Re: [PATCH v3 05/10] lib: add dmabuf token infrastructure

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/13/26 09:24, Christoph Hellwig wrote:
> Naming and placement:
> 
> This is about dma-buf based I/O.  So I'd expect it to be named dma-buf-io
> and no io-dmabuf, and live in drivers/dma-buf and not the unrelated lib/.
> But I'd like to hear from the dma-buf maintainers about that.

Looking at what Ming is saying, it'd make more sense to keep some of the
parts like iterator and the file op more flexible and not automatically
imply dma-buf even if it's the main and for now the only medium. I.e.
ublk/fuse can use a similar interface for mapping buffers to the server
even without dma mappings.

I don't know how the API should look like, maybe passing memfd, and dma-buf
supports mmap, but I think it's better to call the op something like
"register_buffer" instead and keep all it in lib/ for the same reasons.

> Config option:  as this unconditionally when DMA_SHARED_BUFFER is enabled,
> why does it need a separate config option?

More clearly marking relevant code, easier to make optional if needed,
and gives some introspection via /proc/config.

> Interface:  io_dmabuf_token_create / ->create_dmabuf_token filling
> in a structure allocated by the caller feels odd.

It's minimising pointer chasing. "token" is mainly used by io_uring in
the hot path, and io_uring just keeps it as a part of a larger struct.
For the same reasons "map" is allocated by the driver.

I can add an extra parameter to io_dmabuf_token_create() for how
many extra bytes to allocate for the caller's use, if that makes
things any better for you, but it was easier to just pass an
already allocated struct.

   My gut feeling
> would be to move most of io_dmabuf_token_create into a helper called
> by ->create_dmabuf_token so that the token is allocated in the
> driver data structure and returned from create_dmabuf_token.

-- 
Pavel Begunkov