[PATCH 0/3] drm: panfrost: Coherency support

[Robin Murphy]

On 2020-09-16 18:46, Rob Herring wrote:
> On Wed, Sep 16, 2020 at 11:04 AM Alyssa Rosenzweig
> <alyssa.rosenzweig at collabora.com> wrote:
>>
>>> So I get a performance regression with the dma-coherent approach, even if it's
>>> clearly the cleaner.
>>
>> That's bizarre -- this should really be the faster of the two.
> 
> Coherency may not be free. CortexA9 had something like 4x slower
> memcpy if SMP was enabled as an example. I don't know if there's
> anything going on like that specifically here. If there's never any
> CPU accesses mixed in with kmscube, then there would be no benefit to
> coherency.

There will still be CPU benefits in terms of not having to spend time 
cache-cleaning every BO upon allocation, and less overhead on writing 
out descriptors in the first place (due to cacheable vs. non-cacheable).

I haven't tried the NSh hack on Juno, but I don't see any notable 
performance issue as-is - kmscube hits a solid 60FPS from the off (now 
that it works without spewing faults). Given that the hardware on Juno 
can be generously described as "less good", it would certainly be 
interesting to figure out what difference is at play here...

The usual argument against I/O coherency is that it adds latency to 
every access, but if you already have a coherent interconnect anyway 
then the sensible answer to that is implementing decent snoop filters, 
rather than making software more complicated.

Robin.