linux-kernel - Re: [PATCH v2 2/3] clocksource/drivers/timer-clint: Add option to use CSR instead of mtime

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Date: Tue, 9 Apr 2024 15:26:18 +0100
From: Conor Dooley <conor@...nel.org>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Samuel Holland <samuel.holland@...ive.com>,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] clocksource/drivers/timer-clint: Add option to
 use CSR instead of mtime

On Sat, Apr 06, 2024 at 07:21:58PM +0800, Jisheng Zhang wrote:
> 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."
> 
> To use the clint in RISCV-M NOMMU env on Milkv Duo little core, we
> need to fall back to read time CSR instead of mtime register. Add the
> option for this purpose.
> 
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>

> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> index 34faa0320ece..7bbdbf2f96a8 100644
> --- a/drivers/clocksource/Kconfig
> +++ b/drivers/clocksource/Kconfig
> @@ -650,6 +650,15 @@ config CLINT_TIMER
>  	  This option enables the CLINT timer for RISC-V systems.  The CLINT
>  	  driver is usually used for NoMMU RISC-V systems.
>  
> +config CLINT_USE_CSR_INSTEADOF_MTIME
> +	bool "Use TIME CSR instead of the mtime register"
> +	depends on CLINT_TIMER
> +	help
> +	  Use TIME CSR instead of mtime register. Enable this option if
> +	  prefer TIME CSR over MTIME register, or if the implementation
> +	  doesn't implement the mtime register in CLINT, so fall back on
> +	  TIME CSR.

This, as a Kconfig option, seems a bit strange to me. We know at runtime
if we are on a T-Head device without the mtime register and should be
able decide to use the CSR implementation dynamically in that case,
right?

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