Re: [PATCH v1 -next 0/3] RISC-V: ACPI: Enable CPPC based cpufreq support

[Sunil V L]

On Wed, Mar 20, 2024 at 09:12:11PM -0700, Drew Fustini wrote:
> On Tue, Mar 19, 2024 at 03:50:21PM +0530, Sunil V L wrote:
> > On Mon, Mar 18, 2024 at 07:27:34PM -0700, Drew Fustini wrote:
> > > On Mon, Mar 18, 2024 at 11:40:34AM -0700, Drew Fustini wrote:
> > > > On Thu, Feb 08, 2024 at 09:14:11AM +0530, Sunil V L wrote:
> > > > > This series enables the support for "Collaborative Processor Performance
> > > > > Control (CPPC) on ACPI based RISC-V platforms. It depends on the
> > > > > encoding of CPPC registers as defined in RISC-V FFH spec [2].
> > > > > 
> > > > > CPPC is described in the ACPI spec [1]. RISC-V FFH spec required to
> > > > > enable this, is available at [2].
> > > > > 
> > > > > [1] - https://uefi.org/specs/ACPI/6.5/08_Processor_Configuration_and_Control.html#collaborative-processor-performance-control
> > > > > [2] - https://github.com/riscv-non-isa/riscv-acpi-ffh/releases/download/v1.0.0/riscv-ffh.pdf
> > > > > 
> > > > > The series is based on the LPI support series.
> > > > > Based-on: 20240118062930.245937-1-sunilvl@xxxxxxxxxxxxxxxx
> > > > > (https://lore.kernel.org/lkml/20240118062930.245937-1-sunilvl@xxxxxxxxxxxxxxxx/)
> > > > 
> > > > Should the https://github.com/vlsunil/qemu/tree/lpi_exp branch also be
> > > > used for this CPPC series too?
> > > 
> > > I noticed the ventanamicro qemu repo has a dev-upstream branch [1] which
> > > contains 4bb6ba4d0fb9 ("riscv/virt: acpi: Enable CPPC - _CPC and _PSD").
> > > I've built that but I still see 'SBI CPPC extension NOT detected!!' in
> > > the Linux boot log.
> > > 
> > > I'm using upstream opensbi. It seems that sbi_cppc_probe() fails because
> > > cppc_dev is not set. Nothing in the upstream opensbi repo seems to call
> > > sbi_cppc_set_device(), so I am uncertain how it is possible for it to
> > > work. Is there an opensbi branch I should be using?
> > > 
> > > Thanks,
> > > Drew
> > > 
> > > [1] https://github.com/ventanamicro/qemu/tree/dev-upstream
> > 
> > Please use below branches for qemu and opensbi. These are just dummy
> > objects/interfaces added to test the kernel change which are otherwise
> > platform specific features.
> > 
> > https://github.com/vlsunil/qemu/tree/lpi_cppc_exp
> > https://github.com/vlsunil/opensbi/tree/cppc_exp
> 
> I know the opensbi branch is just for the purpose of testing the kernel
> driver. However, I am new to ACPI and I am trying to understand how a
> real system might work.
> 
> The _CPC register address encoding in the RISC-V FFH spec enables the
> SBI CPPC register ID to be specified. But how would SBI firmware know
> what physical address corresponds to the CPPC register?
> 
> If sbi_cppc_test_write() [1] was implemented for a real system, then how
> would it know what physical address to write to for a CPPC register like
> SBI_CPPC_DESIRED_PERF?
> 
The SBI extension provides an abstraction to access the CPPC registers.
SBI implementation for the platform should be aware of how to access a
particular register in the back end when it supports the extension.

Regards,
Sunil