[Intel-gfx] [PATCH 06/13] drm/i915/bdw: implement semaphore signal

[Ben Widawsky]

On Thu, Jan 30, 2014 at 01:35:41PM +0000, Chris Wilson wrote:
> On Thu, Jan 30, 2014 at 02:18:32PM +0100, Daniel Vetter wrote:
> > On Thu, Jan 30, 2014 at 1:46 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > Oh. So they changed how post-sync writes operated - this should be a
> > > separate fix for stable I believe (so that batches are not run before we
> > > have finished invalidating the TLBs required).
> > 
> > We have an igt to exercise tlb invalidation stuff, which runs on all
> > rings. But it only runs a batch, so only uses the CS tlb. Do we need
> > to extend this?
> 
> So the spec says:
> 
> Pipe Control Flush Enable (IVB+)
> If ENABLED, the PIPE_CONTROL command will wait until all previous writes
> of immediate data from post sync circles are complete before executing
> the next command.
> 
> Post Sync Operation
> This field specifies an optional action to be taken upon completion of
> the synchronization operation.
> 
> TLB Invalidate
> If ENABLED, all TLBs belonging to Render Engine will be invalidated once
> the flush operation is complete.
> 
> Command Streamer Stall Enable
> If ENABLED, the sync operation will not occur until all previous flush
> operations pending a completion of those previous flushes will complete,
> including the flush produced from this command. This enables the command
> to act similar to the legacy MI_FLUSH command.
> 
> Going by that, the order is
> 
> flush, stall, TLB invalidate / post-sync op, [pipe control flush]
> 
> Based on my reading of the above (which unless someone has a more
> definitive source) says that without the CONTROL_FLUSH_ENABLE, the CS
> can continue operations as soon as the flush is complete - in parallel
> to the TLB invalidate. Adding CONTROL_FLUSH_ENABLE would then stall the
> CS until the post-sync operation completes. That still leaves the
> possibility that the TLB invalidate is being performed in parallel and
> is itself provides no CS sync.
> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

so.... what the verdict?

-- 
Ben Widawsky, Intel Open Source Technology Center