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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Dec 8, 2017 at 12:47 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> "Rafael J. Wysocki" <rafael@kernel.org> writes:
>
>> On Thu, Dec 7, 2017 at 6:59 AM, Gautham R. Shenoy
>> <ego@linux.vnet.ibm.com> wrote:
>>> From: "Gautham R. Shenoy" <ego@linux.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@vger.kernel.org> #v4.8
>>> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
>>> Tested-and-reviewed-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
>>> Acked-by: Viresh Kumar <viresh.kumar@linaro.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.