Re: [PATCH v2] drm/i915/execlists: Inhibit context save/restore for the fake preempt context

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On 1/23/2018 1:04 PM, Chris Wilson wrote:
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.

v2: BIT(2) for save-inhibit.

N.B. Daniele mentioned this bit mbz for ICL, and has been moved into the
submission process rather than the context image.

Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Michal Winiarski <michal.winiarski@xxxxxxxxx>
Cc: Michel Thierry <michel.thierry@xxxxxxxxx>
Cc: Michal Wajdeczko <michal.wajdeczko@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>
---
  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);


This shouldn't break anything and ICL is not merged yet (plus things may still change) so if you want to merge this,

Reviewed-by: Michel Thierry <michel.thierry@xxxxxxxxx>

  	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..636ced41225d 100644
--- a/drivers/gpu/drm/i915/intel_lrc.h
+++ b/drivers/gpu/drm/i915/intel_lrc.h
@@ -37,6 +37,7 @@
  #define	  CTX_CTRL_INHIBIT_SYN_CTX_SWITCH	(1 << 3)
  #define	  CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT	(1 << 0)
  #define   CTX_CTRL_RS_CTX_ENABLE                (1 << 1)
+#define	  CTX_CTRL_ENGINE_CTX_SAVE_INHIBIT	(1 << 2)

CTX_CTRL_INHIBIT_SYN_CTX_SWITCH should be here, but that's for another patch ;)

  #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)
  #define RING_CONTEXT_STATUS_BUF_HI(engine, i)	_MMIO((engine)->mmio_base + 0x370 + (i) * 8 + 4)

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