Quoting Michał Winiarski (2017-09-25 16:16:33)
> On Mon, Sep 25, 2017 at 12:44:07PM +0100, Chris Wilson wrote:
> > Add another perma-pinned context for using for preemption at any time.
> > We cannot just reuse the existing kernel context, as first and foremost
> > we need to ensure that we can preempt the kernel context itself, so
> > require a distinct context id. Similar to the kernel context, we may
> > want to interrupt execution and switch to the preempt context at any
> > time, and so it needs to be permanently pinned and available.
> >
> > To compensate for yet another permanent allocation, we shrink the
> > existing context and the new context by reducing their ringbuffer to the
> > minimum.
>
> Since we're special treating preempt_context, we should also probably BUG_ON for
> any attempts to allocate requests for it.
Yup.
diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index 4eb1a76731b2..a6fba3e3d9c5 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -586,6 +586,7 @@ i915_gem_request_alloc(struct intel_engine_cs *engine,
int ret;
lockdep_assert_held(&dev_priv->drm.struct_mutex);
+ GEM_BUG_ON(ctx == dev_priv->preempt_context);
/* ABI: Before userspace accesses the GPU (e.g. execbuffer), report
* EIO if the GPU is already wedged.
Hmm, now imagine that with an explanatory text before the bug_on
-Chris
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