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

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

On 5/19/26 07:56, Christoph Hellwig wrote:
> On Mon, May 18, 2026 at 03:23:53PM +0100, Pavel Begunkov wrote:
>> To be fair, it's not that dma-buf specific. This lib/ code only
>> does some resv locking, fence waiting and queuing fences,
> 
> But all the dma resv/fence stuff is pretty tied into the dma-buf
> ecosystem.  I don't think it would really apply to something not
> doing DMA at all.

The point is that those can be separated to reuse the rest.
  >> 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.
> 
> But none of that really sits in the current lib/ code anyway?

It's about naming. E.g. passing a DMABUF_ITER that doesn't have a
dma-buf would be confusing, and then it'll need renaming at all
layers to support the use case.

>>> 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?
> 
> drivers/dma-buf is a pretty natural place for it, I could not thing

_If_ there is no dma mappings, drivers/dma-buf would definitely
be an awkward spot. Just trying to understand your criteria for
placement, let's say of a generic buffer registration code assuming
there is no dma-buf involved at all. Again, just a hypothetical.

> where else you'd place dma-buffers.  I'm not sure how hmm has anything
> to do with it.

Looks there is some confusion. It's was meant as an interjection
with an open question, I didn't mention the HMM subsystem.

-- 
Pavel Begunkov