On Thu, Feb 6, 2020 at 12:10 PM Alexander Graf agraf@csgraf.de wrote:
On 06.02.20 19:28, Atish Patra wrote:
On Tue, Feb 4, 2020 at 11:43 PM Ard Biesheuvel ard.biesheuvel@linaro.org wrote:
On Wed, 5 Feb 2020 at 05:53, Heinrich Schuchardt xypron.glpk@gmx.de wrote:
RISC-V booting currently is based on a per boot stage lottery to determine the active CPU. The Hart State Management (HSM) SBI extension replaces this lottery by using a dedicated primary CPU.
Cf. Hart State Management Extension, Extension ID: 0x48534D (HSM) https://github.com/riscv/riscv-sbi-doc/blob/master/riscv-sbi.adoc
In this scenario the id of the active hart has to be passed from boot stage to boot stage. Using a UEFI variable would provide an easy implementation.
This patch provides a weak function that is called at the end of the setup of U-Boot's UEFI sub-system. By overriding this function architecture specific UEFI variables or configuration tables can be created.
Signed-off-by: Heinrich Schuchardt xypron.glpk@gmx.de Reviewed-by: Atish Patra atish.patra@wdc.com
OK, so I have a couple of questions:
- does RISC-V use device tree? if so, why are you not passing the
active hart via a property in the /chosen node?
Yes. RISC-V uses device tree. Technically, we can pass the active hart by a DT property but that means we have to modify the DT in OpenSBI (RISC-V specific run time service provider). We have been trying to avoid that if possible so that DT is not bounced around. This also limits U-Boot to have its own device tree.
I don't understand how this is different from the UEFI variable scheme proposed here? If you want to create an SBI interface to propagate the active HART that U-Boot then uses to populate the /chosen property, that's probably fine as well.
We don't want to create SBI interface to pass this information.
We already have hooks that allow you to modify the DT right before it gets delivered to the payload. Just add it there?
Hmm. I guess it is true if the DT is loaded from MMC or network as well. How about EDK2 ? If we go DT route, it also has to modify the DT to pass the boot hart.
As it requires DT modification in multiple projects, why not use efi configuration tables as suggested by Ard ?
I'd assume the EFI stub would not care at all about this information, and it would give you a Linux/RISC-V specific way to convey this information that is independent of EFI.
Yes. EFI stub doesn't care about this information. However, it needs to save the information somewhere so that it can pass to the real kernel after exiting boot time services.
DT sounds like a pretty natural choice for that to me :).
Alex