On Fri, Nov 15, 2013 at 09:57:17AM +0000, Mark Rutland wrote: > On Fri, Nov 15, 2013 at 01:44:10AM +0000, Olof Johansson wrote: > > The more I start to see early UEFI/ACPI code, the more I am certain > > that we want none of that crap in the kernel. It's making things > > considerably messier, while we're already very busy trying to convert > > everything over and enable DT -- we'll be preempting that effort just > > to add even more boilerplate everywhere and total progress will be > > hurt. > > We are certainly under a lot of pressure with the device tree migration, > and I agree that adding another information source is going to be a > source of pain. However, I'd argue that we're going to encounter pain > regardless of which approach we take. > > I'm of the opinion that the only way we should support ACPI is as a > first-class citizen. We don't need to modify every driver and subsystem > to support ACPI, only those necessary to support the minimal set of > platforms using ACPI. ACPI is new in the arm space, and we can enforce > quality standards on ACPI _now_ above what we're allowing for DT, and > avoid future problems. It's obvious from the very first submission, from a vendor that has worked closely with "the community" (i.e. one enterprise distro at least), that this is completely new territory for _everyone_ involved. There's no way that we can enforce quality standards for ACPI yet -- we don't know what they look like! Nobody knows how to design a good ACPI implementation or binding. Oh wait, there's people who have been doing this for years. Microsoft. They should be the ones driving this and taking the pain for it. Once the platform is enabled for their needs, we'll sort it out at our end. After all, that has worked reasonably well for x86 platforms. If we knew exactly what we wanted, it'd be a different story. _We don't_. We're into year FOUR of the device tree conversion and we're just now reaching a point where we think we know what a good solution looks like. The first two years were easy -- they were the trivial devices we're now talking about on ACPI. After that it got harder. Going through all of that again with ACPI? No. No way. Microsoft gets to do it and while they're busy sorting it out, we'll boot with device tree. Once they're done, we'll figure out how to enable new hardware. Sure, someone needs to keep them sane and participate in the standardization process, but we don't have to drag the whole kernel through it. > There may even be things which we don't have to deal with at all on ACPI > systems as used in servers (e.g. clock management), but perhaps we will > if people see value in those elements. It's not about seeing value, it's about trying to shoehorn an existing implementation into ACPI right now. People don't program with the ACPI mindset since that's not what they're used to. So everybody needs to re-learn everything all over again. The clock issues still have to be dealt with in ACPI _somewhere_, and they likely have to be dealt with by the same software team at the vendors that do Linux kernel work. So by forcing them to figure out how to do it in ACPI, we're distracting from their efforts to support their hardware better in the kernel in the first place. That's not win-win, it's lose-lose. Or rather, it's lose for all of us, and win for Microsoft since they are DoS:ing us. > > The server guys really want UEFI for their boot protocols, > > installation managers, etc, etc. That's fine, let them do that, but > > that doesn't mean we need to bring the same APIs all the way into the > > kernel. > > > > So, I'm strongly urging that whatever the server guys try to do, it > > will in the end result in the ACPI data being translated into DT > > equivalents, such that the kernel _only_ needs to handle data via DT. > > I'm not sure that translating ACPI tables to dt makes any sense. If AML > is used (even sparingly), I do not believe that we can do any sensible > conversion to device tree. My understanding is that AML includes > functionality for modifying ACPI tables, and I don't see how we can > possibly support that without having to add a lot of boilerplate to all > the code handling AML to add a device tree backend... We can definitely modify device tree contents at runtime, it's just that nobody besides the POWER server guys are doing that today. So that's not a strong argument in ACPI's favor. We should fix device-tree where needed instead. Also, we can either have some of our better people sort out the ACPI-to-DT translation and management, and get it done right, or we can rely on all the vendors to get ACPI right in all their drivers. While some of them will, I suspect we'll be more successful driving this from a common place. It also gives us a place to stick all the fixups for broken firmware, since the first generations of ARM servers are bound to have them. And best of all, it allows us to continue focus on device tree and getting that done right, than splitting all efforts into two. We can't affort that right now. > > Just like PowerPC scrapes the OpenFirmware client interface to build a > > flat device tree, we should add a pre-boot stage that scrapes > > ACPI/UEFI data and constructs an appropriate device-tree. We can still > > bring over ACPI methods and represent those in the DT, but we should > > _not_ have two native interfaces. > > I'm not sure the two cases are comparable. The format of the FDT was > designed to encode the data structure used by OpenFirmware, and thus the > two map to each other pretty well. I do not believe that mapping from > ACPI tables to an FDT blob is anywhere near as simple, and as I mention > above I do not believe that we can just copy over the ACPI methods in > isolation. Sorry, I wasn't implying that there's a one-time trivial conversion to be made in the generic case, but it can still be done in a similar manner even though it will be more complex. Nobody is expecting there to be zero work for new drivers with ACPI; after all, the driver itself still has to be written. We're talking differences from board to board and system to system here, which we can definitely handle through a translator as well. And, as you say, if the first platforms are going to be trivial and easy to implement with ACPI, then doing a translation for them will be simple too. Shipping a firmware with ACPI is really no different from shipping a firmware with a flashed device tree. Whatever bugs or strange things the vendor chooses to do in there, we'll have to live with forever. We all know DT considerably better to a point where I would recommend that they flash a DTB in their UEFI firmware instead of go with ACPI. For simple hardware and basic devices we've got most bindings sorted out by now, and we've decided on backwards compatibility from here on out. > > It might be done via having a skeleton/framework DT for the vendor > > platform that is updated via UEFI/ACPI data, or it might be > > constructed entirely out of tables coming from firmware. I don't care > > about the methods for how it is done, but I do feel strongly that we > > should _not_ introduce a second API for everything. I can't think of a > > single good reason to do it. > > Where does this skeleton/framework come from? Within the kernel? Since there might need to be runtime modifications done to the tree based on ACPI events, it seems to make sense to do it in the kernel, so that translation state and such can be kept around for use by the runtime modification parts. Having it extractable out of the kernel tree for testing purposes would be greatly appreciated, so it can be ran through valgrind, add testcases, etc. > That > sounds like an arcane board file equivalent, and is counter to the > entire reason for using UEFI and ACPI -- having a well-defined > (excluding particular driver bindings, and I'm not arguing well-defined > means nice) stable standard that allows the kernel to boot on an > arbitrary platform without requiring arbitrary platform-specific code > everywhere in the boot path. > > It might not be pretty, and it will certainly require a lot of work, but > I'd prefer it at least for a semblance of uniformity. That is a red herring -- that booting standard has _nothing_ to do with ACPI. Once you've got a standard that is well-defined enough like that, you no longer need the runtime portions of ACPI to deal with it. You can just hardcode it. You need a way to probe _which_ type of standard is used, but from there on out you can assume that things are the same way. > I think that trying to shoe-horn ACPI-derived information into device > tree is fundamentally the wrong approach. I don't think it encourages > best practices, and I don't think it's beneficial to the long term > health of Linux or the ecosystem as a whole. There are no known best practices with ACPI. It's a complete fumbling around in the dark right now. We're complaining about reviewer bandwith on DT -- do we have a single reviewer for ACPI on ARM that actually knows what a good solution looks like? I don't think so. So, until there's strong evidence that ACPI is actually going to be a sane solution, in other words, until someone has shipped a system that uses it (with Windows) and does it successfully without being a hot mess, we shouldn't litter the kernel with it. Vendors can standardise of UEFI if they want, but I would much rather see them bundle DTB images with their firmware today, than rely on early buggy and still-early-on-the-learning-curve ACPI bindings that we then have to live with forever. -Olof -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
