[RFC PATCH] drm/panfrost: Handle IDVS_GROUP_SIZE feature

[Alyssa Rosenzweig]

> > This feature adds an extra IDVS group size field to the JM_CONFIG
> > register. In kbase, the value is configurable via the device tree; kbase
> > uses 0xF as a default if no value is specified. Until we find a device
> > demanding otherwise, let's always set the 0xF default on devices which
> > support this feature mimicking kbase's behaviour.
> 
> This is a performance thing - so I don't think it will break anything if
> this is wrong, it just won't be optimal.

Then interpret my remarks as hardcoding the default until we find a
device where setting to something other than 0xF improves performance
nontrivially. (Read: I am lazy and do not want to write dt-bindings for
something nobody will ever use.)

> > As JM_CONFIG is an undocumented register, it's not clear to me what
> > happens if we fail to include this handling. Index-driven vertex shading
> > already works on Bifrost boards with this feature without this handling.
> > Perhaps this has performance implications? Patch untested for the
> > moment, wanted to give Steven a chance to comment.
> 
> As it's a performance thing you shouldn't see correctness issues with
> not setting it. But 0xF seems to have been chosen as it gave the best
> overall performance (although for individual test content this can
> vary). AFAICT the performance impact isn't massive either.

Good to know, will update the commit message accordingly.

> Reviewed-by: Steven Price <steven.price at arm.com>
> 
> Since you've tagged this RFC I won't merge it now, but it looks correct
> to me.

Thanks for the review... I hope you like reviewing Panfrost patches
because I have a Valhall bring-up series waiting o:)

When I get a chance to uprev the kernel on my G52 board I'll see if I
can benchmark the impact of this change, so far this is only
compile-tested. Even if there's no impact the patch should likely go in
to stay consistent with kbase, but hopefully there's a win from this. At
that point I'll send a v2 with your reviewed-by (and hopefully no
changes other than the commit message) and we'll land that.