Re: [PATCH-next] arm: fix clang build warning in include/asm/memory.h

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Nathan Chancellor <nathan@kernel.org>,
	Yipeng Zou <zouyipeng@huawei.com>,
	ndesaulniers@google.com, morbo@google.com,
	justinstitt@google.com, ssantosh@kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH-next] arm: fix clang build warning in include/asm/memory.h
Date: Fri, 15 Mar 2024 14:46:26 +0000	[thread overview]
Message-ID: <ZfRfQmkG8hBw+VNT@shell.armlinux.org.uk> (raw)
In-Reply-To: <CACRpkdbxnt5RBTM4eqvq9rTCm2DTjH9BhotJQ_xG3aksrXkE6g@mail.gmail.com>

On Fri, Mar 15, 2024 at 02:16:23PM +0100, Linus Walleij wrote:
> On Fri, Mar 15, 2024 at 11:08 AM Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> > On Fri, Mar 15, 2024 at 08:40:28AM +0100, Linus Walleij wrote:
> > > +1 and I would probably use this as well:
> > >
> > > <linux/limits.h>
> > >
> > > #define IDMAP_INVALID_ADDR    PHYS_ADDR_MAX
> > >
> > > if (IS_ENABLED(CONFIG_PHYS_ADDR_T_64BIT) && addr > PHYS_ADDR_MAX)
> > >     addr = IDMAP_INVALID_ADDR;
> > >
> > > Because then it is clear what is going on: we are capping to the max physical
> > > address.
> > >
> > > PHYS_ADDR_MAX is defined to (~(phys_addr_t)0) which on
> > > ARM is (~(u32)0).
> >
> > ... which is _not_ the same as (u32)~0, so using PHYS_ADDR_MAX is not
> > appropriate.
> 
> Attention to detail! Thanks Russell.
> 
> However U32_MAX is
> #define U32_MAX         ((u32)~0U)
> 
> So that should work and also convey the attention I think?

If you want to use that to define IDMAP_INVALID_ADDR then yes. If you're
thinking about getting rid of IDMAP_INVALID_ADDR, then I think that
would reduce code readability, because the idea here is that addresses
above the 32-bit virtual space are invalid for IDMAP, and just using
U32_MAX is a rather opaque way to convey that.

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