_READ(engine, RING_MI_MODE);
> +	}
>   
>   	/* Now awake, let it get started */
>   	if (ring->tail != ring->head) {
> @@ -259,16 +275,16 @@ static int xcs_resume(struct intel_engine_cs *engine)
>   	return 0;
>   
>   err:
> -	drm_err(&engine->i915->drm,
> -		"%s initialization failed; "
> -		"ctl %08x (valid? %d) head %08x [%08x] tail %08x [%08x] start %08x [expected %08x]\n",
> -		engine->name,
> -		ENGINE_READ(engine, RING_CTL),
> -		ENGINE_READ(engine, RING_CTL) & RING_VALID,
> -		ENGINE_READ(engine, RING_HEAD), ring->head,
> -		ENGINE_READ(engine, RING_TAIL), ring->tail,
> -		ENGINE_READ(engine, RING_START),
> -		i915_ggtt_offset(ring->vma));
> +	ENGINE_TRACE(engine,
> +		     "initialization failed; "
> +		     "ctl %08x (valid? %d) head %08x [%08x] tail %08x [%08x] start %08x [expected %08x]\n",
> +		     ENGINE_READ(engine, RING_CTL),
> +		     ENGINE_READ(engine, RING_CTL) & RING_VALID,
> +		     ENGINE_READ(engine, RING_HEAD), ring->head,
> +		     ENGINE_READ(engine, RING_TAIL), ring->tail,
> +		     ENGINE_READ(engine, RING_START),
> +		     i915_ggtt_offset(ring->vma));
> +	GEM_TRACE_DUMP();
>   	return -EIO;
>   }
>   
> diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
> index b64192d9c7daa..f24a25c0685e1 100644
> --- a/drivers/gpu/drm/i915/i915_utils.h
> +++ b/drivers/gpu/drm/i915/i915_utils.h
> @@ -254,6 +254,14 @@ wait_remaining_ms_from_jiffies(unsigned long timestamp_jiffies, int to_wait_ms)
>   	}
>   }
>   
> +/**
> + * until_timeout_ns - Keep retrying (busy spin) until the duration has passed
> + */
> +#define until_timeout_ns(end, timeout_ns) \
> +	for ((end) = ktime_get() + (timeout_ns); \
> +	     ktime_before(ktime_get(), (end)); \
> +	     cpu_relax())
> + >   /**
>    * __wait_for - magic wait macro
>    *