Hi James, On Sun, Aug 23, 2020 at 01:04:43PM -0700, James Jones wrote: > On 8/20/20 1:15 AM, Ezequiel Garcia wrote: > > On Mon, 2020-08-17 at 20:49 -0700, James Jones wrote: > >> On 8/17/20 8:18 AM, Brian Starkey wrote: > >>> On Sun, Aug 16, 2020 at 02:22:46PM -0300, Ezequiel Garcia wrote: > >>>> This heap is basically a wrapper around DMA-API dma_alloc_attrs, > >>>> which will allocate memory suitable for the given device. > >>>> > >>>> The implementation is mostly a port of the Contiguous Videobuf2 > >>>> memory allocator (see videobuf2/videobuf2-dma-contig.c) > >>>> over to the DMA-BUF Heap interface. > >>>> > >>>> The intention of this allocator is to provide applications > >>>> with a more system-agnostic API: the only thing the application > >>>> needs to know is which device to get the buffer for. > >>>> > >>>> Whether the buffer is backed by CMA, IOMMU or a DMA Pool > >>>> is unknown to the application. > >>>> > >>>> I'm not really expecting this patch to be correct or even > >>>> a good idea, but just submitting it to start a discussion on DMA-BUF > >>>> heap discovery and negotiation. > >>>> > >>> > >>> My initial reaction is that I thought dmabuf heaps are meant for use > >>> to allocate buffers for sharing across devices, which doesn't fit very > >>> well with having per-device heaps. > >>> > >>> For single-device allocations, would using the buffer allocation > >>> functionality of that device's native API be better in most > >>> cases? (Some other possibly relevant discussion at [1]) > >>> > >>> I can see that this can save some boilerplate for devices that want > >>> to expose private chunks of memory, but might it also lead to 100 > >>> aliases for the system's generic coherent memory pool? > >>> > >>> I wonder if a set of helpers to allow devices to expose whatever they > >>> want with minimal effort would be better. > >> > >> I'm rather interested on where this goes, as I was toying with using > >> some sort of heap ID as a basis for a "device-local" constraint in the > >> memory constraints proposals Simon and I will be discussing at XDC this > >> year. It would be rather elegant if there was one type of heap ID used > >> universally throughout the kernel that could provide a unique handle for > >> the shared system memory heap(s), as well as accelerator-local heaps on > >> fancy NICs, GPUs, NN accelerators, capture devices, etc. so apps could > >> negotiate a location among themselves. This patch seems to be a step > >> towards that in a way, but I agree it would be counterproductive if a > >> bunch of devices that were using the same underlying system memory ended > >> up each getting their own heap ID just because they used some SW > >> framework that worked that way. > >> > >> Would appreciate it if you could send along a pointer to your BoF if it > >> happens! > > > > Here is it: > > > > https://linuxplumbersconf.org/event/7/contributions/818/ > > > > It would be great to see you there and discuss this, > > given I was hoping we could talk about how to meet a > > userspace allocator library expectations as well. > > Thanks! I hadn't registered for LPC and it looks like it's sold out, > but I'll try to watch the live stream. > > This is very interesting, in that it looks like we're both trying to > solve roughly the same set of problems but approaching it from different > angles. From what I gather, your approach is that a "heap" encompasses > all the allocation constraints a device may have. > > The approach Simon Ser and I are tossing around so far is somewhat > different, but may potentially leverage dma-buf heaps a bit as well. > > Our approach looks more like what I described at XDC a few years ago, > where memory constraints for a given device's usage of an image are > exposed up to applications, which can then somehow perform boolean > intersection/union operations on them to arrive at a common set of > constraints that describe something compatible with all the devices & > usages desired (or fail to do so, and fall back to copying things around > presumably). I believe this is more flexible than your initial proposal > in that devices often support multiple usages (E.g., different formats, > different proprietary layouts represented by format modifiers, etc.), > and it avoids adding a combinatorial number of heaps to manage that. > > In my view, heaps are more like blobs of memory that can be allocated > from in various different ways to satisfy constraints. I realize heaps > mean something specific in the dma-buf heap design (specifically, > something closer to an association between an "allocation mechanism" and > "physical memory"), but I hope we don't have massive heap/allocator > mechanism proliferation due to constraints alone. Perhaps some > constraints, such as contiguous memory or device-local memory, are > properly expressed as a specific heap, but consider the proliferation > implied by even that simple pair of examples: How do you express > contiguous device-local memory? Do you need to spawn two heaps on the > underlying device-local memory, one for contiguous allocations and one > for non-contiguous allocations? Seems excessive. > > Of course, our approach also has downsides and is still being worked on. > For example, it works best in an ideal world where all the allocators > available understand all the constraints that exist. Shouldn't allocators be decoupled of constraints ? In my imagination I see devices exposing constraints, and allocators exposing parameters, with a userspace library to reconcile the constraints and produce allocator parameters from them. > Dealing with a > reality where there are probably a handful of allocators, another > handful of userspace libraries and APIs, and still more applications > trying to make use of all this is one of the larger remaining challenges > of the design. > > We'll present our work at XDC 2020. Hope you can check that out as well! > > >>> 1. https://lore.kernel.org/dri-devel/57062477-30e7-a3de-6723-a50d03a402c4@xxxxxxxx/ > >>> > >>>> Given Plumbers is just a couple weeks from now, I've submitted > >>>> a BoF proposal to discuss this, as perhaps it would make > >>>> sense to discuss this live? > >>>> > >>>> Not-signed-off-by: Ezequiel Garcia <ezequiel@xxxxxxxxxxxxx> -- Regards, Laurent Pinchart
