linux-kernel - Re: [PATCH 06/12] openrisc: Use of_get_cpu

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Message-ID: <YV4lM/YJ4V4EAlZb@antec>
Date:   Thu, 7 Oct 2021 07:37:39 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Rob Herring <robh@...nel.org>, Rich Felker <dalias@...c.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, Guo Ren <guoren@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-riscv@...ts.infradead.org,
        Will Deacon <will@...nel.org>, Jonas Bonn <jonas@...thpole.se>,
        Florian Fainelli <f.fainelli@...il.com>,
        Yoshinori Sato <ysato@...rs.osdn.me>, linux-sh@...r.kernel.org,
        x86@...nel.org, Russell King <linux@...linux.org.uk>,
        linux-csky@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Catalin Marinas <catalin.marinas@....com>,
        Palmer Dabbelt <palmer@...belt.com>,
        devicetree@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Ray Jui <rjui@...adcom.com>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        openrisc@...ts.librecores.org, Borislav Petkov <bp@...en8.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        Scott Branden <sbranden@...adcom.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Frank Rowand <frowand.list@...il.com>,
        James Morse <james.morse@....com>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 06/12] openrisc: Use of_get_cpu_hwid()

Hi Segher,

On Wed, Oct 06, 2021 at 04:27:28PM -0500, Segher Boessenkool wrote:
> On Thu, Oct 07, 2021 at 05:44:00AM +0900, Stafford Horne wrote:
> > You have defined of_get_cpu_hwid to return u64, will this create compiler
> > warnings when since we are storing a u64 into a u32?
> > 
> > It seems only if we make with W=3.
> 
> Yes.  This is done by -Wconversion, "Warn for implicit conversions that
> may alter a value."

Yeah, that is what I found out when I looked into it.

> > I thought we usually warned on this.
> 
> This warning is not in -Wall or -Wextra either, it suffers too much from
> false positives.  It is very natural to just ignore the high bits of
> modulo types (which is what "unsigned" types *are*).  Or the bits that
> "fall off" on a conversion.  The C standard makes this required
> behaviour, it is useful, and it is the only convenient way of getting
> this!

Thanks for the background, It does make sense. I guess I was confused with java
which requires casting when you store to a smaller size.  I.e.

    Test.java:5: error: incompatible types: possible lossy conversion from int to short
	s = i;

-Stafford