Re: [PATCH v2 4/5] drm/panfrost: Add a PANFROST_BO_GPUONLY flag

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

On Fri, 1 Oct 2021 16:13:42 +0100
Steven Price <steven.price@arm.com> wrote:

> On 01/10/2021 15:34, Boris Brezillon wrote:
> > This lets the driver reduce shareability domain of the MMU mapping,
> > which can in turn reduce access time and save power on cache-coherent
> > systems.
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>  
> 
> This seems reasonable to me - it matches the kbase
> BASE_MEM_COHERENT_SYSTEM (only backwards obviously) and it worked
> reasonably well for the blob.
> 
> But I'm wondering if we need to do anything special to deal with the
> fact we will now have some non-coherent mappings on an otherwise
> coherent device.
> 
> There are certainly some oddities around how these buffers will be
> mapped into user space if requested, e.g. panfrost_gem_create_object()
> sets 'map_wc' based on pfdev->coherent which is arguably wrong for
> GPUONLY. So there are two things we could consider:
> 
> a) Actually prevent user space mapping GPUONLY flagged buffers. Which
> matches the intention of the name.

I intended to do that, just forgot to add wrappers around
drm_gem_shmem_{mmap,vmap}() to forbid CPU-mappings on gpuonly buffers.

> 
> b) Attempt to provide user space with the tools to safely interact with
> the buffers (this is the kbase approach). This does have the benefit of
> allowing *mostly* GPU access. An example here is the tiler heap where
> the CPU could zero out as necessary but mostly the GPU has ownership and
> the CPU never reads the contents. GPUONLY/DEVONLY might not be the best
> name in that case.

Uh, right, I forgot we had to zero the tiler heap on Midgard (most of
the time done with a WRITE_VALUE job, but there's an exception on some
old Midgard GPUs IIRC).
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