Re: [RFC][PATCH 2/2] dma-buf: heaps: Fix the name used when exporting dmabufs to be the actual heap name

[Daniel Vetter]

On Mon, Feb 8, 2021 at 9:51 PM John Stultz <john.stultz@xxxxxxxxxx> wrote:
> On Mon, Feb 8, 2021 at 2:08 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > On Sat, Feb 06, 2021 at 05:47:48AM +0000, John Stultz wrote:
> > > By default dma_buf_export() sets the exporter name to be
> > > KBUILD_MODNAME. Unfortunately this may not be identical to the
> > > string used as the heap name (ie: "system" vs "system_heap").
> > >
> > > This can cause some minor confusion with tooling, and there is
> > > the future potential where multiple heap types may be exported
> > > by the same module (but would all have the same name).
> > >
> > > So to avoid all this, set the exporter exp_name to the heap name.
> > >
> > > Cc: Daniel Vetter <daniel@xxxxxxxx>
> > > Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> > > Cc: Liam Mark <lmark@xxxxxxxxxxxxxx>
> > > Cc: Chris Goldsworthy <cgoldswo@xxxxxxxxxxxxxx>
> > > Cc: Laura Abbott <labbott@xxxxxxxxxx>
> > > Cc: Brian Starkey <Brian.Starkey@xxxxxxx>
> > > Cc: Hridya Valsaraju <hridya@xxxxxxxxxx>
> > > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > Cc: Sandeep Patil <sspatil@xxxxxxxxxx>
> > > Cc: Daniel Mentz <danielmentz@xxxxxxxxxx>
> > > Cc: Ørjan Eide <orjan.eide@xxxxxxx>
> > > Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > > Cc: Ezequiel Garcia <ezequiel@xxxxxxxxxxxxx>
> > > Cc: Simon Ser <contact@xxxxxxxxxxx>
> > > Cc: James Jones <jajones@xxxxxxxxxx>
> > > Cc: linux-media@xxxxxxxxxxxxxxx
> > > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > > Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> >
> > Looks reasonable to me.
> >
> > I guess the main worry is "does this mean heap names become uapi", in
> > which case I'm maybe not so sure anymore how this will tie into the
> > overall gpu memory accounting story.
> >
> > Since for dma-buf heaps one name per buffer is perfectly fine, since
> > dma-buf heaps aren't very dynamic. But on discrete gpu drivers buffers
> > move, so baking in the assumption that "exporter name = resource usage for
> > this buffer" is broken.
>
> I suspect I'm missing a subtlety in what you're describing. My sense
> of the exporter name doesn't account for a buffer's usage, it just
> describes what code allocated it and implicitly which dmabuf_ops
> handles it.  Maybe could you give a more specific example of what
> you're hoping to avoid?

Just paranoia really - on the linux side where we allocate most
buffers (even shared ones) with the driver, that allocator info isn't
that meaningful, it really just tells you which code
allocated/exported that dma-buf.

But on Android, where all shared buffers come from specific heaps, it
is rather meaningful information. So I wondered whether e.g. the
android dmabuf debug tool uses that to collect per-heap stats, but
sounds like no right now. Plus with the chat we've had I think we have
a long-term plan for how to expose that information properly.

> To me this patch is mostly just a consistency/least-surprise thing, so
> the heaps exporter name matches the string used for the heap's chardev
> device (the interface used to allocate it) in output like
> debugfs/dma_buf/bufinfo.

Yeah for debug this makes sense. a-b: me if you want that somewhere on
the patches.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch