Re: [PATCH 02/18] drm/i915/ringbuffer: Brute force context restore

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

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

> An issue encountered with switching mm on gen7 is that the GPU likes to
> hang (with the VS unit busy) when told to force restore the current
> context. We can simply workaround this by substituting the
> MI_FORCE_RESTORE flag with a round-trip through the kernel_context,
> forcing the context to be saved and restored; thereby reloading the
> PP_DIR registers and updating the modified page directory!
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> Cc: Matthew Auld <matthew.william.auld@gmail.com>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 30 ++++++++++++++++++++++---
>  1 file changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 65811e2fa7da..332d97bc5c27 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -1458,6 +1458,7 @@ static inline int mi_set_context(struct i915_request *rq, u32 flags)
>  		(HAS_LEGACY_SEMAPHORES(i915) && IS_GEN7(i915)) ?
>  		INTEL_INFO(i915)->num_rings - 1 :
>  		0;
> +	bool force_restore = false;
>  	int len;
>  	u32 *cs;
>  
> @@ -1471,6 +1472,12 @@ static inline int mi_set_context(struct i915_request *rq, u32 flags)
>  	len = 4;
>  	if (IS_GEN7(i915))
>  		len += 2 + (num_rings ? 4*num_rings + 6 : 0);
> +	if (flags & MI_FORCE_RESTORE) {
> +		GEM_BUG_ON(flags & MI_RESTORE_INHIBIT);
> +		flags &= ~MI_FORCE_RESTORE;
> +		force_restore = true;
> +		len += 2;
> +	}
>  
>  	cs = intel_ring_begin(rq, len);
>  	if (IS_ERR(cs))
> @@ -1496,6 +1503,20 @@ static inline int mi_set_context(struct i915_request *rq, u32 flags)
>  	}
>  
>  	*cs++ = MI_NOOP;
> +	if (force_restore) {
> +		/*
> +		 * The HW doesn't handle being told to restore the current
> +		 * context very well. Quite often it likes goes to go off and
> +		 * sulk, especially when it is meant to be reloading PP_DIR.
> +		 * A very simple fix to force the reload is to simply switch
> +		 * away from the current context and back again.
> +		 */
> +		*cs++ = MI_SET_CONTEXT;
> +		*cs++ = i915_ggtt_offset(to_intel_context(i915->kernel_context,
> +							  engine)->state) |
> +			MI_MM_SPACE_GTT |
> +			MI_RESTORE_INHIBIT;

Why inhibit? You dont really switch to kernel but rather overwrite
current with kernel ctx.

-Mika

> +	}
>  	*cs++ = MI_SET_CONTEXT;
>  	*cs++ = i915_ggtt_offset(rq->hw_context->state) | flags;
>  	/*
> @@ -1585,11 +1606,14 @@ static int switch_context(struct i915_request *rq)
>  
>  		to_mm->pd_dirty_rings &= ~intel_engine_flag(engine);
>  		engine->legacy_active_ppgtt = to_mm;
> -		hw_flags = MI_FORCE_RESTORE;
> +
> +		if (to_ctx == from_ctx) {
> +			hw_flags = MI_FORCE_RESTORE;
> +			from_ctx = NULL;
> +		}
>  	}
>  
> -	if (rq->hw_context->state &&
> -	    (to_ctx != from_ctx || hw_flags & MI_FORCE_RESTORE)) {
> +	if (rq->hw_context->state && to_ctx != from_ctx) {
>  		GEM_BUG_ON(engine->id != RCS);
>  
>  		/*
> -- 
> 2.17.1
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