Re: [PATCH] drm/i915: Only mark the ctx as initialised after a SET_CONTEXT operation

[Ben Widawsky <ben@bwidawsk.net>]

On Fri, May 30, 2014 at 02:16:30PM +0100, Chris Wilson wrote:
> Fallout from
> 
> commit 46470fc932ac8a0e8317a220b3f4ea4ed903338e
> Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> Date:   Wed May 21 19:01:06 2014 +0300
> 
>     drm/i915: Add null state batch to active list
> 
> undid the earlier fix of only marking the ctx as initialised after it is
> saved by the hardware during a SET_CONTEXT operation.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Damien Lespiau <damien.lespiau@intel.com>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> Cc: Ben Widawsky <ben@bwidawsk.net>
> ---
>  drivers/gpu/drm/i915/i915_gem_context.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index 5a71ef1975b3..34a0b49e6add 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -641,6 +641,7 @@ static int do_switch(struct intel_engine_cs *ring,
>  	struct intel_context *from = ring->last_context;
>  	struct i915_hw_ppgtt *ppgtt = ctx_to_ppgtt(to);
>  	u32 hw_flags = 0;
> +	bool uninitialized = false;
>  	int ret, i;
>  
>  	if (from != NULL && ring == &dev_priv->ring[RCS]) {
> @@ -739,18 +740,19 @@ static int do_switch(struct intel_engine_cs *ring,
>  		i915_gem_context_unreference(from);
>  	}
>  
> +	uninitialized = !to->is_initialized && from == NULL;
> +	to->is_initialized = true;

Aren't you missing some error paths if you do this?  I think we need to
set the state after we've successfully emitted the MI_SET_CONTEXT
command (though really, until we move the ringbuffer tail, it's all
lies, on that note, I'm scared to look at if reset dtrt).

> +
>  done:
>  	i915_gem_context_reference(to);
>  	ring->last_context = to;
>  
> -	if (ring->id == RCS && !to->is_initialized && from == NULL) {
> +	if (uninitialized) {
>  		ret = i915_gem_render_state_init(ring);
>  		if (ret)
>  			DRM_ERROR("init render state: %d\n", ret);
>  	}
>  
> -	to->is_initialized = true;
> -
>  	return 0;
>  
>  unpin_out:

I just realized the from == NULL check is something which seems fragile
to me given how much churn we might see with init/reset paths. It'd
probably be good to eventually switch to some global state in dev_priv
which we can track across init/reset/fini

Otherwise it looks correct to me.

-- 
Ben Widawsky, Intel Open Source Technology Center
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