[Intel-gfx] [PATCH] drm/i915/execlists: Inhibit context save/restore for the fake preempt context

[Chris Wilson]

We only use the preempt context to inject an idle point into execlists.
We never need to reference its logical state, so tell the GPU never to
load it or save it.

Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski at intel.com>
Cc: Michel Thierry <michel.thierry at intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Mika Kuoppala <mika.kuoppala at intel.com>
---
Please check the register definition. I know the save-inhibit bit used
to exist, I don't know if it is BIT(1) in RING_CONTEXT_CONTROL -- it
feels like it should be, and the numbers indicate that it does
something beneficial.
-Chris
---
 drivers/gpu/drm/i915/intel_lrc.c | 4 ++++
 drivers/gpu/drm/i915/intel_lrc.h | 1 +
 2 files changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 22d471a4228d..c28f267a8417 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -2285,6 +2285,10 @@ populate_lr_context(struct i915_gem_context *ctx,
 	if (!engine->default_state)
 		regs[CTX_CONTEXT_CONTROL + 1] |=
 			_MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT);
+	if (ctx->hw_id == PREEMPT_ID)
+		regs[CTX_CONTEXT_CONTROL + 1] |=
+			_MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT |
+					   CTX_CTRL_ENGINE_CTX_SAVE_INHIBIT);
 
 	i915_gem_object_unpin_map(ctx_obj);
 
diff --git a/drivers/gpu/drm/i915/intel_lrc.h b/drivers/gpu/drm/i915/intel_lrc.h
index 6d4f9b995a11..e6e2c1774600 100644
--- a/drivers/gpu/drm/i915/intel_lrc.h
+++ b/drivers/gpu/drm/i915/intel_lrc.h
@@ -36,6 +36,7 @@
 #define RING_CONTEXT_CONTROL(engine)		_MMIO((engine)->mmio_base + 0x244)
 #define	  CTX_CTRL_INHIBIT_SYN_CTX_SWITCH	(1 << 3)
 #define	  CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT	(1 << 0)
+#define	  CTX_CTRL_ENGINE_CTX_SAVE_INHIBIT	(1 << 1)
 #define   CTX_CTRL_RS_CTX_ENABLE                (1 << 1)
 #define RING_CONTEXT_STATUS_BUF_BASE(engine)	_MMIO((engine)->mmio_base + 0x370)
 #define RING_CONTEXT_STATUS_BUF_LO(engine, i)	_MMIO((engine)->mmio_base + 0x370 + (i) * 8)
-- 
2.15.1