OpenSBI system reset device tree binding

[Heinrich Schuchardt]

On 03.03.21 10:06, Anup Patel wrote:
> Hi Heinrich,
>
>> -----Original Message-----
>> From: Heinrich Schuchardt <xypron.glpk at gmx.de>
>> Sent: 03 March 2021 12:45
>> To: Anup Patel <Anup.Patel at wdc.com>
>> Cc: OpenSBI <opensbi at lists.infradead.org>
>> Subject: OpenSBI system reset device tree binding
>>
>> Hello Anup,
>>
>> now that we have the system reset extension in OpenSBI the next logical
>> step for me would be to implement a U-Boot driver using it to provide the
>> UEFI ResetSystem() runtime service.
>
> Yes, that's the right way.
>
>>
>> Similarly a reset driver could be built into Linux.
>
> I have already send-out couple of revision of Linux patches.
>
> May be you can review that and take it as reference for U-Boot driver ??

Are you referring to this series:

https://patchwork.kernel.org/project/kvm/list/?series=427585 ?

>
>>
>> First thing that is needed is a device tree binding describing that we have an
>> OpenSBI system reset available for the board. A comparable binding exists in
>> the ARM world for PSCI. Cf.
>> Documentation/devicetree/bindings/arm/psci.yaml in the Linux code.
>
> The ARM world is bit different. The PSCI came late in ARM world so they
> Have ARM 32bit systems which run without PSCI. Probably, that’s why PSCI
> is detected/probed via DT node.
>
>>
>> Has such a binding been defined for OpenSBI yet?
>
> In RISC-V S-mode world, the SBI is assumed to be always there so we
> don't need a DT node to detect SBI presence. Instead, we have SBI calls
> to get SBI spec version and detect available SBI extensions. This also
> simplify Guest/VM DT generation for hypervisors.

The OpenSBI project was started after RISC-V support entered U-Boot.
Hence U-Boot can be built with and without SBI
(qemu-riscv64_smode_defconfig vs qemu-riscv64_defconfig,
sipeed_maix_bitm_defconfig vs sipeed_maix_smode_defconfig).

How would an operating system know if there is an SBI present if it is
not stated in the device-tree?

Once we know there is an SBI we can check if it has the reset extension.
So this bit of information is not needed in the device-tree.

Best regards

Heinrich