On 03/06/2017 07:52 AM, Daniel Vetter wrote: > On Mon, Mar 06, 2017 at 03:43:53PM +0200, Laurent Pinchart wrote: >> Hi Daniel, >> >> On Monday 06 Mar 2017 11:32:04 Daniel Vetter wrote: >>> On Fri, Mar 03, 2017 at 10:50:20AM -0800, Laura Abbott wrote: >>>> On 03/03/2017 08:41 AM, Laurent Pinchart wrote: >>>>> On Thursday 02 Mar 2017 13:44:42 Laura Abbott wrote: >>>>>> When CMA was first introduced, its primary use was for DMA allocation >>>>>> and the only way to get CMA memory was to call dma_alloc_coherent. This >>>>>> put Ion in an awkward position since there was no device structure >>>>>> readily available and setting one up messed up the coherency model. >>>>>> These days, CMA can be allocated directly from the APIs. Switch to >>>>>> using this model to avoid needing a dummy device. This also avoids >>>>>> awkward caching questions. >>>>> >>>>> If the DMA mapping API isn't suitable for today's requirements anymore, >>>>> I believe that's what needs to be fixed, instead of working around the >>>>> problem by introducing another use-case-specific API. >>>> >>>> I don't think this is a usecase specific API. CMA has been decoupled from >>>> DMA already because it's used in other places. The trying to go through >>>> DMA was just another layer of abstraction, especially since there isn't >>>> a device available for allocation. >>> >>> Also, we've had separation of allocation and dma-mapping since forever, >>> that's how it works almost everywhere. Not exactly sure why/how arm-soc >>> ecosystem ended up focused so much on dma_alloc_coherent. >> >> I believe because that was the easy way to specify memory constraints. The API >> receives a device pointer and will allocate memory suitable for DMA for that >> device. The fact that it maps it to the device is a side-effect in my opinion. >> Agreed. The device Ion wanted to use was never a real device though so any constraints it satisfied were making assumptions about what memory would be allocated. >>> I think separating allocation from dma mapping/coherency is perfectly >>> fine, and the way to go. >> >> Especially given that in many cases we'll want to share buffers between >> multiple devices, so we'll need to map them multiple times. >> >> My point still stands though, if we want to move towards a model where >> allocation and mapping are decoupled, we need an allocation function that >> takes constraints (possibly implemented with two layers, a constraint >> resolution layer on top of a pool/heap/type/foo-based allocator), and a >> mapping API. IOMMU handling being integrated in the DMA mapping API we're >> currently stuck with it, which might call for brushing up that API. > > Hm, maybe I wasn't clear, but that's exactly what I assume will happen: > > The constraint resolver is the unix device memory allocation thing, which > happens entirely in userspace. There's a lot more than just "where to > allocate" to negotiate, e.g. pixel format, stride/size > limits/requirements, tiling formats. A lot of it the kernel doesn't even > know. > > Allocation then needs to happen through the kernel ofc, but that doesn't > mean we need to have all the constraint resolving in the kernel. As long > as the kernel exposes the device /dev node -> ion heap stuff, userspace > can figure this out. Or an alternative way would be to have a cascade of > ion heaps to keep things a notch more opaque. Either way, no actaul > constraint resolving in the kernel itself, and except for a bunch more > stuff in sysfs maybe, also no other uapi changes. Once we have a place to > allocate stuff which isn't the device driver at least, aka ION. > > And then once allocated you use the dma apis to instantiate the iommus > mappings. > > Anyway, at least from my understanding I think there's 0 risk with merging > ION wrt the constraint resolving side (at least as discussed around XDC > last year), and for setups that need cma, it might finally enable to get > things moving forward. > > Or do I miss something big here? > -Daniel > This all sounds like what I was thinking. I think some of the concerns may be that the details of constraint solving are mostly handwaving right now. Thanks, Laura
