[Mesa-dev] [PATCHv3] r600g: Use a fake reloc to sleep for fences

[Simon Farnsworth]

On Wednesday 8 February 2012 18:28:05 Michel Dänzer wrote:
> On Fre, 2012-02-03 at 17:32 +0000, Simon Farnsworth wrote:
> > r300g is able to sleep until a fence completes rather than busywait
> > because it creates a special buffer object and relocation that stays
> > busy until the CS containing the fence is finished.
> > 
> > Copy the idea into r600g, and use it to sleep if the user asked for an
> > infinite wait, falling back to busywaiting if the user provided a
> > timeout.
> > 
> > Signed-off-by: Simon Farnsworth <simon.farnsworth at onelan.co.uk>
> > ---
> > 
> > v3: At Dave Airlie's suggestion on dri-devel, make use of the BO to give
> > us a fighting chance of recovering after a GPU reset.
> > 
> > We know that the fence will never be signalled by hardware if the dummy
> > BO has gone idle - we use that information to escape the loop. This
> > might be a useful addition to the 8.0 branch.
> 
> That's a nifty trick. :)
> 
It's just a shame that the rest of recovering from a GPU reset isn't as simple 
- at least this stops us busywaiting eternally after the kernel's detected a 
CP stall.

> > diff --git a/src/gallium/drivers/r600/r600_hw_context.c
> > b/src/gallium/drivers/r600/r600_hw_context.c index 8eb8e6d..acfa494
> > 100644
> > --- a/src/gallium/drivers/r600/r600_hw_context.c
> > +++ b/src/gallium/drivers/r600/r600_hw_context.c
> > @@ -1623,6 +1623,13 @@ void r600_context_emit_fence(struct r600_context
> > *ctx, struct r600_resource *fen> 
> >  	ctx->pm4[ctx->pm4_cdwords++] = 0;                       /* DATA_HI
> >  	*/
> >  	ctx->pm4[ctx->pm4_cdwords++] = PKT3(PKT3_NOP, 0, 0);
> >  	ctx->pm4[ctx->pm4_cdwords++] = r600_context_bo_reloc(ctx, fence_bo,
> >  	RADEON_USAGE_WRITE);> 
> > +
> > +	/* Create a dummy BO so that fence_finish without a timeout can sleep
> > waiting for completion */ +	*sleep_bo = (struct r600_resource*)
> > +			pipe_buffer_create(&ctx->screen->screen, PIPE_BIND_CUSTOM,
> > +					   PIPE_USAGE_STAGING, 1);
> > +	/* Add the fence as a dummy relocation. */
> > +	r600_context_bo_reloc(ctx, *sleep_bo, RADEON_USAGE_READWRITE);
> 
> Sorry for only thinking of this now, but what's the advantage of doing
> this here, rather than in r600_create_fence()? Seems like that would be
> simpler and cleaner.

I've done it here because this is the bit of code that deals with all the 
hardware-related stuff, and it already knows about writing out relocations. I 
have no particular opinion either way, though, and wouldn't get upset about 
moving it. Thoughts?
-- 
Simon Farnsworth
Software Engineer
ONELAN Ltd
http://www.onelan.com
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