On 12/06/2024 22:35, Mauro Carvalho Chehab wrote: > Em Wed, 12 Jun 2024 17:22:34 +0900 > Tomasz Figa <tfiga@xxxxxxxxxxxx> escreveu: > >> On Wed, Jun 12, 2024 at 4:54 PM Mauro Carvalho Chehab >> <mchehab@xxxxxxxxxx> wrote: >>> >>> Em Wed, 12 Jun 2024 08:46:50 +0200 >>> Hans Verkuil <hverkuil@xxxxxxxxx> escreveu: >>> >>>> On 6/12/24 06:12, Tomasz Figa wrote: >>>>> On Wed, May 15, 2024 at 1:19 AM Daniel Almeida >>>>> <daniel.almeida@xxxxxxxxxxxxx> wrote: >>>>>> >>>>>> Hi Hans, all, >>>>>> >>>>>> I’d like to attend in person and discuss the use of Rust in the subsystem, especially in light of [0] and [1]. >>>>>> >>>>>> Please note that these are new submissions that are unrelated with what was discussed last year. >>>>>> >>>>>> 30 minutes will do. >>>>>> >>>>>> [0] https://lwn.net/ml/linux-media/20240227215146.46487-1-daniel.almeida@xxxxxxxxxxxxx/ >>>>>> [1] https://lwn.net/Articles/970565 >>>>> >>>>> Somewhat related to the topic: I see potential for a quite big >>>>> redesign of the videobuf2 framework going forward and recently with >>>>> more Rust adoption I'm starting to think it could benefit from being >>>>> implemented in Rust, since we would have to rewrite it quite a bit >>>>> anyway. Especially since it's a part of the subsystem that has to deal >>>>> with memory management, object lifetime and asynchronousness quite a >>>>> lot and we had a history of issues there. So it could be interesting >>>>> to hear everyone's thoughts. >>>> >>>> I think it is far too soon to write a framework like that in Rust. >>> >>> Agreed. I don't object redesigns in C to make it better - which could have >>> some colateral effect of making things easier for a future Rust adoption, >>> but such changes should be justified by themselves, and not because of a >>> language change. >> >> No, the thought of redesign doesn't come from the language change, >> it's the other way around. Since rewriting a lot of the code already, >> why not do it in a language that is generally considered better. > > As Hans said, Rast has experimental support. We can't have drivers > depending on experimental stuff. Indeed. While discussing Rust for experimental drivers or codec libraries is interesting (and I am doing a Rust course, so hopefully I have a better understanding of what's involved by the upcoming media summit), using it for core media frameworks is simply a hard NACK until Linus blesses Rust as a second kernel language. So don't spend your valuable time on that. > >> >>> >>> See: redesigns at the core will potentially affect lots of drivers, >>> so it needs very good technical reasons why doing it. Plus, it requires >>> comprehensive tests with different types of hardware/drivers to reduce the >>> risk of regressions. Depending on the changes, it may require extra tests >>> with devices that are outside complex camera world: radio, analog and digital >>> TV drivers - and even some input devices that use VB2 - to ensure that >>> nothing broke. >> >> We don't have to do it in an all-or-nothing way. We can start with an >> experimental new implementation in Rust, which could be gradually >> tested. It could even be done the same way as the vb -> vb2 >> transition, although I suspect it wouldn't really be necessary, as I >> would like to see it more like a drop-in replacement. In general I >> think the API exposed outside of the framework wouldn't really change >> that much, it's more about the internal design. It makes no sense to have a C and a Rust version of vb2. This framework is critical to all drivers, and we're not going to support two versions and fix bugs/add features in two places. Again, it's a hard NACK. Don't waste time on that. If there are ideas to make vb2 better, then I am all for that. I just want to mention two things here: For most drivers, using vb2 is just fine: the work a driver needs to do is quite straightforward. Exceptions are codec drivers and possibly complex camera drivers when they need to use requests (not certain yet). Internally vb2 is quite complex, but that's because what it does is quite complex. And that's fine. If the internal structure can be improved to make it less complex, then I'm all for that, but there is no magic bullet (including using Rust instead of C) that suddenly makes everything simple. Generally I prefer to have the complexity in core frameworks, that will only make life easier for the driver developers. To summarize: Until Rust is accepted by Linus as a second kernel language, as media maintainer I will NACK core media frameworks written in Rust. I won't spend time on it, it's an immediate NACK from me. Note that this doesn't imply that once Linus *does* accept Rust, that we are OK with core frameworks written in Rust. But that will be a separate discussion once that happens. Regards, Hans > > Having two implementations of the same logic doesn't sound reasonable, > as