Re: [PATCH 0/8] dma-buf: heaps: Support carved-out heaps and ECC related-flags

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, May 16, 2024 at 09:51:35AM -0700, John Stultz wrote:
> On Thu, May 16, 2024 at 3:56 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote:
> > > But it makes me a little nervous to add a new generic allocation flag
> > > for a feature most hardware doesn't support (yet, at least). So it's
> > > hard to weigh how common the actual usage will be across all the
> > > heaps.
> > >
> > > I apologize as my worry is mostly born out of seeing vendors really
> > > push opaque feature flags in their old ion heaps, so in providing a
> > > flags argument, it was mostly intended as an escape hatch for
> > > obviously common attributes. So having the first be something that
> > > seems reasonable, but isn't actually that common makes me fret some.
> > >
> > > So again, not an objection, just something for folks to stew on to
> > > make sure this is really the right approach.
> >
> > Another good reason to go with full heap names instead of opaque flags on
> > existing heaps is that with the former we can use symlinks in sysfs to
> > specify heaps, with the latter we need a new idea. We haven't yet gotten
> > around to implement this anywhere, but it's been in the dma-buf/heap todo
> > since forever, and I like it as a design approach. So would be a good idea
> > to not toss it. With that display would have symlinks to cma-ecc and cma,
> > and rendering maybe cma-ecc, shmem, cma heaps (in priority order) for a
> > SoC where the display needs contig memory for scanout.
> 
> So indeed that is a good point to keep in mind, but I also think it
> might re-inforce the choice of having ECC as a flag here.
> 
> Since my understanding of the sysfs symlinks to heaps idea is about
> being able to figure out a common heap from a collection of devices,
> it's really about the ability for the driver to access the type of
> memory. If ECC is just an attribute of the type of memory (as in this
> patch series), it being on or off won't necessarily affect
> compatibility of the buffer with the device.  Similarly "uncached"
> seems more of an attribute of memory type and not a type itself.
> Hardware that can access non-contiguous "system" buffers can access
> uncached system buffers.

Yeah, but in graphics there's a wide band where "shit performance" is
defacto "not useable (as intended at least)".

So if we limit the symlink idea to just making sure zero-copy access is
possible, then we might not actually solve the real world problem we need
to solve. And so the symlinks become somewhat useless, and we need to
somewhere encode which flags you need to use with each symlink.

But I also see the argument that there's a bit a combinatorial explosion
possible. So I guess the question is where we want to handle it ...

Also wondering whether we should get the symlink/allocator idea off the
ground first, but given that that hasn't moved in a decade it might be too
much. But then the question is, what userspace are we going to use for all
these new heaps (or heaps with new flags)?

Cheers, Sima

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch