linux-kernel - Re: [PATCH v3 1/7] arm64/sysreg: Convert CCSIDR

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Message-ID: <87cz8hez0i.wl-maz@kernel.org>
Date:   Sun, 18 Dec 2022 11:23:41 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Akihiko Odaki <akihiko.odaki@...nix.com>
Cc:     linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        asahi@...ts.linux.dev, Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Sven Peter <sven@...npeter.dev>,
        Hector Martin <marcan@...can.st>
Subject: Re: [PATCH v3 1/7] arm64/sysreg: Convert CCSIDR_EL1 to automatic generation

On Sun, 18 Dec 2022 05:14:06 +0000,
Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
> 
> Convert CCSIDR_EL1 to automatic generation as per DDI0487I.a. The field
> definition is for case when FEAT_CCIDX is not implemented. Fields WT,
> WB, RA and WA are defined as per A.j since they are now reserved and
> may have UNKNOWN values in I.a, which the file format cannot represent.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
> ---
>  arch/arm64/include/asm/sysreg.h |  1 -
>  arch/arm64/tools/sysreg         | 11 +++++++++++
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 7d301700d1a9..910e960661d3 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -425,7 +425,6 @@
>  
>  #define SYS_CNTKCTL_EL1			sys_reg(3, 0, 14, 1, 0)
>  
> -#define SYS_CCSIDR_EL1			sys_reg(3, 1, 0, 0, 0)
>  #define SYS_AIDR_EL1			sys_reg(3, 1, 0, 0, 7)
>  
>  #define SYS_RNDR_EL0			sys_reg(3, 3, 2, 4, 0)
> diff --git a/arch/arm64/tools/sysreg b/arch/arm64/tools/sysreg
> index 384757a7eda9..acc79b5ccf92 100644
> --- a/arch/arm64/tools/sysreg
> +++ b/arch/arm64/tools/sysreg
> @@ -871,6 +871,17 @@ Sysreg	SCXTNUM_EL1	3	0	13	0	7
>  Field	63:0	SoftwareContextNumber
>  EndSysreg
>  
> +Sysreg	CCSIDR_EL1	3	1	0	0	0
> +Res0	63:32
> +Field	31:31	WT
> +Field	30:30	WB
> +Field	29:29	RA
> +Field	28:28	WA

For fields described as a single bit, the tool supports simply
indicating the bit number (28 rather than 28:28).

However, I strongly recommend against describing fields that have been
dropped from the architecture.  This only happens when these fields
are never used by any implementation, so describing them is at best
useless.

> +Field	27:13	NumSets
> +Field	12:3	Associavity
> +Field	2:0	LineSize
> +EndSysreg
> +

I don't think we have a good solution for overlapping fields that
depend on other factors, either contextual (such as a mode that
changes the layout of a sysreg), or architecture warts such as
FEAT_CCIDX (which changes the layout of a well-known sysreg).

At least, put a comment here that indicates the context of the
description.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.