[PATCH] drm/panfrost: Add "compute shader only" hint

[Alyssa Rosenzweig]

Ah-ha! Thank you for the detailed explanation; this makes a lot more
sense now :)

> In practice all this only really matters on the T62x GPU. All other GPUs
> have only one core group[1]. So it only really makes sense to use JS2 on
> the T62x where you want to use both JS1 and JS2 to run two independent
> jobs: one on each core group.
> 
> Oh, and there is one hardware workaround (BASE_HW_ISSUE_8987) that uses
> JS2. This is to avoid vertex and compute jobs landing on the same slot.
> This affects T604 "dev15" only and is because some state was not
> properly cleared between jobs.

On my end at least, our work with compute has been focused on
RK3399/T860. In userspace, it's tempting to limit T6xx/T720 to e.g.
OpenGL ES 2.0 (between this/other errata, and of course fake-MRT), at
least for the interim.

For completeness, we'll maybe need to deal with this Eventually, but it
doesn't sound like this needs to be handled right now? (It sounds like
there's no changes needed for compute on T700+, since #8987 doesn't
apply and there's only one core group.). In that case, maybe it's not
worth it to start mucking with the UABI for a feature that may or may
not ever be used.
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