[Mesa-dev] [PATCH 05/10] i965: Use Global GTT for Sandybridge post-sync non-zero workaround.

[Daniel Vetter]

On Mon, Jan 13, 2014 at 01:02:19PM -0800, Eric Anholt wrote:
> Kenneth Graunke <kenneth at whitecape.org> writes:
> 
> > On 01/09/2014 10:03 PM, Eric Anholt wrote:
> >> Eric Anholt <eric at anholt.net> writes:
> >> 
> >>> Kenneth Graunke <kenneth at whitecape.org> writes:
> >>>
> >>>> The kernel doesn't even set up the aliasing PPGTT on Sandybridge, so any
> >>>> writes marked as PPGTT will likely just get dropped on the floor.
> >>>
> >>> The hardware bug is that writes not marked as GTT are still looked up in
> >>> the GTT anyway.
> >>>
> >>> The kernel does set up the PPGTT, which is how we found we needed to put
> >>> in the kernel workaround based on DOMAIN_INSTRUCTION (of binding the
> >>> target buffer to the gtt as well as the ppgtt, since the writes landed
> >>> in the wrong place)
> >>>
> >>> I don't think this patch will change anything, but it seems reasonable
> >>> if the commit message is updated.
> >> 
> >> Actually, thinking about it more, I'd rather not explicitly use global
> >> GTT, unless the function is also renamed to
> >> gen6_emit_post_sync_nonzero_workaround, since now this function on
> >> non-gen6 would reference GTT memory in its instruction, but the kernel
> >> wouldn't put anything in the GTT.
> >> 
> >> (I'd rather just leave the workaround as is, myself).
> >
> > Okay, sounds like this is unnecessary.  But...the next patch (helper
> > function for writes) causes this to use PIPE_CONTROL_GLOBAL_GTT_WRITE on
> > SNB only, and PPGTT on Gen7+.
> 
> Oh, right.  I'm fine with this as-is, then (r-b).

Hm, I didn't spot the code to set the address space bit correctly, but
unconditionally setting the ppgtt bit isn't the right thing to do.
Currently we have ppgtt (the hw mode, not the separate address spaces)
enabled on snb, ivb and hsw, but disabled on byt and bdw. There's also a
module option to override that default.

If the write actually matters you need to consult the
I915_PARAM_HAS_ALIASING_PPGTT driver param. If it is set then you should
use ppgtt as the address space selector. This will even hold for full
ppgtt (so a better name would be USES_PPGTT_FOR_EXECBUF, but abi and all
that).

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch