Hi Daniel, On Fri, Feb 21, 2025 at 3:12 PM Daniel Stone <daniel@xxxxxxxxxxxxx> wrote: > > Hi Sumit, > > On Fri, 21 Feb 2025 at 11:24, Sumit Garg <sumit.garg@xxxxxxxxxx> wrote: > > On Tue, 18 Feb 2025 at 21:52, Daniel Stone <daniel@xxxxxxxxxxxxx> wrote: > > > dma-heaps was created to solve the problem of having too many > > > 'allocate $n bytes from $specialplace' uAPIs. The proliferation was > > > painful and making it difficult for userspace to do what it needed to > > > do. Userspace doesn't _yet_ make full use of it, but the solution is > > > to make userspace make full use of it, not to go create entirely > > > separate allocation paths for unclear reasons. > > > > > > Besides, I'm writing this from a platform that implements SVP not via > > > TEE. I've worked on platforms which implement SVP without any TEE, > > > where the TEE implementation would be at best a no-op stub, and at > > > worst flat-out impossible. > > > > Can you elaborate the non-TEE use-case for Secure Video Path (SVP) a > > bit more? As to how the protected/encrypted media content pipeline > > works? Which architecture support does your use-case require? Is there > > any higher privileged level firmware interaction required to perform > > media content decryption into restricted memory? Do you plan to > > upstream corresponding support in near future? > > You can see the MTK SVP patches on list which use the MTK SMC to mediate it. > > There are TI Jacinto platforms which implement a 'secure' area > configured statically by (IIRC) BL2, with static permissions defined > for each AXI endpoint, e.g. CPU write + codec RW + dispc read. I've > heard of another SoC vendor doing the same, but I don't think I can > share those details. There is no TEE interaction. > > I'm writing this message from an AMD laptop which implements > restricted content paths outside of TEE. I don't have the full picture > of how SVP is implemented on AMD systems, but I do know that I don't > have any TEE devices exposed. > > > Let me try to elaborate on the Secure Video Path (SVP) flow requiring > > a TEE implementation (in general terms a higher privileged firmware > > managing the pipeline as the kernel/user-space has no access > > permissions to the plain text media content): > > > > - [...] > > Yeah, I totally understand the TEE usecase. I think that TEE is a good > design to implement this. I think that TEE should be used for SVP > where it makes sense. > > Please understand that I am _not_ arguing that no-one should use TEE for SVP! > > > > So, again, let's > > > please turn this around: _why_ TEE? Who benefits from exposing this as > > > completely separate to the more generic uAPI that we specifically > > > designed to handle things like this? > > > > The bridging between DMA heaps and TEE would still require user-space > > to perform an IOCTL into TEE to register the DMA-bufs as you can see > > here [1]. Then it will rather be two handles for user-space to manage. > > Yes, the decoder would need to do this. That's common though: if you > want to share a buffer between V4L2 and DRM, you have three handles: > the V4L2 buffer handle, the DRM GEM handle, and the dmabuf you use to > bridge the two. > > > Similarly during restricted memory allocation/free we need another > > glue layer under DMA heaps to TEE subsystem. > > Yep. > > > The reason is simply which has been iterated over many times in the > > past threads that: > > > > "If user-space has to interact with a TEE device for SVP use-case > > then why it's not better to ask TEE to allocate restricted DMA-bufs > > too" > > The first word in your proposition is load-bearing. > > Build out the usecase a little more here. You have a DRMed video > stream coming in, which you need to decode (involving TEE for this > usecase). You get a dmabuf handle to the decoded frame. You need to > pass the dmabuf across to the Wayland compositor. The compositor needs > to pass it to EGL/Vulkan to import and do composition, which in turn > passes it to the GPU DRM driver. The output of the composition is in > turn shared between the GPU DRM driver and the separate KMS DRM > driver, with the involvement of GBM. > > For the platforms I'm interested in, the GPU DRM driver needs to > switch into protected mode, which has no involvement at all with TEE - > it's architecturally impossible to have TEE involved without moving > most of the GPU driver into TEE and destroying performance. The > display hardware also needs to engage protected mode, which again has > no involvement with TEE and again would need to have half the driver > moved into TEE for no benefit in order to do so. The Wayland > compositor also has no interest in TEE: it tells the GPU DRM driver > about the protected status of its buffers, and that's it. > > What these components _are_ opinionated about, is the way buffers are > allocated and managed. We built out dmabuf modifiers for this usecase, > and we have a good negotiation protocol around that. We also really > care about buffer placement in some usecases - e.g. some display/codec > hardware requires buffers to be sourced from contiguous memory, other > hardware needs to know that when it shares buffers with another > device, it needs to place the buffers outside of inaccessible/slow > local RAM. So we built out dma-heaps, so every part of the component > in the stack can communicate their buffer-placement needs in the same > way as we do modifiers, and negotiate an acceptable allocation. > > That's my starting point for this discussion. We have a mechanism to > deal with the fact that buffers need to be shared between different IP > blocks which have their own constraints on buffer placement, avoiding > the current problem of having every subsystem reinvent their own > allocation uAPI which was burying us in impedance mismatch and > confusion. That mechanism is dma-heaps. It seems like your starting > point from this discussion is that you've implemented a TEE-centric > design for SVP, and so all of userspace should bypass our existing > cross-subsystem special-purpose allocation mechanism, and write > specifically to one implementation. I believe that is a massive step > backwards and an immediate introduction of technical debt. > > Again, having an implementation of SVP via TEE makes a huge amount of > sense. Having _most_ SVP implementations via TEE still makes a lot of > sense. Having _all_ SVP implementations eventually be via TEE would > still make sense. But even if we were at that point - which we aren't > - it still doesn't justify telling userspace 'use the generic dma-heap > uAPI for every device-specific allocation constraint, apart from SVP > which has a completely different way to allocate some bytes'. I must admit that I don't see how this makes a significant difference, but then I haven't hacked much in the stacks you're talking about, so I'm going to take your word for it. I've experimented with providing a dma-heap replacing the TEE API. The implementation is more complex than I first anticipated, adding about 400 lines to the patch set. From user space, it looks like another dma-heap. I'm using the names you gave earlier, protected,secure-video, protected,trusted-ui, and protected,secure-video-record. However, I wonder if we shouldn't use "restricted" instead of "protected" since we had agreed to call it restricted memory earlier. I'll soon post this in a v6 and an updated demo. Cheers, Jens
