linux-kernel - Re: [PATCH] cpufreq: powernv: Check negative value returned by cpufreq_table_find_index

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Message-ID: <CAJZ5v0jEg7FELH6mBY16mB5sfo7s2mbH2SGheH4KVeKLnStjjg@mail.gmail.com>
Date:   Wed, 21 Feb 2018 10:27:39 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] cpufreq: powernv: Check negative value returned by cpufreq_table_find_index_dl()

On Wed, Feb 21, 2018 at 6:54 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 21-02-18, 16:39, Michael Ellerman wrote:
>> Viresh Kumar <viresh.kumar@...aro.org> writes:
>
>> > AFAICT, you will get -1 here only if the freq table had no valid
>> > frequencies (or the freq table is empty). Why would that happen ?
>>
>> Bugs?
>
> The cupfreq driver shouldn't have registered itself in that case (i.e.
> if the cpufreq table is empty).

To be precise, ->init() should fail as that's where the table is
created.  The registration fails as a result then.

But what if the bug is that ->init() doesn't fail when it should?

I guess the core could double check the frequency table after ->init()
if ->target_index is not NULL.

The overall point here is that if you get a negative index in
->fast_switch(), that's way too late anyway and we should be able to
catch that error much earlier.