On Wed, Nov 18, 2020 at 07:19:07PM -0800, John Stultz wrote: > On Wed, Nov 18, 2020 at 5:22 PM Hyesoo Yu <hyesoo.yu@xxxxxxxxxxx> wrote: > > > > On Tue, Nov 17, 2020 at 07:00:54PM -0800, John Stultz wrote: > > > So I suspect Rob will push back on this as he has for other dt > > > bindings related to ion/dmabuf heaps (I tried to push a similar > > > solution to exporting multiple CMA areas via dmabuf heaps). > > > > > > The proposal he seemed to like best was having an in-kernel function > > > that a driver would call to initialize the heap (associated with the > > > CMA region the driver is interested in). Similar to Kunihiko Hayashi's > > > patch here: > > > - https://lore.kernel.org/lkml/1594948208-4739-1-git-send-email-hayashi.kunihiko@xxxxxxxxxxxxx/ > > > > > > The one sticking point for that patch (which I think is a good one), > > > is that we don't have any in-tree users, so it couldn't be merged yet. > > > > > > A similar approach might be good here, but again we probably need to > > > have at least one in-tree user which could call such a registration > > > function. > > > > Thanks for your review. > > > > The chunk heap is not considered for device-specific reserved memory and specific driver. > > It is similar to system heap, but it only collects high-order pages by using specific cma-area for performance. > > So, yes I agree, the chunk heap isn't device specific. It's just that > the CMA regions usually are tied to devices. > > The main objection to this style of solution has been due to the fact > that the DTS is supposed to describe the physical hardware (in an OS > agnostic way), rather than define configuration info for Linux > software drivers. > > Obviously this can be quibbled about, as the normal way of tying > devices to CMA has some assumptions of what the driver will use that > region for, rather than somehow representing a physical tie between a > memory reservation and a device. Nonetheless, Rob has been hesitant to > take any sort of ION/DmaBuf Heap DT devices, and has been more > interested in some device having the memory reservation reference and > the driver for that doing the registration. > > > It is strange that there is in-tree user who registers chunk heap. > > (Wouldn't it be strange for some users to register the system heap?) > > Well, as there's no reservation/configuration needed, the system heap > can register itself. > > The CMA heap currently only registers the default CMA heap, as we > didn't want to expose all CMA regions and there's otherwise no way to > pick and choose. Yub. dma-buf really need a way to make exclusive CMA area. Otherwise, default CMA would be shared among drivers and introduce fragmentation easily since we couldn't control other drivers. In such aspect, I don't think current cma-heap works if userspace needs big memory chunk. Here, the problem is there is no in-kernel user to bind the specific CMA area because the owner will be random in userspace via dma-buf interface. > > > Is there a reason to use dma-heap framework to add cma-area for specific device ? > > > > Even if some in-tree users register dma-heap with cma-area, the buffers could be allocated in user-land and these could be shared among other devices. > > For exclusive access, I guess, the device don't need to register dma-heap for cma area. > > > > It's not really about exclusive access. More just that if you want to > bind a memory reservation/region (cma or otherwise), at least for DTS, > it needs to bind with some device in DT. > > Then the device driver can register that region with a heap driver. > This avoids adding new Linux-specific software bindings to DT. It > becomes a driver implementation detail instead. The primary user of > the heap type would probably be a practical pick (ie the display or > isp driver). If it's the only solution, we could create some dummy driver which has only module_init and bind it from there but I don't think it's a good idea. > > The other potential solution Rob has suggested is that we create some > tag for the memory reservation (ie: like we do with cma: "reusable"), > which can be used to register the region to a heap. But this has the > problem that each tag has to be well defined and map to a known heap. Do you think that's the only solution to make progress for this feature? Then, could you elaborate it a bit more or any other ideas from dma-buf folks?
