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

[Gautham R Shenoy]

Hi Balbir,

On Sun, Dec 17, 2017 at 02:04:03PM +1100, Balbir Singh wrote:
> On Wed, Dec 13, 2017 at 5:57 PM, Gautham R. Shenoy
> <ego at linux.vnet.ibm.com> wrote:
> > From: "Gautham R. Shenoy" <ego at linux.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 at linux.vnet.ibm.com>
> > ---
> 
> This looks better

Thanks for the Review.

> 
> Acked-by: Balbir Singh <bsingharora at gmail.com>
> 
> Balbir Singh
>