Re: [PATCH v4 1/7] drm/panthor: Add support for atomic page table updates

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 21 Aug 2025 16:02:09 +0100
Steven Price <steven.price@arm.com> wrote:

> On 21/08/2025 12:51, Boris Brezillon wrote:
> > On Wed, 16 Jul 2025 16:43:24 +0100
> > Steven Price <steven.price@arm.com> wrote:  
> [...]
> >> Although in general I'm a bit wary of relying on the whole lock region
> >> feature - previous GPUs have an errata. But maybe I'm being over
> >> cautious there.  
> > 
> > We're heavily relying on it already to allow updates of the VM while
> > the GPU is executing stuff. If that's problematic on v10+, I'd rather
> > know early :D.  
> 
> I think I'm just scarred by my experiences over a decade ago... ;)
> 
> I'm not aware of any issues with the modern[1] GPUs. The issue used to
> be that the lock region could get accidentally unlocked by a cache flush
> from another source - specifically the cache flush on job start flag.
> 
> It's also not a major issue if you keep the page tables consistent, the
> lock region in theory allows a region to be in an inconsistent state -
> but generally there's no need for that. AFAIK we mostly keep the tables
> consistent anyway.

Right, it's not a problem until we introduce sparse binding support, at
which point atomicity becomes important, and given remapping is not a
thing the io-pagetable layer provides (remap has to be unmap+map), I
need to rely on region locking to make it work, or we'll have to eat the
fault-but-not-really-because-its-being-remapped overhead/complexity.
Honestly, I'd rather rely on region locking if it's working, because
it's far simpler ;-).