linux-kernel - Re: [PATCH] cpufreq: powernv: Define methods to parse positive & negative pstates

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Date:   Wed, 06 Dec 2017 23:41:32 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Gautham R Shenoy <ego@...ux.vnet.ibm.com>
Cc:     Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        linux-pm@...r.kernel.org,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        viresh.kumar@...aro.org, rjw@...ysocki.net,
        linux-kernel@...r.kernel.org, huntbag@...ux.vnet.ibm.com,
        linuxppc-dev@...ts.ozlabs.org, akshay.adiga@...ux.vnet.ibm.com
Subject: Re: [PATCH] cpufreq: powernv: Define methods to parse positive & negative pstates

Gautham R Shenoy <ego@...ux.vnet.ibm.com> writes:

> Hi Michael,
>
> On Wed, Dec 06, 2017 at 09:54:27PM +1100, Michael Ellerman wrote:
>> Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com> writes:
>> 
>> > From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>> >
>> > Pstates are 8bit values but on POWER8 they are negative and on POWER9
>> > they are positive. This patch adds helper routines to differentiate
>> > the sign to read the correct pstate value.
>> 
>> This sounds like it could be a bad bug, but I can't really tell from the
>> change log. What is the actual impact of not having this patch?
>
> On some POWER9 platforms, there can be more than 128 pstates. 
>
> Without this patch, on such platforms, if the value of the current
> frequency corresponds to a pstate greater than 128, then the code will
> interpret it as a negative number, and report that the "pstate is out
> of bound" while returning a nominal frequency.

OK, that's good detail to have in the change log, please include it.

>> Should it have a Fixes/Cc-stable tag?
>
> This doesn't fix any prior commit, but is fixes a newly discovered
> bug.

OK. You could say it "fixes" the commit that added Power9 support to the
driver, but it seems there wasn't really a commit that did that
specifically.

> I will resend the patch Cc'ing stable.

You don't have to Cc stable, that was just a suggestion. Though it
sounds like the symptoms are probably bad enough to warrant it.

cheers