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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Wed, 21 Feb 2018 11:17:15 +0100
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Subject: Re: [PATCH] cpufreq: powernv: Check negative value returned by cpufreq_table_find_index_dl()
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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On Wed, Feb 21, 2018 at 11:02 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> On 21-02-18, 10:27, Rafael J. Wysocki wrote:
>> 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.
>
> I don't want to end up doing double checking as some of it is already
> done at init, but let me check on what can be done.

The driver is expected to call cpufreq_table_validate_and_show() at
->init() time and fail ->init() if that fails.

That's kind of fragile, because it depends on the driver to do the right thing.