linux-kernel - Re: [PATCH v2 2/4] MIPS: Introduce config options for LLSC availability

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Message-Id: <2c26a07f-fa68-48f1-8f3b-3b5e4f77130b@app.fastmail.com>
Date: Fri, 21 Jun 2024 11:45:41 +0100
From: "Jiaxun Yang" <jiaxun.yang@...goat.com>
To: "Maciej W. Rozycki" <macro@...am.me.uk>
Cc: "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
 "Jonas Gorski" <jonas.gorski@...il.com>,
 "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] MIPS: Introduce config options for LLSC availability

在2024年6月21日六月 上午1:00，Maciej W. Rozycki写道：
> On Wed, 12 Jun 2024, Jiaxun Yang wrote:
>
>> Introduce CPU_HAS_LLSC and CPU_MAY_HAVE_LLSC to determine availability
>> of LLSC and Kconfig level.
>
>  Taking the subsequent patches in this series into account this seems to 
> create a parallel universe in which the availability of LL/SC for certain 
> features is handled at the Kconfig level while in the other universe it's 
> handled via <asm/mach-*/cpu-feature-overrides.h>.
>
>  I think this ought not to be done in two places independently and the 
> pieces in <asm/mach-*/cpu-feature-overrides.h> need to be removed, likely 
> in the same change even, *however* not without double-checking whether 
> there is not a case among them where a platform actually has LL/SC support 
> disabled despite the CPU used there having architectural support for the 
> feature.  Otherwise we may end up with a case where a platform has LL/SC 
> support disabled via its <asm/mach-*/cpu-feature-overrides.h> setting and 
> yet we enable ARCH_SUPPORTS_ATOMIC_RMW or ARCH_HAVE_NMI_SAFE_CMPXCHG for 
> it via Kconfig.

IMO it's necessary for platforms who know what are they doing such as ATH25,
which we took care in this series.

I'll add a build time assertion to ensure when CONFIG_CPU_HAS_LLSC is selected
kernel_uses_llsc is statically 1, so any incorrect overrides can be spotted
at build time.

It's better to clean up platform's overrides at some point, I'll leave
it to a future patch.

>
>  The note from <asm/mach-ip32/cpu-feature-overrides.h> seems a candidate 
> to move to arch/mips/Kconfig at the relevant place on this occasion too.
> There may be more such notes and they ought not to be lost.

Noted, I'll add a Kconfig symbol WAR_R5000_LLSC and rescue this comment.

Thanks
>
>   Maciej

-- 
- Jiaxun