[PATCH v6] clocksource/drivers/timer-clint: Add T-Head C9xx clint

[Daniel Lezcano]

On Mon, Jun 23, 2025 at 07:15:36PM -0400, Jesse Taube wrote:
> From: Jisheng Zhang <jszhang at kernel.org>
> 
> To use the T-HEAD C9xx clint in RISCV-M NOMMU env, we need to take
> care two points:
> 
> 1.The mtimecmp in T-Head C9xx clint only supports 32bit read/write,
> implement such support.
> 
> 2. As pointed out by commit ca7810aecdba ("lib: utils/timer: mtimer:
> add a quirk for lacking mtime register") of opensbi:
> 
> "T-Head developers surely have a different understanding of time CSR and
> CLINT's mtime register with SiFive ones, that they did not implement
> the mtime register at all -- as shown in openC906 source code, their
> time CSR value is just exposed at the top of their processor IP block
> and expects an external continous counter, which makes it not
> overrideable, and thus mtime register is not implemented, even not for
> reading. However, if CLINTEE is not enabled in T-Head's MXSTATUS
> extended CSR, these systems still rely on the mtimecmp registers to
> generate timer interrupts. This makes it necessary to implement T-Head
> C9xx CLINT support in OpenSBI MTIMER driver, which skips implementing
> reading mtime register and falls back to default code that reads time
> CSR."
> 
> So, we need to fall back to read time CSR instead of mtime register.
> Add riscv_csr_time_available static key for this purpose.
> 
> Signed-off-by: Jisheng Zhang <jszhang at kernel.org>
> Signed-off-by: Jesse Taube <Mr.Bossman075 at gmail.com>
> ---
> Treat this as a completely new patch, as it is mostly rewritten.
> Original:
> https://lore.kernel.org/all/20240410142347.964-3-jszhang@kernel.org/
> V3 -> V4:
>  - Add riscv,csr-clint
>  - Allow using of CSRs in S mode
>  - Change if return else return to if return return
>  - Change static_branch_likely to static_branch_unlikely
>  - Fix 32-bit clint_get_cycles64 csr_available check being inverted
>  - Fix is_c900_clint being uninitialized
> V4 -> V5:
>  - Remove riscv,csr-clint
>  - Remove riscv_csr_time_available
>  - Replace all static_branch_unlikely(&riscv_csr_time_available) with
>     riscv_has_extension_likely(RISCV_ISA_EXT_ZICNTR)
> V5 -> V6:
>  - Add csr_clock_next_event
>  - Add csr_get_cycles64 to sched_clock_register
>  - Break get_cycles64 into csr_get_cycles64 and clint_get_cycles64
>  - Change default behavior to use CSR time register
> 
> Posible future work:
>  - Check for SSTC support and use stimercmp
>  - SSTC depends on smode which cpufeature doesnt check for.
> ---

I would like to have Anup Patel opinion on these changes

  -- Daniel

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