it doubles the maintainership effort: all changes done on one > implementation needs to be moved to the other one. > > Btw, we also have seem this problem before with VB and, up to some > sense, with VB2, as some drivers used to have their own buffer > handling implementation that usually started from a VB or VB2 fork. > > So, if VB2 has issues, let's fix it in C code. > >>>> To be >>>> honest, I won't even consider it until Linus officially accepts Rust as a >>>> second language in the kernel, instead of as an experiment. >>> >>> This is not enough: if the core starts to use a second language, all media >>> developers will be affected and will be required to have expertise on such >>> language. >> >> Let's be realistic, how many developers are actively touching vb2 these days? > > How many developers don't need VB2? Hopefully none :-) > >>> That's not something that should happen without careful >>> analysis and plans that should include a gradual roll-up, lost of tests >>> with the affected drivers including the legacy ones and some strategy to >>> quickly solve regression issues. >> >> That said, I agree. It needs proper discussion and planning. That's >> why I'm proposing this as a topic. :) >> Moreover the redesign itself also needs proper discussion and is more >> of a long term goal, not something to land in the next few days. >> >>> >>> It is not a matter of what language is better. Instead, it is a matter of >>> not affecting code maintenance during the (probably long) transition period >>> and beyond. >>> >>> If you see the past history, the transition from V4L to V4L2 took more than 10 >>> years - being possible to be done only with the help of libv4l, plus a >>> lot of backward-compat code that we added. Still there were several >>> regressions and we even had to quickly patch the Kernel and/or some apps >>> that were using the uAPI on different ways. >> >> That's a different situation, because UAPI is involved. > > It is different, but similar, up to some sense, as a change at VB2 > implementation will likely affect its kAPI, its behavior or both. > > The point I'm underlining is that core redesigns do affect existing > drivers usually on unexpected ways. > >> >>> >>> Yet, the transition from VB1 to VB2 was also painful, and took a lot of time. >>> >> >> Yes, vb -> vb2 would be a more appropriate comparison. >> >>> On both cases, there were very good technical reasons for the transition, >>> in terms of missing needed features, broken memory models and serious >>> troubles that utterly causing VB1 to not work well on non-x86 hardware. >>> >> >> It's a very similar situation now, vb2 doesn't work well on modern >> hardware, but I still have hopes that it can be fixed without >> affecting the driver-facing behavior. (We would probably need to >> develop some unit tests that validate the driver-facing behavior to >> ensure that.) >> >>> In the end, the authors of the core changes need to acquire legacy hardware >>> and to do lots of driver-specific changes to avoid breaking existing stuff. >>> Hans and I had to dedicate a lot of time and efforts on such transitions, >>> as it required a lot of work. >>> >>> I can tell you: there's no fun on such changes: typically, companies won't >>> pay someone to do changes on drivers for legacy hardware, specially >>> when there are no real benefits, which is the case here, as the final result >>> is just to keep the existing drivers to work with existing hardware, >>> usually without any new features. So, the ones behind such core changes >>> have to commit fixing drivers usually on their spare time. >>> >> >> I don't get that argument. Wouldn't the same apply to any core change? > > It depends of the type of change. For instance, an addition of a new > V4L2 control should not cause regressions to existing drivers. The > same would be true if one adds a new memory allocation component for > VB2 (e. g. something similar to videobuf2-vmalloc.c/videobuf2-dma-sg.c/..): > only drivers using the new way would be affected. > >> I think the reason we have driver maintainers is that they can help >> with testing. Moreover, we need to invest into testing infrastructure >> (which is what people have been doing recently via Media CI) to make >> such changes less painful. Otherwise the subsystem will just bit-rot >> and become useful for modern use cases. > > Using CI to check for uAPI/kAPI changes is helpful, but it doesn't cover > actual drivers. For that, we would need to invest on a CI solution > integrated with lots of different hardware pieces, to check for actual > driver regressions. > > On one of my previous work, the company I used to work had that: they > had some monitors display some things, and the camera captured input > were compared to what the monitor were actually displaying. Doable, but > expensive. > > Regards, > Mauro > > Thanks, > Mauro
