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

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

On 5/18/26 13:53, Christoph Hellwig wrote:
> On Mon, May 18, 2026 at 11:14:09AM +0100, Pavel Begunkov wrote:
>>> 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.
> 
> Let's get the current version landed.  If we come up with some kind of
> non-dma dmabuf in the future we can refactor it and move it around.
> I'm a little skeptic we'll be able to share code as long as dmabuf
> is allergic to physical addresses, though.

To be fair, it's not that dma-buf specific. This lib/ code only
does some resv locking, fence waiting and queuing fences, otherwise
all the attaching is done by the driver behind callbacks. Switching
it to some memfd could be pretty simple. But The main thing it'd
need to share is iterator handling like forwarding in the block
layer, and it should be fine as it's already passed as a completely
opaque object with no knowledge about pages / dma / etc. for the
middle layers.

> lib/ is most certainly the wrong place for something that absolutely
> is not library functionality but directly interacts with a few
> subsystems.

It only interacts with dma-buf, and even for dma-buf attachments
are created by the driver. Block, nvme, io_uring are users, either
using the helpers or implementing callbacks.

Ok. Let's assume for the argument's sake it's not dma-buf
specific, if not lib/, where would you put it? I was also
assuming that dma-buf being under drivers/ is rather a relic
of the past rather than the desired location, hmm?


-- 
Pavel Begunkov