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

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Message-ID: <aHY0mWEx8_z_nJoU@mai.linaro.org>
Date: Tue, 15 Jul 2025 12:59:37 +0200
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Jesse Taube <mr.bossman075@...il.com>
Cc: linux-riscv@...ts.infradead.org,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Thomas Gleixner <tglx@...utronix.de>,
	Samuel Holland <samuel.holland@...ive.com>,
	Conor Dooley <conor@...nel.org>, Icenowy Zheng <uwu@...nowy.me>,
	Jisheng Zhang <jszhang@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6] clocksource/drivers/timer-clint: Add T-Head C9xx clint

On Mon, Jun 23, 2025 at 07:15:36PM -0400, Jesse Taube wrote:
> From: Jisheng Zhang <jszhang@...nel.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@...nel.org>
> Signed-off-by: Jesse Taube <Mr.Bossman075@...il.com>
> ---
> Treat this as a completely new patch, as it is mostly rewritten.

[ ... ]

> 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

[ ... ]

Any comments for these changes from the arch maintainer ?

Thanks

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