Re: [RFC] drm/i915: check that rpm ref is held when writing to ringbuf in stolen mem

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jan 27, 2016 at 01:50:17PM +0000, Chris Wilson wrote:
> On Wed, Jan 27, 2016 at 01:13:54PM +0000, Daniele Ceraolo Spurio wrote:
> > 
> > 
> > On 27/01/16 09:38, Chris Wilson wrote:
> > >On Wed, Jan 27, 2016 at 08:55:40AM +0000, daniele.ceraolospurio@intel.com wrote:
> > >>From: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> > >>
> > >>While running some tests on the scheduler patches with rpm enabled I
> > >>came across a corruption in the ringbuffer, which was root-caused to
> > >>the GPU being suspended while commands were being emitted to the
> > >>ringbuffer. The access to memory was failing because the GPU needs to
> > >>be awake when accessing stolen memory (where my ringbuffer was located).
> > >>Since we have this constraint it looks like a sensible idea to check that
> > >>we hold a refcount when we emit commands.
> > >>
> > >>Cc: John Harrison <John.C.Harrison@Intel.com>
> > >>Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> > >>---
> > >>  drivers/gpu/drm/i915/intel_lrc.c | 5 +++++
> > >>  1 file changed, 5 insertions(+)
> > >>
> > >>diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > >>index 3761eaf..f9e8d74 100644
> > >>--- a/drivers/gpu/drm/i915/intel_lrc.c
> > >>+++ b/drivers/gpu/drm/i915/intel_lrc.c
> > >>@@ -1105,6 +1105,11 @@ int intel_logical_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
> > >>  	if (ret)
> > >>  		return ret;
> > >>+	// If the ringbuffer is in stolen memory we need to be sure that the
> > >>+	// gpu is awake before writing to it
> > >>+	if (req->ringbuf->obj->stolen && num_dwords > 0)
> > >>+		assert_rpm_wakelock_held(dev_priv);
> > >The assertion you want is that when iomapping through the GTT that we
> > >hold a wakeref.
> > >-Chris
> > 
> > If I'm not missing anything, we iomap the ringbuffer at request
> > allocation time;
> 
> Strictly, the ring is pinned for whilst we access it for writing the
> request i.e. only during request constuction. It can be unpinned at any
> point afterwards. It is unpinned late today to paper over a few other
> issues with context pinning and the overhead of having to do the iomap.
> 
> > however, with the scheduler a request could
> > potentially wait in the queue for a time long enough to allow RPM to
> > kick in, especially if the request is waiting on a fence object
> > coming from a different driver. In this situation the rpm reference
> > taken to cover the request allocation would have already been
> > released and so we need to ensure that a new one has been taken
> > before writing to the ringbuffer; that's why I originally placed the
> > assert in ring_begin.
> 
> No, once the request is queued we are not modifying the ring. If the
> scheduler needs to manipulate it (which it shouldn't) then it has to
> acquire its own pin for its access (or extend the original pinning to
> suit which would also be less than ideal).

Yeah, with execlist all the scheduling should happen at the context level.
Well, all the scheduling should always happen at the context level, but
with execlist the hw makes it much smoother. Scheduler touching the rings
in the ctx object sounds like a bug.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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