[Intel-gfx] [RFC v2] drm/i915: Engine discovery query

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-04-11 10:46:31)
> +/**
> + * struct drm_i915_engine_info
> + *
> + * Describes one engine known to the driver, whether or not is physical engine
> + * only, or it can also be targetted from userspace, and what are its
> + * capabilities where applicable.
> + */
> +struct drm_i915_engine_info {
> +       /** Engine flags. */
> +       __u64 flags;
> +#define I915_ENGINE_FLAG_PHYSICAL      (1 << 0)
> +#define I915_ENGINE_FLAG_ABI           (1 << 1)
> +
> +       /** Engine class as in enum drm_i915_gem_engine_class. */
> +       __u16 class;
> +
> +       /** Engine instance number. */
> +       __u16 instance;
> +
> +       __u32 rsvd0;
> +
> +       /** Capabilities of this engine. */
> +       __u64 capabilities;
> +#define I915_VCS_CLASS_CAPABILITY_HEVC (1 << 0)
> +
> +       __u64 rsvd1[2];
> +};
> +
> +/**
> + * struct drm_i915_query_engine_info
> + *
> + * Engine info query enumerates all the engines known to the driver returning
> + * the array of struct drm_i915_engine_info structures.
> + */
> +struct drm_i915_query_engine_info {
> +       /** Number of struct drm_i915_engine_info structs following. */
> +       __u32 num_engines;
> +
> +       /** MBZ */
> +       __u32 rsvd[3];

Ok, I can understand having some reserved fields before the [], but
within struct drm_i915_engine_info isn't the size and layout determined
by flags? (Thinking about the tight protocol packing of the 80s and
slightly less tight packing for the 90s ;) Trade-off between ease of
decoder and running out of space?
-Chris