[Intel-gfx] [PATCH 11/11] [v2] drm/i915/bdw: Ensure a context is loaded before RC6

[Ben Widawsky]

RC6 works a lot like HW contexts in that when the GPU enters RC6 it
saves away the state to a context, and loads it upon wake.

It's to be somewhat expected that BIOS will not set up valid GPU state.
As a result, if loading bad state can cause the GPU to get angry, it
would make sense then that we need to load state first. There are two
ways in which we can do this:

1. Create 3d state in the driver, load it up, then enable RC6.
1b. Reuse a known good state, [and if needed,] just bind objects where
needed. Then enable RC6.
2. Hold off enabling RC6 until userspace has had a chance to complete
batches.

There has been discussions in the past with #1 as it has been
recommended for fixes elsewhere. I'm not opposed to it, I'd just like to
do the easy thing now to enable the platform.

This patch is a hack that implement option #2. It will defer enabling
rc6 until the first batch from userspace has been retired. It suffers
two flaws. The first is, if the driver is loaded, but a batch is not
submitted/completed, we'll never enter rc6. The other is, it expects
userspace to submit a batch with 3d state first. Both of these things
are not actual flaws for most users because most users will boot to a
graphical composited desktop. Both mesa, and X will always emit the
necessary 3d state.

Once a context is loaded and we enable rc6, the default context should
inherit the proper state because we always inhibit the restore for the
default context. This assumes certain things about the workaround/issue
itself to which I am not privy (primarily that the indirect state
objects don't actually need to exist).

With that, there are currently 4 options for BDW:
1. Don't use RC6.
2. Use RC6 and expect a hang on the first batch submitted for every
context.
3. Use RC6 and use this patch.
4. Wait for another workaround implementation.

NOTE: This patch could be used against other platforms as well.

v2: Re-add accidentally dropped hunk (Ben)

The fact that RC6 residency continues to increment, and that this patch
prevents a hang on BDW silicon has been:
Tested-by: Kenneth Graunke <kenneth at whitecape.org>

Cc: David E. Box <david.e.box at intel.com>
Cc: Kristen Carlson Accardi <kristen at linux.intel.com>
Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem.c      | 12 ++++++++++++
 drivers/gpu/drm/i915/intel_display.c |  5 +++++
 2 files changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 3618bb0..07170a7 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2420,6 +2420,7 @@ void i915_gem_reset(struct drm_device *dev)
 void
 i915_gem_retire_requests_ring(struct intel_ring_buffer *ring)
 {
+	static bool rc6_enabled = false;
 	uint32_t seqno;
 
 	if (list_empty(&ring->request_list))
@@ -2443,6 +2444,17 @@ i915_gem_retire_requests_ring(struct intel_ring_buffer *ring)
 		if (!i915_seqno_passed(seqno, obj->last_read_seqno))
 			break;
 
+		/* Wa: can't find the w/a name.
+		 * This doesn't actually implement the w/a, but it a workaround
+		 * for the workaround. It defers using rc6 until we know valid
+		 * state exists.
+		 */
+		if (IS_BROADWELL(dev) && intel_enable_rc6(dev) &&
+		    !rc6_enabled && ring->id == RCS &&) {
+			intel_enable_gt_powersave(ring->dev);
+			rc6_enabled = true;
+		}
+
 		i915_gem_object_move_to_inactive(obj);
 	}
 
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index f19e6ea..72c8e1d 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -10983,6 +10983,11 @@ void intel_modeset_init_hw(struct drm_device *dev)
 
 	intel_reset_dpio(dev);
 
+	if (IS_BROADWELL(dev)) {
+		DRM_DEBUG_DRIVER("Deferring RC6 enabling until first batch is complete\n");
+		return;
+	}
+
 	mutex_lock(&dev->struct_mutex);
 	intel_enable_gt_powersave(dev);
 	mutex_unlock(&dev->struct_mutex);
-- 
1.8.5.5