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

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Date:   Fri, 8 Dec 2017 15:08:17 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        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

On Fri, Dec 8, 2017 at 12:47 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> "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.

No problem. :-)

Just please let me know when you're ready.