[Intel-gfx] [REPOST] [PATCH] drm/i915/ppgtt: Load address space after mi_set_context

[Ville Syrjälä]

On Thu, Jun 12, 2014 at 08:25:52AM -0700, Ben Widawsky wrote:
> On GEN8 the PDPs are saved and restored with context, which means we
> must set them after the context switch has occurred. If we do not do
> this, we end up saving the new PDPs for the old context.
> 
> Example of a problem
> LRI PDPs 1
> MI_SET_CONTEXT bar
> LRI_PDPs 2
> MI_SET_CONTEXT foo // save PDPs 2 to bar's context
> 		  //  load foos PDPs
> LRI PDPs 1
> MI_SET_CONTEXT bar // save PDPs 1 to foo's context
> 
> It's all wacky. This should allow full PPGTT on Broadwell to work.

Hmm. I had this impression too but now I'm not sure. The PDPs are listed
as being in the execlist context. Do we save/restore that in ring buffer
mode too? IIRC on ivb/hsw it got skipped.

And if the PDPs are really part of the context we shouldn't need to
load them at all I think, except when we skip the restore (unitialized
or default context). Or am I missing something here?

And what about ivb/hsw? Doesn't this reordering risk the context restore
accessing the wrong PPGTT. That's assuming the context restore can
already chase some pointers.

> 
> Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
> ---
>  drivers/gpu/drm/i915/i915_gem_context.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index 3ffe308..b2434e0 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -623,13 +623,12 @@ static int do_switch(struct intel_engine_cs *ring,
>  	 */
>  	from = ring->last_context;
>  
> -	if (USES_FULL_PPGTT(ring->dev)) {
> -		ret = ppgtt->switch_mm(ppgtt, ring, false);
> -		if (ret)
> -			goto unpin_out;
> -	}
> -
>  	if (ring != &dev_priv->ring[RCS]) {
> +		if (USES_FULL_PPGTT(ring->dev)) {
> +			ret = ppgtt->switch_mm(ppgtt, ring, false);
> +			if (ret)
> +				goto unpin_out;
> +		}
>  		if (from)
>  			i915_gem_context_unreference(from);
>  		goto done;
> @@ -660,6 +659,19 @@ static int do_switch(struct intel_engine_cs *ring,
>  	if (ret)
>  		goto unpin_out;
>  
> +	if (USES_FULL_PPGTT(ring->dev)) {
> +		ret = ppgtt->switch_mm(ppgtt, ring, false);
> +		/* The hardware context switch is emitted, but we haven't
> +		 * actually changed the state - so it's probably safe to bail
> +		 * here. Still, let the user know something dangerous has
> +		 * happened.
> +		 */
> +		if (ret) {
> +			DRM_ERROR("Failed to change address space on context switch\n");
> +			goto unpin_out;
> +		}
> +	}
> +
>  	for (i = 0; i < MAX_L3_SLICES; i++) {
>  		if (!(to->remap_slice & (1<<i)))
>  			continue;
> -- 
> 2.0.0
> 
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-- 
Ville Syrjälä
Intel OTC