linux-kernel - Re: [v3 PATCH 1/3] powernv-cpufreq: Add helper to extract pstate from PMSR

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Date:   Sun, 17 Dec 2017 14:04:03 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "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 <akshay.adiga@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        linux-pm@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [v3 PATCH 1/3] powernv-cpufreq: Add helper to extract pstate from PMSR

On Wed, Dec 13, 2017 at 5:57 PM, Gautham R. Shenoy
<ego@...ux.vnet.ibm.com> wrote:
> From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>
> On POWERNV platform, the fields for pstates in the Power Management
> Status Register (PMSR) and the Power Management Control Register
> (PMCR) are 8-bits wide. On POWER8 the pstates are negatively numbered
> while on POWER9 they are positively numbered.
>
> The device-tree exports pstates as 32-bit entries. The device-tree
> implementation sign-extends the 8-bit pstate values to obtain the
> corresponding 32-bit entry.
>
> Eg: On POWER8, a pstate value 0x82 [-126] is represented in the
> device-tree as 0xfffffff82 while on POWER9, the same value 0x82 [130]
> is represented in the device-tree as 0x00000082.
>
> The powernv-cpufreq driver implementation represents pstates using the
> integer type. In multiple places in the driver, the code interprets
> the pstates extracted from the PMSR as a signed byte and assigns it to
> a integer variable to get the sign-extention.
>
> On POWER9 platforms which have greater than 128 pstates, this results
> in the driver performing incorrect sign-extention, and thereby
> treating a legitimate pstate (say 130) as an invalid pstates (since it
> is interpreted as -126).
>
> This patch fixes the issue by implementing a helper function to
> extract Pstates from PMSR register, and correctly sign-extend it to be
> consistent with the values provided by the device-tree.
>
> Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> ---

This looks better

Acked-by: Balbir Singh <bsingharora@...il.com>

Balbir Singh