[Mesa-dev] [RFC v2 2/2] drm/i915: Select engines via class and instance in execbuffer2

[Chris Wilson]

On Thu, Apr 27, 2017 at 10:10:34AM +0100, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> Building on top of the previous patch which exported the concept
> of engine classes and instances, we can also use this instead of
> the current awkward engine selection uAPI.
> 
> This is primarily interesting for the VCS engine selection which
> is a) currently done via disjoint set of flags, and b) the
> current I915_EXEC_BSD flags has different semantics depending on
> the underlying hardware which is bad.
> 
> Proposed idea here is to reserve 16-bits of flags, to pass in
> the engine class and instance (8 bits each), and a new flag
> named I915_EXEC_CLASS_INSTACE to tell the kernel this new engine
> selection API is in use.
> 
> The new uAPI also removes access to the weak VCS engine
> balancing as currently existing in the driver.
> 
> Example usage to send a command to VCS0:
> 
>   eb.flags = i915_execbuffer2_engine(DRM_I915_ENGINE_CLASS_VIDEO_DECODE, 0);
> 
> Or to send a command to VCS1:
> 
>   eb.flags = i915_execbuffer2_engine(DRM_I915_ENGINE_CLASS_VIDEO_DECODE, 1);
> 
> v2:
>  * Fix unknown flags mask.
>  * Use I915_EXEC_RING_MASK for class. (Chris Wilson)
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Ben Widawsky <ben at bwidawsk.net>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Jon Bloomfield <jon.bloomfield at intel.com>
> Cc: Daniel Charles <daniel.charles at intel.com>
> Cc: "Rogozhkin, Dmitry V" <dmitry.v.rogozhkin at intel.com>
> Cc: Oscar Mateo <oscar.mateo at intel.com>
> Cc: "Gong, Zhipeng" <zhipeng.gong at intel.com>
> Cc: intel-vaapi-media at lists.01.org
> Cc: mesa-dev at lists.freedesktop.org
> ---
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 29 +++++++++++++++++++++++++++++
>  include/uapi/drm/i915_drm.h                | 11 ++++++++++-
>  2 files changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index af1965774e7b..ecd1486642a7 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1492,6 +1492,32 @@ gen8_dispatch_bsd_engine(struct drm_i915_private *dev_priv,
>  	return file_priv->bsd_engine;
>  }
>  
> +extern u8 user_class_map[DRM_I915_ENGINE_CLASS_MAX];
> +
> +static struct intel_engine_cs *
> +eb_select_engine_class_instance(struct drm_i915_private *i915,
> +				struct drm_i915_gem_execbuffer2 *args)
> +{
> +	struct intel_engine_cs *engine;
> +	enum intel_engine_id id;
> +	u8 class, instance;
> +
> +	class = args->flags & I915_EXEC_RING_MASK;
> +	if (class >= DRM_I915_ENGINE_CLASS_MAX)
> +		return NULL;
> +	class = user_class_map[class];
> +
> +	instance = (args->flags >> I915_EXEC_INSTANCE_SHIFT) &&
> +		   I915_EXEC_INSTANCE_MASK;
> +
> +	for_each_engine(engine, i915, id) {
> +		if (engine->class == class && engine->instance == instance)
> +			return engine;
> +	}

I am underwhelmed. No, i915->class_engine[class][instance] ?

Still, at what point do we kill busy-ioctl per-engine reporting? Should
we update all tracepoints to use class:instance (I think that's a better
abi going forward).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre