Re: [PATCH v5 1/1] RISC-V: Add common csr_read_num() and csr_write_num() functions

[Paul Walmsley <pjw@kernel.org>]

Hi Anup,

On Wed, 4 Feb 2026, Anup Patel wrote:

> From: Anup Patel <apatel@ventanamicro.com>
> 
> In RISC-V, there is no CSR read/write instruction which takes CSR
> number via register so add common csr_read_num() and csr_write_num()
> functions which allow accessing certain CSRs by passing CSR number
> as parameter. These common functions will be first used by the
> ACPI CPPC driver and RISC-V PMU driver.
> 
> Also, the RISC-V ACPI FFH specification allows arbitrary CSR number
> as CPPC register and the RISC-V SBI specification allows arbitrary
> CSR number as PMU hardware counter. This means ACPI CPPC driver and
> RISC-V PMU driver no longer need to do sanity checks on CSR number
> which are now done by the common csr_read_num() and csr_write_num()
> functions.

I recall that when we discussed this patch on a call a few months ago, it 
was brought up that it seemed useful to restrict which CSRs could be read 
or written at runtime, on a per-vendor basis.  That way, the FFH interface 
couldn't be used to read or write custom CSRs that had nothing to do with 
CPPC or PMU functions. 

Are you still planning to implement that change?


- Paul

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