Re: [PATCH] drm/i915/execlists: Document runtime pm for intel_lrc_irq_handler()

[Chris Wilson <chris@chris-wilson.co.uk>]

On Wed, Apr 12, 2017 at 08:12:17AM +0100, Tvrtko Ursulin wrote:
> 
> On 11/04/2017 18:58, Chris Wilson wrote:
> >We indirectly hold the runtime-pm for the intel_lrc_irq_handler() by
> >virtue of dev_priv->gt.awake keeping a wakeref whilst the requests are
> >busy. As this is not obvious from the code, add a comment.
> >
> >Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> >Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >---
> > drivers/gpu/drm/i915/intel_lrc.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> >diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> >index 0dc1cc4ad6e7..e16cc28dc783 100644
> >--- a/drivers/gpu/drm/i915/intel_lrc.c
> >+++ b/drivers/gpu/drm/i915/intel_lrc.c
> >@@ -515,6 +515,15 @@ static void intel_lrc_irq_handler(unsigned long data)
> > 	struct execlist_port *port = engine->execlist_port;
> > 	struct drm_i915_private *dev_priv = engine->i915;
> >
> >+	/* We can skip acquiring intel_runtime_pm_get() here as it was taken
> >+	 * on our behalf by the request (see i915_gem_mark_busy ()) and it will
> >+	 * not be relinquished until the device is idle (see
> >+	 * i915_gem_idle_work_handler()). As a precaution, we make sure
> >+	 * that all ELSP are drained i.e. we have processed the CSB,
> >+	 * before allowing ourselves to idle and calling intel_runtime_pm_put().
> >+	 */
> >+	GEM_BUG_ON(!dev_priv->gt.awake);
> >+
> > 	intel_uncore_forcewake_get(dev_priv, engine->fw_domains);
> >
> > 	/* Prefer doing test_and_clear_bit() as a two stage operation to avoid
> >
> 
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> It feels a big to c&p, but not sure if it worth putting a comment
> "see comment in intel_lrc_irq_handler on why we don't have to " to
> i915_guc_irq_handler or maybe dequeue?

For guc, the requirement for rpm is much less strict - the interface
with the guc is through ordinary memory not device memory, so we don't
need rpm_get (aiui). There is the mmio readback for !llc to ensure that
all GTT writes are flushed to system memory prior to dispatching the
workqueue -- that technically doesn't require rpm either.

I'm a bit more hesistant about adding such a comment to the guc atm, as
I think I will be more misleading then helpful.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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