linux-kernel - RE: [PATCH v5 1/8] x86/resctrl: Prepare for new domain scope

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Date:   Wed, 30 Aug 2023 14:11:14 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>
CC:     "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        "Jonathan Corbet" <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        "Babu Moger" <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: RE: [PATCH v5 1/8] x86/resctrl: Prepare for new domain scope

> >+static int get_domain_id_from_scope(int cpu, enum resctrl_scope scope)
> >+{
> >+    switch (scope) {
> >+    case RESCTRL_L3_CACHE:
> >+            return get_cpu_cacheinfo_id(cpu, 3);
> >+    case RESCTRL_L2_CACHE:
> >+            return get_cpu_cacheinfo_id(cpu, 2);
> >+    default:
> >+            WARN_ON_ONCE(1);
> >+            break;
> >+    }
> >+
> >+    return -1;
> >+}
>
> Is there some reason the "return -1" is outside of the default switch
> case?
>
> Other switch statements in this patch do have returns inside the default
> case, is this one different in some way?

I've sometimes had compilers complain about code written:

static int get_domain_id_from_scope(int cpu, enum resctrl_scope scope)
{
        switch (scope) {
        case RESCTRL_L3_CACHE:
                return get_cpu_cacheinfo_id(cpu, 3);
        case RESCTRL_L2_CACHE:
                return get_cpu_cacheinfo_id(cpu, 2);
        default:
                WARN_ON_ONCE(1);
                return -1;
        }
}

because they failed to notice that every path in the switch does a "return and they
issue a warning that the function has no return value because they don't realize
that the end of the function can't be reached.

So it's defensive programming against possible complier issues.

-Tony