[Intel-gfx] [PATCH] drm/i915: don't track relative-constants-mode

[Chris Wilson]

On Fri, Aug 26, 2016 at 07:25:57PM +0100, Dave Gordon wrote:
> 'relative_constants_mode' has always been tracked per-device, but this
> has actually been wrong ever since hardware contexts were introduced, as
> the INSTPM register is saved (and automatically restored) as part of the
> render ring context. The software per-device value could therefore get
> out of sync with the hardware per-context value.
> 
> Rather than track this correctly (per-context in LRC modes, per-device
> in legacy ringbuffer mode), a simpler solution is just to give up trying
> to track it at all, and always prefix each render batch with the single
> instruction needed to explicitly set it to the required mode for the
> current batch.

Agreed. However always emitting it wasteful for those who never touch
it, i.e. everybody. Rather than track the last value, just track if it
ever changed from default then reassign it before every batch?

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 99d50c0a710c..394c9cd3ddd3 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1578,7 +1578,7 @@ execbuf_submit(struct i915_execbuffer_params *params,
                intel_ring_emit(ring, instp_mask << 16 | instp_mode);
                intel_ring_advance(ring);
 
-               dev_priv->relative_constants_mode = instp_mode;
+               dev_priv->relative_constants_mode = -1;
        }
 
        if (args->flags & I915_EXEC_GEN7_SOL_RESET) {


-- 
Chris Wilson, Intel Open Source Technology Centre