[Intel-gfx] [PATCH] drm/i915: Decouple execbuf uAPI from internal implementation

[Chris Wilson]

On Thu, Jan 14, 2016 at 03:02:59PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> At the moment execbuf ring selection is fully coupled to
> internal ring ids which is not a good thing on its own.
> 
> This dependency is also spread between two source files and
> not spelled out at either side which makes it hidden and
> fragile.
> 
> This patch decouples this dependency by introducing an explicit
> translation table of execbuf uAPI to ring id close to the only
> call site (i915_gem_do_execbuffer).
> 
> This way we are free to change driver internal implementation
> details without breaking userspace. All state relating to the
> uAPI is now contained in, or next to, i915_gem_do_execbuffer.

I was hesistant at first, since "why?" isn't fully developed, but the
code won me over.

> +#define I915_USER_RINGS (4)
> +
> +static const enum intel_ring_id user_ring_map[I915_USER_RINGS + 1] = {
> +	[I915_EXEC_DEFAULT]	= RCS,
> +	[I915_EXEC_RENDER]	= RCS,
> +	[I915_EXEC_BLT]		= BCS,
> +	[I915_EXEC_BSD]		= VCS,
> +	[I915_EXEC_VEBOX]	= VECS
> +};

I was wondering whether packing here mattered at all, but we're under a
cacheline so highly unlikely.

>  static int
>  i915_gem_do_execbuffer(struct drm_device *dev, void *data,
>  		       struct drm_file *file,
> @@ -1393,6 +1393,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
>  	struct i915_execbuffer_params params_master; /* XXX: will be removed later */
>  	struct i915_execbuffer_params *params = &params_master;
>  	const u32 ctx_id = i915_execbuffer2_get_context_id(*args);
> +	unsigned int user_ring_id = args->flags & I915_EXEC_RING_MASK;
>  	u32 dispatch_flags;
>  	int ret;
>  	bool need_relocs;
> @@ -1414,49 +1415,39 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
>  	if (args->flags & I915_EXEC_IS_PINNED)
>  		dispatch_flags |= I915_DISPATCH_PINNED;
>  
> -	if ((args->flags & I915_EXEC_RING_MASK) > LAST_USER_RING) {
> -		DRM_DEBUG("execbuf with unknown ring: %d\n",
> -			  (int)(args->flags & I915_EXEC_RING_MASK));
> +	if (user_ring_id > I915_USER_RINGS) {
> +		DRM_DEBUG("execbuf with unknown ring: %u\n", user_ring_id);
>  		return -EINVAL;
>  	}
>  
> -	if (((args->flags & I915_EXEC_RING_MASK) != I915_EXEC_BSD) &&
> +	if ((user_ring_id != I915_EXEC_BSD) &&
>  	    ((args->flags & I915_EXEC_BSD_MASK) != 0)) {
>  		DRM_DEBUG("execbuf with non bsd ring but with invalid "
>  			"bsd dispatch flags: %d\n", (int)(args->flags));
>  		return -EINVAL;
> -	} 
> -
> -	if ((args->flags & I915_EXEC_RING_MASK) == I915_EXEC_DEFAULT)
> -		ring = &dev_priv->ring[RCS];
> -	else if ((args->flags & I915_EXEC_RING_MASK) == I915_EXEC_BSD) {
> -		if (HAS_BSD2(dev)) {
> -			int ring_id;
> -
> -			switch (args->flags & I915_EXEC_BSD_MASK) {
> -			case I915_EXEC_BSD_DEFAULT:
> -				ring_id = gen8_dispatch_bsd_ring(dev, file);
> -				ring = &dev_priv->ring[ring_id];
> -				break;
> -			case I915_EXEC_BSD_RING1:
> -				ring = &dev_priv->ring[VCS];
> -				break;
> -			case I915_EXEC_BSD_RING2:
> -				ring = &dev_priv->ring[VCS2];
> -				break;
> -			default:
> -				DRM_DEBUG("execbuf with unknown bsd ring: %d\n",
> -					  (int)(args->flags & I915_EXEC_BSD_MASK));
> -				return -EINVAL;
> -			}
> -		} else
> -			ring = &dev_priv->ring[VCS];
> -	} else
> -		ring = &dev_priv->ring[(args->flags & I915_EXEC_RING_MASK) - 1];
> +	}
> +
> +	if (user_ring_id == I915_EXEC_BSD && HAS_BSD2(dev)) {

HAS_BSD2(dev_priv)

> +		unsigned int bsd_idx = args->flags & I915_EXEC_BSD_MASK;
> +
> +		if (bsd_idx == I915_EXEC_BSD_DEFAULT) {
> +			bsd_idx = gen8_dispatch_bsd_ring(dev_priv, file);
> +		} else if (bsd_idx >= I915_EXEC_BSD_RING1 &&
> +			   bsd_idx <= I915_EXEC_BSD_RING2) {
> +			bsd_idx--;
> +		} else {
> +			DRM_DEBUG("execbuf with unknown bsd ring: %u\n",
> +				  bsd_idx);
> +			return -EINVAL;
> +		}
> +
> +		ring = &dev_priv->ring[_VCS(bsd_idx)];
> +	} else {
> +		ring = &dev_priv->ring[user_ring_map[user_ring_id]];
> +	}
>  
>  	if (!intel_ring_initialized(ring)) {
> -		DRM_DEBUG("execbuf with invalid ring: %d\n",
> -			  (int)(args->flags & I915_EXEC_RING_MASK));
> +		DRM_DEBUG("execbuf with invalid ring: %u\n", user_ring_id);
>  		return -EINVAL;
>  	}

One question, how does this look if we move this section to its own
function:

ring = eb_select_ring(dev_priv, file, args);
if (IS_ERR(ring))
	return PTR_ERR(ring);

Do you keep your code size reduction?

> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index 7349d9258191..fdc220fc2b18 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -148,14 +148,14 @@ struct  i915_ctx_workarounds {
>  struct  intel_engine_cs {
>  	const char	*name;
>  	enum intel_ring_id {
> -		RCS = 0x0,
> -		VCS,
> +		RCS = 0,
>  		BCS,
> -		VECS,
> -		VCS2
> +		VCS,
> +		VCS2,	/* Keep instances of the same type engine together. */
> +		VECS

Technically, this breaks userspace ABI elsewhere. :| Though the only
user of the id mask is only looking for RCS==0 vs the reset.

So I think we would cope, but to be extra safe we could just avoid
reshuffling the ids. Let me sleep on the implications. We may say just
break that ABI whilst we still can and do a reverse-map to EXEC bit.
Hmm.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre