Hi,
On Wed, 28 Feb 2024 10:26:47 -0600 Rob Herring robh@kernel.org 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@bootlin.com
[1]: https://lore.kernel.org/all/20230317053415.2254616-1-frowand.list@gmail.com/
Best regards, Hervé Codina