[Intel-gfx] [PATCH] drm/i915/bdw: The TLB invalidation mechanism has been removed from INSTPM

[Ben Widawsky]

On Thu, Mar 13, 2014 at 01:40:28AM +0000, Damien Lespiau wrote:
> While wandering in the spec, I noticed that BDW removes those 2 bits
> from INSTPM. I couldn't find any direct way to invalidate the TLB (ie
> without the ring working already). Maybe someone will be more lucky.
> 
> At least, we now know we may be a problem.
> 
> Signed-off-by: Damien Lespiau <damien.lespiau at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index c50388a..4eb3e06 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -981,8 +981,14 @@ void intel_ring_setup_status_page(struct intel_ring_buffer *ring)
>  	I915_WRITE(mmio, (u32)ring->status_page.gfx_addr);
>  	POSTING_READ(mmio);
>  
> -	/* Flush the TLB for this page */
> -	if (INTEL_INFO(dev)->gen >= 6) {
> +	/*
> +	 * Flush the TLB for this page
> +	 *
> +	 * FIXME: These two bits have disappeared on gen8, so a question
> +	 * arises: do we still need this and if so how should we go about
> +	 * invalidating the TLB?
> +	 */
> +	if (INTEL_INFO(dev)->gen >= 6 && INTEL_INFO(dev)->gen < 8) {
>  		u32 reg = RING_INSTPM(ring->mmio_base);
>  
>  		/* ring should be idle before issuing a sync flush*/

I'm missing something on the original patch,
884020bf3d2a3787a1cc6df902e98e0eec60330b. How were we emitting
breadcrumbs without flushing the TLB? All bathcbuffers should be
bookended by a TLB invalidate already, so I'm not sure the logic holds.
Chris could explain that one a bit further?

The only reason I bring this up is I'd like to rip this out completely
and have Thiago retest, or at least change the comment/commit message to
be to reflect whatever light Chris sheds on the matter.

Anyway, the bits are definitely gone, and I also can't find a non-ring
based replacement.
Reviewed-by: Ben Widawsky <ben at bwidawsk.net>

-- 
Ben Widawsky, Intel Open Source Technology Center