Hi, On Wed, 28 Feb 2024 10:26:47 -0600 Rob Herring <robh@xxxxxxxxxx> wrote: ... > > > > > > Yes, that version unflattened the bootloader passed DT. Now within > > > unflatten_devicetree(), the bootloader DT is ignored if ACPI is > > > enabled and we unflatten an empty tree. That will prevent the kernel > > > getting 2 h/w descriptions if/when a platform does such a thing. Also, > > > kexec still uses the bootloader provided DT as before. > > > > That avoids the main instance of my concern, and means that this'll boot > > without issue, but IIUC this opens the door to dynamically instantiating DT > > devices atop an ACPI base system, which I think in general is something that's > > liable to cause more problems than it solves. > > > > I understand that's desireable for the selftests, though I still don't believe > > it's strictly necessary -- there are plenty of other things that only work if > > the kernel is booted in a specific configuration. > > Why add to the test matrix if we don't have to? > > > Putting the selftests aside, why do we need to do this? Is there any other > > reason to enable this? > > See my Plumbers talk... > > Or in short, there's 3 main usecases: > > - PCI FPGA card with devices instantiated in it > - SoCs which expose their peripherals via a PCI endpoint. > - Injecting test devices with QEMU (testing, but not what this series > does. Jonathan Cameron's usecase) > > In all cases, drivers already exist for the devices, and they often only > support DT. DT overlays is the natural solution for this, and there's > now kernel support for it (dynamically generating PCI DT nodes when they > don't exist). The intent is to do the same thing on ACPI systems. > > I don't see another solution other than 'go away, you're crazy'. There's > ACPI overlays, but that's only a debug feature. Also, that would > encourage more of the DT bindings in ACPI which I find worse than this > mixture. There's swnodes, but that's just board files and platform_data > 2.0. > > I share the concerns with mixing, but I don't see a better solution. The > scope of what's possible is contained enough to avoid issues. > I tested on a x86 system. My use case is 'SoCs which expose their peripherals via a PCI endpoint' described by Rob. Indeed, I have a Microchip Lan9662 board (the one mentioned by Rob in his Plumbers talk) and the root DT node creation is obviously needed. I have previously used Frank Rowan's patches [1] that did this DT root node creation. This series perfectly replace them and the root DT node is successfully created. Tested-by: Herve Codina <herve.codina@xxxxxxxxxxx> [1]: https://lore.kernel.org/all/20230317053415.2254616-1-frowand.list@xxxxxxxxx/ Best regards, Hervé Codina -- Hervé Codina, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
