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 <dan...@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
