[Intel-gfx] [PATCH 6/6] drm/i915: Kick start the rings

[Mateo Lozano, Oscar]

> Subject: [Intel-gfx] [PATCH 6/6] drm/i915: Kick start the rings
> 
> On g4x, we have an issue where the register write to setup the rings do not
> always take. However, it appears that the current check also passes only by
> chance, a second reading of the register returns a different broekn value - but
> the GPU appears to function. Based on that observation, lets try feeding a nop
> into the ring and seeing if it advances as part of our startup sanity checks.
> 
> References: https://bugs.freedesktop.org/show_bug.cgi?id=76554
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c
> b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index a8c73a0d935d..bc52645fa8d5 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -514,10 +514,14 @@ static int init_ring_common(struct intel_ring_buffer
> *ring)
>  			((ring->size - PAGE_SIZE) & RING_NR_PAGES)
>  			| RING_VALID);
> 
> +	iowrite32(MI_NOOP, ring->virtual_start + 0);
> +	iowrite32(MI_NOOP, ring->virtual_start + 4);
> +	ring->write_tail(ring, 8);

Maybe add a comment in the code about why we are doing this? (otherwise, it looks a bit magical)

>  	/* If the head is still not zero, the ring is dead */
>  	if (wait_for((I915_READ_CTL(ring) & RING_VALID) != 0 &&
>  		     I915_READ_START(ring) == i915_gem_obj_ggtt_offset(obj)
> &&
> -		     (I915_READ_HEAD(ring) & HEAD_ADDR) == 0, 50)) {
> +		     (I915_READ_HEAD(ring) & HEAD_ADDR) == 8, 50)) {
>  		DRM_ERROR("%s initialization failed "
>  			  "ctl %08x (valid? %d) head %08x tail %08x start %08x
> [expected %08lx]\n",
>  			  ring->name,
> --
> 1.9.1
> 
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