[Mesa-dev] [PATCH] i965: Skip register write checks if cmd_parser_version >= 2.

[Daniel Vetter]

On Wed, Oct 19, 2016 at 02:26:15PM -0700, Kenneth Graunke wrote:
> On Wednesday, October 19, 2016 8:35:11 PM PDT Chris Wilson wrote:
> > On Wed, Oct 19, 2016 at 12:25:51PM -0700, Kenneth Graunke wrote:
> > > If the kernel advertises a new enough command parser version, then we
> > > can just assume that register writes will work and not bother executing
> > > commands on the GPU to test it.
> > 
> > We do not guarantee that capabilities will not be removed, especially
> > whitelisted registers, or that all platforms will implement the same set.
> > -Chris
> 
> I only care about Haswell, Ivybridge, and Baytrail here, so "not all
> platforms will implement the same set" is not a concern.
> 
> If you make the command parser start rejecting these, you /will/
> break/cripple userspace, and as I understand it the kernel community
> frowns upon that.
> 
> Mesa already does a CMD_PARSER_VERSION >= check for:
> 
> - Enabling L3 atomics when there's a DC partition (version >= 4)
> - Enabling ARB_query_buffer_object on Haswell
>   (need TIMESTAMP register, CS GPR registers, and MI_LRR: version >= 7)
> 
> - Enabling hardware conditional rendering support
>   (need MI_PREDICATE registers, so version >= 2)
> - Enabling compute shaders and ES3.1 compatibility on Haswell
>   (need GPGPU dispatch compute indirect registers, so version >= 5)
> 
> A few things were implemented before the command parser landed, so they
> rely on our ad-hoc check alone:
> 
> - ARB_transform_feedback{2,3,_instanced}
> - ARB_draw_indirect
> - AMD_performance_monitor / INTEL_performance_query
> 
> The only reason I implemented it this way is because the kernel did
> not offer me an interface I could use to query this.  Now it does.
> I can't see any reason why some basic cases need to guess-and-check
> while others can use the query.  It should be consistent.
> 
> I could perhaps see OACONTROL being removed, since arguably the feature
> it exposes currently provides broken counter data without Rob's work.
> But the others...you're not going to remove.
> 
> If you start taking away features, the version getparam is entirely
> meaningless.  We can't infer anything from it, and need to go back
> to ad-hoc checking of every feature we ever use.  That's asinine.

Yeah, we won't/can't remove registers (on a given platform at least) from
the allowed set. Ack on the patch.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch