Hi James, On Sun, Aug 23, 2020 at 03:53:50PM -0700, James Jones wrote: > On 8/23/20 1:46 PM, Laurent Pinchart wrote: > > 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. > > Perhaps another level of abstraction would help. I'll have to think > about that. > > However, as far as I can tell, it wouldn't remove the need to > communicate a lot of constraints from multiple engines/devices/etc. to > the allocator (likely a single allocator. I'd be interested to know if > anyone has a design that effectively uses multiple allocators to satisfy > a single allocation request, but I haven't come up with a good one) > somehow. Either the constraints are directly used as the parameters, or > there's a translation/second level of abstraction, but either way much > of the information needs to make it to the allocator, or represent the > need to use a particular allocator. Simple things like pitch and offset > alignment can be done without help from a kernel-level allocator, but > others such as cache coherency, physical memory bank placement, or > device-local memory will need to make it all the way down to the kernel > some how I believe. I fully agree that we'll need kernel support, but I don't think the constraints reporting API and the allocator API need to speak the same language. For instance, drivers will report alignment constraints, the userspace allocator library will translate that to a pitch value, and pass it to the allocator as an allocation parameter. The allocator won't know about alignment constraints. That's a simple example, let's see how it turns out with more complex constraints. With a centralized userspace library we have the ability to decouple the two sides, which I believe can be useful to keep the complexity of constraints and allocation parameters (as) low (as possible). > >> 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
