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Date: Thu, 23 May 2024 11:45:28 +0200
From: Daniel Vetter <daniel@ffwll.ch>
To: Maxime Ripard <mripard@kernel.org>
Cc: John Stultz <jstultz@google.com>, Rob Herring <robh@kernel.org>,
	Saravana Kannan <saravanak@google.com>,
	Sumit Semwal <sumit.semwal@linaro.org>,
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Subject: Re: [PATCH 0/8] dma-buf: heaps: Support carved-out heaps and ECC
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On Wed, May 22, 2024 at 03:18:02PM +0200, Maxime Ripard wrote:
> On Tue, May 21, 2024 at 02:06:19PM GMT, Daniel Vetter wrote:
> > 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)".
> 
> Right, but "not useable" is still kind of usage dependent, which
> reinforces the need for flags (and possibly some way to discover what
> the heap supports).
> 
> Like, if I just want to allocate a buffer for a single writeback frame,
> then I probably don't have the same requirements than a compositor that
> needs to output a frame at 120Hz.
> 
> The former probably doesn't care about the buffer attributes aside that
> it's accessible by the device. The latter probably can't make any kind
> of compromise over what kind of memory characteristics it uses.
> 
> If we look into the current discussions we have, a compositor would
> probably need a buffer without ECC, non-secure, and probably wouldn't
> care about caching and being physically contiguous.
> 
> Libcamera's SoftISP would probably require that the buffer is cacheable,
> non-secure, without ECC and might ask for physically contiguous buffers.
> 
> As we add more memory types / attributes, I think being able to discover
> and enforce a particular set of flags will be more and more important,
> even more so if we tie heaps to devices, because it just gives a hint
> about the memory being reachable from the device, but as you said, you
> can still get a buffer with shit performance that won't be what you
> want.
> 
> > 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)?
> 
> For ECC here, the compositors are the obvious target. Which loops backs
> into the discussion with John. Do you consider dma-buf code have the
> same uapi requirements as DRM?

Imo yes, otherwise we'll get really funny stuff like people bypass drm's
userspace requirement for e.g. content protected buffers by just shipping
the feature in a dma-buf heap ... Been there, done that.

Also I think especially with interop across components there's a huge
difference between a quick test program toy and the real thing. And
dma-buf heaps are kinda all about cross component interop.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch