Re: [PATCH v5 00/11] Support repeated mappings in GPUVM and Panthor

[Janne Grunau]

On Wed, Mar 25, 2026 at 06:51:55AM +1000, Dave Airlie wrote:
> On Sat, 14 Mar 2026 at 01:19, Adrián Larumbe
> <adrian.larumbe@xxxxxxxxxxxxx> wrote:
> >
> > This patch series adds OP_MAP_REPEAT flag, which lets the user map a BO
> > region over an address range repeatedly with just one map operation.
> >
> > Sparse resources in the Vulkan API let the user leave regions of a
> > resource unmapped (from the API perspective.) Accesses to such regions
> > must not result in program termination, but loads produce undefined
> > values.
> >
> > To implement this feature on Mali hardware, Vulkan sparse unmap is
> > implemented by mapping the specified region to a "dummy bo" so that the
> > accesses do not fault. A newly created sparse resource starts off
> > unmapped, and therefore also has to be mapped to the "dummy bo".  This
> > "dummy bo" is small (a page size) in comparison to the sizes of va
> > ranges that we might want to map to it, and a large number of vm_bind
> > ops can be necessary. For example, if the user were to create a
> > 100e6-byte sparse resident resource, we'd have to poke VM_BIND with
> > ceil(100e6/0x1000)=24415 map operations.
> 
> So other drivers pass a NULL (xe) operation to their VM BIND which
> then goes into the kernel and is handled in the backend to write the
> special sparse PT entry.

Other drivers (asahi) have repeated repeated mapping of a single page
baked in their UAPI, see DRM_ASAHI_BIND_SINGLE_PAGE.
The out of tree downstream driver uses an earlier version of the DRM
gpuvm changes to support DRM_ASAHI_BIND_SINGLE_PAGE.
The upstream mesa driver uses this to implement Vulkan sparse resources.

> Can't you just do the same thing in the driver backend, have a dummy
> bo allocated in the kernel and map the pages to it when you see a NULL
> mapping,
> 
> or does this mess up some accounting or refcounts?

I suppose this might have worked as well. I don't know what led to the
asahi UAPI but I suspect leaving things up to user-space manage to
manage was one goal.

Janne