Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> In order to ensure seqno/irq coherency, we current read a ring register.
> We are not sure quite how it works, only that is does. Experiments show
> that e.g. doing a clflush(seqno) instead is not sufficient, but we can
> remove the forcewake dance from the mmio access.
>
> v2: Baytrail wants a clflush too.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/intel_ringbuffer.c | 15 +++++++++++++--
> 1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 8cd8aabcc3ff..935add1422ae 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -1492,10 +1492,21 @@ gen6_ring_get_seqno(struct intel_engine_cs *ring, bool lazy_coherency)
> {
> /* Workaround to force correct ordering between irq and seqno writes on
> * ivb (and maybe also on snb) by reading from a CS register (like
> - * ACTHD) before reading the status page. */
> + * ACTHD) before reading the status page.
> + *
> + * Note that this effectively effectively stalls the read by the
> time
effectively twice there.
Reviewed-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
> + * it takes to do a memory transaction, which more or less ensures
> + * that the write from the GPU has sufficient time to invalidate
> + * the CPU cacheline. Alternatively we could delay the interrupt from
> + * the CS ring to give the write time to land, but that would incur
> + * a delay after every batch i.e. much more frequent than a delay
> + * when waiting for the interrupt (with the same net latency).
> + */
> if (!lazy_coherency) {
> struct drm_i915_private *dev_priv = ring->dev->dev_private;
> - POSTING_READ(RING_ACTHD(ring->mmio_base));
> + POSTING_READ_FW(RING_ACTHD(ring->mmio_base));
> +
> + intel_flush_status_page(ring, I915_GEM_HWS_INDEX);
> }
>
> return intel_read_status_page(ring, I915_GEM_HWS_INDEX);
> --
> 2.7.0.rc3
>
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