Re: [RFC PATCH v3 0/3] new subsystem for compute accelerator devices

[Jason Gunthorpe]

On Sun, Nov 06, 2022 at 11:02:22PM +0200, Oded Gabbay wrote:
> Another important change is that I have reverted back to use IDR for minor
> handling instead of xarray. This is because I have found that xarray doesn't
> handle well the scenario where you allocate a NULL entry and then exchange it
> with a real pointer. It appears xarray still considers that entry a "zero"
> entry. This is unfortunate because DRM works that way (first allocates a NULL
> entry and then replaces the entry with a real pointer).

This is what XA_ZERO_ENTRY is for.

Some APIs, like xa_alloc automatically promote NULL to XA_ZERO_ENTRY,
others require it to be explicit.

If you use the usual pattern of xa_alloc(NULL), xa_store(!NULL) then
you should be fine, as far as I know. So long as the xarray was tagged
as allocating.

Jason