linux-kernel - Re: [v2 PATCH] cpufreq: powernv: Correctly parse the sign of pstates on POWER8 vs POWER9

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Date:   Fri, 08 Dec 2017 22:47:38 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        huntbag@...ux.vnet.ibm.com, akshay.adiga@...ux.vnet.ibm.com,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Stable <stable@...r.kernel.org>
Subject: Re: [v2 PATCH] cpufreq: powernv: Correctly parse the sign of pstates on POWER8 vs POWER9

"Rafael J. Wysocki" <rafael@...nel.org> writes:

> On Thu, Dec 7, 2017 at 6:59 AM, Gautham R. Shenoy
> <ego@...ux.vnet.ibm.com> wrote:
>> From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>>
>> On POWERNV platform, Pstates are 8-bit values. On POWER8 they are
>> negatively numbered while on POWER9 they are positively
>> numbered. Thus, on POWER9, the maximum number of pstates could be as
>> high as 256.
>>
>> The current code interprets pstates as a signed 8-bit value. This
>> causes a problem on POWER9 platforms which have more than 128 pstates.
>> On such systems, on a CPU that is in a lower pstate whose number is
>> greater than 128, querying the current pstate returns a "pstate X is
>> out of bound" error message and the current pstate is reported as the
>> nominal pstate.
>>
>> This patch fixes the aforementioned issue by correctly differentiating
>> the sign whenever a pstate value read, depending on whether the
>> pstates are positively numbered or negatively numbered.
>>
>> Fixes: commit 09ca4c9b5958 ("cpufreq: powernv: Replacing pstate_id with frequency table index")
>> Cc: <stable@...r.kernel.org> #v4.8
>> Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
>> Tested-and-reviewed-by: Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>
>> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
>
> I'm going to apply this, or please let me know if you want to route it
> differently.

Do you mind waiting for now, we're still debating how to fix it.

cheers