[Intel-gfx] [PATCH 2/2] drm/i915: Rework order of operations in {__intel, logical}_ring_prepare()

[Chris Wilson]

On Fri, Jun 12, 2015 at 06:09:08PM +0100, Dave Gordon wrote:
> The original idea of preallocating the OLR was implemented in
> 
> > 9d773091 drm/i915: Preallocate next seqno before touching the ring
> 
> and the sequence of operations was to allocate the OLR, then wrap past
> the end of the ring if necessary, then wait for space if necessary.
> But subsequently intel_ring_begin() was refactored, in
> 
> > 304d695 drm/i915: Flush outstanding requests before allocating new seqno
> 
> to ensure that pending work that might need to be flushed used the old
> and not the newly-allocated request. This changed the sequence to wrap
> and/or wait, then allocate, although the comment still said
> 	/* Preallocate the olr before touching the ring */
> which was no longer true as intel_wrap_ring_buffer() touches the ring.
> 
> However, with the introduction of dynamic pinning, in
> 
> > 7ba717c drm/i915/bdw: Pin the ringbuffer backing object to GGTT on-demand
> 
> came the possibility that the ringbuffer might not be pinned to the GTT
> or mapped into CPU address space when intel_ring_begin() is called. It
> gets pinned when the request is allocated, so it's now important that
> this comes *before* anything that can write into the ringbuffer, in this
> case intel_wrap_ring_buffer(), as this will fault if (a) the ringbuffer
> happens not to be mapped, and (b) tail happens to be sufficiently close
> to the end of the ring to trigger wrapping.

On the other hand, the request allocation can itself write into the
ring. This is not the right fix, that is the elimination of olr itself
and passing the request into intel_ring_begin. That way we can explicit
in our ordering into ring access.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre