Re: [PATCH v4 0/3] improve handling of the driver's default (kernel) context

From: Daniel Vetter <daniel@ffwll.ch>
To: Dave Gordon <david.s.gordon@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH v4 0/3] improve handling of the driver's default (kernel) context
Date: Thu, 21 Jan 2016 09:21:06 +0100	[thread overview]
Message-ID: <20160121082106.GK19130@phenom.ffwll.local> (raw)
In-Reply-To: <1453230175-19330-1-git-send-email-david.s.gordon@intel.com>

On Tue, Jan 19, 2016 at 07:02:52PM +0000, Dave Gordon wrote:
> During startup, the driver creates a unique "intel_context" that will
> provide a home for orphan requests (i.e. those generated by the driver
> internally, not associated with user batchbuffers).
> 
> However, one of the infelicities of the current code is that the driver
> keeps a per-engine pointer to the default context for that engine (this
> is probably from a decision made early in execlists implementation,
> when it was thought that a context was engine-specific, and not revised
> when it was decided that an "intel_context" represents a multiplex of
> engine-level contexts). All these per-engine pointers actually end up
> pointing to the same unique object.
> 
> To compound this, the refcounting is bogus; the driver holds just one
> reference to the default context, even though there are multiple pointers
> to it (RCS is considered to hold this "on behalf of" the other engines,
> but this can lead to problems during unload as it makes the code sensitive
> to the order of engine teardown -- RCS should be done last, but it isn't).
> 
> Also, some of the execlists batch submission code treats the default
> context differently from any other; testing for it by comparing against
> the per-engine pointer is quite clumsy, especially in debugfs where
> contexts are enumerated from a global list and therefore not automatically
> known to be associated with a particular engine.
> 
> Therefore, we should replace the per-engine pointers with a single
> driver-wide reference, which will make the refcounting sane and simplify
> the code that uses this context for non-user-related requests.
> 
> THIS patchset does NOT address the execlists code's special handling of
> the default context, but it will simplify the planned future cleanup of
> that code.
> 
> v3: don't flag the context created during startup (and fully instantiated
> on all engines) for driver-internal purposes as "is_global_default",
> 1448630935-27377-1-git-send-email-chris@chris-wilson.co.uk notwithstanding.
> (We now call it "the kernel context", and compare against the device-wide
> pointer).  [Chris Wilson] 
> 
> v4: Rebased

entire series vacuumed up, thanks.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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