Re: [PATCH v3 1/3] drm/i915: simplify allocation of driver-internal requests

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Dave Gordon <david.s.gordon@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH v3 1/3] drm/i915: simplify allocation of driver-internal requests
Date: Thu, 7 Jan 2016 16:56:39 +0000	[thread overview]
Message-ID: <20160107165639.GS652@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <568E5B5F.8020109@intel.com>

On Thu, Jan 07, 2016 at 12:34:39PM +0000, Dave Gordon wrote:
> On 07/01/16 11:58, Chris Wilson wrote:
> >On Thu, Jan 07, 2016 at 10:20:50AM +0000, Dave Gordon wrote:
> >>There are a number of places where the driver needs a request, but isn't
> >>working on behalf of any specific user or in a specific context. At
> >>present, we associate them with the per-engine default context. A future
> >>patch will abolish those per-engine context pointers; but we can already
> >>eliminate a lot of the references to them, just by making the allocator
> >>allow NULL as a shorthand for "an appropriate context for this ring",
> >>which will mean that the callers don't need to know anything about how
> >>the "appropriate context" is found (e.g. per-ring vs per-device, etc).
> >>
> >>So this patch renames the existing i915_gem_request_alloc(), and makes
> >>it local (static inline), and replaces it with a wrapper that provides
> >>a default if the context is NULL, and also has a nicer calling
> >>convention (doesn't require a pointer to an output parameter). Then we
> >>change all callers to use the new convention:
> >>OLD:
> >>	err = i915_gem_request_alloc(ring, user_ctx, &req);
> >>	if (err) ...
> >>NEW:
> >>	req = i915_gem_request_alloc(ring, user_ctx);
> >>	if (IS_ERR(req)) ...
> >>OLD:
> >>	err = i915_gem_request_alloc(ring, ring->default_context, &req);
> >>	if (err) ...
> >>NEW:
> >>	req = i915_gem_request_alloc(ring, NULL);
> >>	if (IS_ERR(req)) ...
> >
> >Nak. You haven't fixed i915_gem_request_alloc() at all.
> >
> >http://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=breadcrumbs&id=82c72e1a2b4385f0ab07dccee45acef38303e96f
> >is the patch I have been carrying ever since.
> >-Chris
> 
> I think you'll find that the version of i915_gem_request_alloc()
> I've implemented is equivalent to yours, with the *additional* (and
> better) semantic of not requiring the caller to specify
> (ring->default_param) as the context parameter (which is the main
> point, as far as I'm concerned; making the calling convention nicer
> was just incidental).

No. Specifying the context is crucial to request allocation. The issue
in the current function call chains are the requests appear out of
nowhere rather than being created with explicit context.

Hiding the context is bad.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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