linux-kernel - Re: [PATCH v2 2/4] arm64/sysreg: Support feature-specific fields with 'Prefix' descriptor

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Message-ID: <c835fd10-5590-42de-b968-4696917b41be@sirena.org.uk>
Date: Tue, 21 Oct 2025 20:22:32 +0100
From: Mark Brown <broonie@...nel.org>
To: Sascha Bischoff <Sascha.Bischoff@....com>
Cc: "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, nd <nd@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"maz@...nel.org" <maz@...nel.org>,
	"oliver.upton@...ux.dev" <oliver.upton@...ux.dev>,
	Joey Gouly <Joey.Gouly@....com>,
	Suzuki Poulose <Suzuki.Poulose@....com>,
	"yuzenghui@...wei.com" <yuzenghui@...wei.com>,
	"will@...nel.org" <will@...nel.org>,
	"lpieralisi@...nel.org" <lpieralisi@...nel.org>
Subject: Re: [PATCH v2 2/4] arm64/sysreg: Support feature-specific fields
 with 'Prefix' descriptor

On Thu, Oct 09, 2025 at 04:54:48PM +0000, Sascha Bischoff wrote:

> The Prefix descriptor can be used in the following way:
> 
>         Sysreg  EXAMPLE 0    1    2    3    4
>         Prefix    FEAT_A
> 	Field   63:0    Foo
> 	EndPrefix
> 	Prefix    FEAT_B
> 	Field   63:1    Bar
>  	Res0    0
>         EndPrefix
>         Field   63:0    Baz
>         EndSysreg

This seems like a reasonable solution to the practical problem, and the
implementation seems OK:

Reviewed-by: Mark Brown <broonie@...nel.org>

I think trying to do too much more would open up far too many cans of
worms to be reasonable to block things on.

One nit:

>  # Parse a "<msb>[:<lsb>]" string into the global variables @msb and @lsb
> @@ -132,10 +149,7 @@ $1 == "EndSysregFields" && block_current() == "SysregFields" {
>  	if (next_bit >= 0)
>  		fatal("Unspecified bits in " reg)
>  
> -	define(reg "_RES0", "(" res0 ")")
> -	define(reg "_RES1", "(" res1 ")")
> -	define(reg "_UNKN", "(" unkn ")")
> -	print ""
> +	define_resx_unkn(prefix, reg, res0, res1, unkn)

This refactoring into a function seems like it could usefully have been
a separate patch.

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