On Mon, Apr 28, 2025 at 7:52 AM Maxime Ripard <mrip...@kernel.org> wrote: > On Fri, Apr 25, 2025 at 12:39:40PM -0700, John Stultz wrote: > > To your larger point about policy, I do get the tension that you want > > to be able to programmatically derive or evaluate heap names, so that > > applications can consistently derive a pathname to get what they want. > > We've discussed it in the past, I don't really want to. But it was clear > from the last discussion that you (plural) wanted to infer heap > semantics from the names. I'm ok with that, but then if we want to make > it work we need to have well defined names.
So my name keeps on getting attached to that, but I don't think I was involved in the LPC conversation when that got decided. > And it's actually what I really want to discuss here: we've discussed at > length how bad the heaps name are (and not only here), but I don't think > we have any documented policy on what makes a good name? I very much think having a policy/guidance for better names is a good goal. I just want to make sure it doesn't become a strict policy that lead folks to make mistaken assumptions about a static solution being viable in userland (like folks nostalgicly using "eth0" or a fixed network device name in scripts expecting it to work on a different system) > For example, I'm not sure exposing the allocator name is a good idea: > it's an implementation detail and for all userspace cares about, we > could change it every release if it provided the same kind of buffers. That is a fair point. > Taking your camera buffers example before, then we could also expose a > memory region id, and let the platform figure it out, or use the usecase > as the name. > > But if we don't document that, how can we possibly expect everyone > including downstream to come up with perfect names every time. And FTR, > I'm willing to write that doc down once the discussion settles. So again, yeah, I very much support having better guidance on the names. I think the number of device constraints and device combinations makes a raw enumeration of things difficult. This is why the per-device use->heap mapping "glue" seems necessary to me. And, I do get that this runs into a similar problem with enumerating and defining "uses" (which boil down to a combination of devices-in-a-pipeline and access use patterns), but for Andorid it has so far been manageable. Personally, I think the best idea I've heard so far to resolve this from userland was Christian's suggestion that devices expose links to compatible heaps, and then userland without a use->heap mapping could for the set of devices they plan to use in a pipeline, figure out the common heap name and use that to allocate. However, that pushes the problem down a bit, requiring drivers (instead of userland) to know what heaps they can work with and what the names might be (which again, your goal for standardizing the heap names isn't a bad thing!). Though, this approach also runs into trouble as it opens a question of: should it only encode strict constraint satisfaction, or something more subtle, as while something might work with multiple heaps, its possible it won't be performant enough unless it picks a specific one on device A or a different one on device B. And getting that sort of device-specific details embedded into a driver isn't great either. > > But I also think that there is so much variety in both the devices and > > uses that there is no way that all use cases and all devices can be > > satisfied with such a static or even programmatic mapping. From my > > perspective, there just is going to have to be some device specific > > glue logic that maps use->heap name. Same reason we have fstab and the > > passwd file. > > fstab and passwd can be generated at (first) boot time / install. fstab > is also being somewhat less important with the auto-partition discovery. > How would you generate that configuration file at boot? > > I'm not really asking this as a theoretical question. Being able to > auto-discover which heap a driver/device would allocate from is central > for the cgroup work I mentioned earlier. > > And I'm really not sure how distros or applications developpers are > supposed to keep up with the raw volume of devices that go out every > year, each and every one of them having different heap names, etc. > Possibly different from one version of the firmware to another. For generic distros, I don't have a good answer here. Historically the focus has always been on single device usage, so having the driver do the allocation was fine, and if you were using multiple devices you could just copy the memory between the driver allocated buffers. But as we've moved to disaggregated IP blocks and device pipelines, all those potential copies wreck performance and power. I'm not sure generic distros have the concept of a device pipeline very well abstracted (obviously mesa and the wayland/X have had to deal with it, and the video and camera side is dealing with it more and more). Maybe a more established notion of use -> pipeline/device collections, is needed as a starting point? Then using Christian's suggestion, one could at least enumerate use -> heap that would be functional. And maybe device makers could then supplement explicit optimized mapping overrides for their device? I just think leaving individual applications (or even individual frameworks like mesa) to embed assumptions about heap names -> functionality is going to be a problematic approach. thanks -john
