[PATCH 1/2] drm/panfrost: Allow passing extra information about BOs used by a job

[Boris Brezillon]

On Mon, 16 Sep 2019 17:20:28 -0500
Rob Herring <robh at kernel.org> wrote:

> On Fri, Sep 13, 2019 at 6:17 AM Boris Brezillon
> <boris.brezillon at collabora.com> wrote:
> >
> > The READ/WRITE flags are particularly useful if we want to avoid
> > serialization of jobs that read from the same BO but never write to it.  
> 
> Any data on performance differences?

Unfortunately no. When I initially added support for BO flags I thought
it would fix a regression I had on one glmark2 test (ideas), but the
problem ended up being something completely different (overhead of
calling ioctl(WAIT_BO) on already idle BOs).

I just ran glmark2 again, and there doesn't seem to be a noticeable
improvement with those 2 patches applied (and mesa patched to use the
new flags). This being said, the improvement is likely to be workload
dependent, so I wouldn't consider these patches as useless, but I'm
fine putting them on hold until we see a real need.

Maybe Steven has some real use cases that could help outline the
benefit of these patches.

> 
> > The NO_IMPLICIT_FENCE might be useful when the user knows the BO is
> > shared but jobs are using different portions of the buffer.  
> 
> Why don't we add this when it is useful rather than might be?

I don't have a need for that one yet, but etanviv has it in place so I
thought I'd add both at the same time.
Note that it could help us reduce the number of fence returned by
panfrost_job_dependency(), but I'm not sure it makes a difference in
terms of perfs.