Re: [PATCH 4/9] dma-buf: heaps: Add debugfs support

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Sep 02, 2025 at 03:37:45PM -0700, John Stultz wrote:
> On Tue, Sep 2, 2025 at 8:46 AM Thierry Reding <thierry.reding@gmail.com> wrote:
> >
> > From: Thierry Reding <treding@nvidia.com>
> >
> > Add a callback to struct dma_heap_ops that heap providers can implement
> > to show information about the state of the heap in debugfs. A top-level
> > directory named "dma_heap" is created in debugfs and individual files
> > will be named after the heaps.
> >
> 
> I know its debugfs, but this feels a little loosey-goosey as an uAPI.

Well, the whole point of debugfs is that it's not really an ABI. Nothing
should ever rely on the presence of these files.

> Is there any expected format for the show function?
> 
> What would other dmabuf heaps ideally export via this interface?

I've thought about this a bit and I'm not sure it makes sense to
standardize on this. I think on one hand having a list of buffers
exported by the dma-buf heap is probably the lowest common denominator,
but then there might be a bunch of other things that are very heap-
specific that some heap might want to export.

> Is there some consistent dma_heap-ish concept for it to justify it
> being under a dma_heap directory, and not just an independent debugfs
> file for the driver implementing the dmabuf heap?

Well, I think just the fact that it's a dma-heap would qualify its
corresponding debugfs to be in a well-known location. We could of course
pick some arbitrary location, but that's just a recipe for chaos because
then everybody puts these whereever they want. There's really no
standard place for driver-specific debugfs files to go, so putting it
into some "subsystem"-specific directory seems like the better option.

Thierry

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