Re: [PATCH 04/13] drm/i915: Fail engine initialization if LRCA is incorrectly aligned

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Jan 08, 2016 at 11:29:43AM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> LRCA can change only when it goes from unpinned to pinned so it
> makes sense to check its alignment at that point rather than at
> every batch buffer submission.
> 
> Furthermore, if we check it at pin time we can actually
> gracefuly fail the engine initialization rather than just
> spamming the logs at runtime with WARNs.
> 
> v2: Return ENODEV for bad alignment. (Chris Wilson)
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 84977a6e6f3f..ff146a15d395 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -302,8 +302,6 @@ uint64_t intel_lr_context_descriptor(struct intel_context *ctx,
>  	uint64_t lrca = i915_gem_obj_ggtt_offset(ctx_obj) +
>  			LRC_PPHWSP_PN * PAGE_SIZE;
>  
> -	WARN_ON(lrca & 0xFFFFFFFF00000FFFULL);
> -
>  	desc |= lrca;
>  	desc |= (u64)intel_execlists_ctx_id(ctx_obj) << GEN8_CTX_ID_SHIFT;
>  
> @@ -1030,6 +1028,7 @@ static int intel_lr_context_do_pin(struct intel_engine_cs *ring,
>  {
>  	struct drm_device *dev = ring->dev;
>  	struct drm_i915_private *dev_priv = dev->dev_private;
> +	u64 lrca;
>  	int ret = 0;
>  
>  	WARN_ON(!mutex_is_locked(&ring->dev->struct_mutex));
> @@ -1038,6 +1037,12 @@ static int intel_lr_context_do_pin(struct intel_engine_cs *ring,
>  	if (ret)
>  		return ret;
>  
> +	lrca = i915_gem_obj_ggtt_offset(ctx_obj) + LRC_PPHWSP_PN * PAGE_SIZE;
> +	if (WARN_ON(lrca & 0xFFFFFFFF00000FFFULL)) {

Essentially this checks that it's page-aligned (which is a fundamental
assumption of how we place objects we depend upon everywhere) and that it
fits within the 4G hw limit of the global gtt (again we assume our code is
correct that way). tbh I'd just drop entirely, it's a useless check.
-Daniel

> +		ret = -ENODEV;
> +		goto unpin_ctx_obj;
> +	}
> +
>  	ret = intel_pin_and_map_ringbuffer_obj(ring->dev, ringbuf);
>  	if (ret)
>  		goto unpin_ctx_obj;
> -- 
> 1.9.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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