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

[Chris Wilson]

On Tue, Apr 18, 2017 at 05:56:15PM +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);

To save a bit of space, we can use the ring selector as a class selector
if bit18 is set, with 19-27 as instance. That limits us to 64 classes -
hopefully not a problem for near future. At least I might have you sold
you on a flexible execbuf3 by then.

(As a digression, some cryptic notes for an implementation I did over Easter:
/*
 * Execbuf3!
 *
 * ringbuffer 
 *  - per context
 *  - per engine
 *  - PAGE_SIZE ctl [ro head, rw tai] + user pot
 *  - kthread [i915/$ctx-$engine] (optional?)
 *  - assumes NO_RELOC-esque awareness
 *
 * SYNC flags [wait/signal], handle [semaphore/fence]
 *
 * BIND handle, offset [user provided]
 * ALLOC[32,64] handle, flags, *offset [kernel provided, need RELOC]
 * RELOC[32,64] handle, target_handle, offset, delta
 * CLEAR flags, handle
 * UNBIND handle
 *
 * BATCH flags, handle, offset
 * [or SVM flags, address]
 *   PIN flags (MAY_RELOC), count, handle[count]
 *   FENCE flags, count, handle[count]
 * SUBMIT handle [fence/NULL with error]
 */

At the moment it is just trying to do execbuf2, but more compactly and
with fewer ioctls. But one of the main selling points is that we can
extend the information passed around more freely than execbuf2.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre