On Mon, May 13, 2024 at 11:06:24AM -0400, Nicolas Dufresne wrote: > Le lundi 13 mai 2024 à 15:51 +0200, Maxime Ripard a écrit : > > On Mon, May 13, 2024 at 09:42:00AM -0400, Nicolas Dufresne wrote: > > > Le lundi 13 mai 2024 à 10:29 +0200, Maxime Ripard a écrit : > > > > On Wed, May 08, 2024 at 10:36:08AM +0200, Daniel Vetter wrote: > > > > > On Tue, May 07, 2024 at 04:07:39PM -0400, Nicolas Dufresne wrote: > > > > > > Le mardi 07 mai 2024 à 21:36 +0300, Laurent Pinchart a écrit : > > > > > > > Shorter term, we have a problem to solve, and the best option we have > > > > > > > found so far is to rely on dma-buf heaps as a backend for the frame > > > > > > > buffer allocatro helper in libcamera for the use case described above. > > > > > > > This won't work in 100% of the cases, clearly. It's a stop-gap measure > > > > > > > until we can do better. > > > > > > > > > > > > Considering the security concerned raised on this thread with dmabuf heap > > > > > > allocation not be restricted by quotas, you'd get what you want quickly with > > > > > > memfd + udmabuf instead (which is accounted already). > > > > > > > > > > > > It was raised that distro don't enable udmabuf, but as stated there by Hans, in > > > > > > any cases distro needs to take action to make the softISP works. This > > > > > > alternative is easy and does not interfere in anyway with your future plan or > > > > > > the libcamera API. You could even have both dmabuf heap (for Raspbian) and the > > > > > > safer memfd+udmabuf for the distro with security concerns. > > > > > > > > > > > > And for the long term plan, we can certainly get closer by fixing that issue > > > > > > with accounting. This issue also applied to v4l2 io-ops, so it would be nice to > > > > > > find common set of helpers to fix these exporters. > > > > > > > > > > Yeah if this is just for softisp, then memfd + udmabuf is also what I was > > > > > about to suggest. Not just as a stopgap, but as the real official thing. > > > > > > > > > > udmabuf does kinda allow you to pin memory, but we can easily fix that by > > > > > adding the right accounting and then either let mlock rlimits or cgroups > > > > > kernel memory limits enforce good behavior. > > > > > > > > I think the main drawback with memfd is that it'll be broken for devices > > > > without an IOMMU, and while you said that it's uncommon for GPUs, it's > > > > definitely not for codecs and display engines. > > > > > > In the context of libcamera, the allocation and the alignment done to the video > > > frame is done completely blindly. In that context, there is a lot more then just > > > the allocation type that can go wrong and will lead to a memory copy. The upside > > > of memfd, is that the read cache will help speeding up the copies if they are > > > needed. > > > > dma-heaps provide cacheable buffers too... > > Yes, and why we have cache hints in V4L2 now. There is no clue that softISP code > can read to make the right call. The required cache management in undefined > until all the importer are known. I also don't think heaps currently care to > adapt the dmabuf sync behaviour based on the different importers, or the > addition of a new importer. On top of which, there is insufficient information > on the device to really deduce what is needed. > > > > Another important point is that this is only used if the application haven't > > > provided frames. If your embedded application is non-generic, and you have > > > permissions to access the right heap, the application can solve your specific > > > issue. But in the generic Linux space, Linux kernel API are just insufficient > > > for the "just work" scenario. > > > > ... but they also provide semantics around the memory buffers that no > > other allocation API do. There's at least the mediatek secure playback > > series and another one that I've started to work on to allocate ECC > > protected or unprotected buffers that are just the right use case for > > the heaps, and the target frameworks aren't. > > Let's agree we are both off topic now. The libcamera softISP is currently purely > software, and cannot write to any form of protected memory. As for ECC, I would > hope this usage will be coded in the application and that this application has > been authorized to access the appropriate heaps. > > And finally, none of this fixes the issue that the heap allocation are not being > accounted properly and allow of an easy memory DoS. So uaccess should be granted > with care, meaning that defaulting a "desktop" library to that, means it will > most of the time not work at all. I think that issue should be fixed, regardless of whether or not we end up using dma heaps for libcamera. If we do use them, maybe there will be a higher incentive for somebody involved in this conversation to tackle that problem first :-) And maybe, as a result, the rest of the Linux community will consider with a more open mind usage of dma heaps on desktop systems. -- Regards, Laurent Pinchart
