linux-kernel - Re: [PATCH v2] ACPI / LPIT: Add Low Power Idle Table (LPIT) support

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Date:   Thu, 05 Oct 2017 22:56:30 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        "Zheng, Lv" <lv.zheng@...el.com>, Len Brown <lenb@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ACPI / LPIT: Add Low Power Idle Table (LPIT) support

On Thursday, October 5, 2017 10:43:33 PM CEST Srinivas Pandruvada wrote:
> On Thu, 2017-10-05 at 21:39 +0300, Andy Shevchenko wrote:
> > On Thu, Oct 5, 2017 at 9:16 PM, Srinivas Pandruvada
> > <srinivas.pandruvada@...ux.intel.com> wrote:
> > > 
> > > Added functionality to read LPIT table, which provides:
> > > 
> > > - Sysfs interface to read residency counters via
> > > /sys/devices/system/cpu/cpuidle/low_power_idle_cpu_residency_us
> > > /sys/devices/system/cpu/cpuidle/low_power_idle_system_residency_us
> > > 
> > > Here the count "low_power_idle_cpu_residency_us" shows the time
> > > spent
> > > by CPU package in low power state. This is read via MSR interface,
> > > which
> > > points to MSR for PKG C10.
> > > 
> > > Here the count "low_power_idle_system_residency_us" show the count
> > > the
> > > system was in low power state. This is read via MMIO interface.
> > > This
> > > is mapped to SLP_S0 residency on modern Intel systems. This
> > > residency
> > > is achieved only when CPU is in PKG C10 and all functional blocks
> > > are
> > > in low power state.
> > > 
> > > It is possible that none of the above counters present or anyone of
> > > the
> > > counter present or all counters present.
> > > 
> > > For example: On my Kabylake system both of the above counters
> > > present.
> > > After suspend to idle these counts updated and prints:
> > > 6916179
> > > 6998564
> > > 
> > > This counter can be read by tools like turbostat to display. Or it
> > > can
> > > be used to debug, if modern systems are reaching desired low power
> > > state.
> > > 
> > > - Provides an interface to read residency counter memory address
> > > This address can be used to get the base address of PMC memory
> > > mapped IO.
> > > This is utilized by intel_pmc_core driver to print more debug
> > > information.
> > 
> > > 
> > > +               switch (residency_info_mem.gaddr.bit_width) {
> > > +               case 8:
> > > +                       count =
> > > readb(residency_info_mem.iomem_addr);
> > > +                       break;
> > > +               case 16:
> > > +                       count =
> > > readw(residency_info_mem.iomem_addr);
> > > +                       break;
> > > +               case 32:
> > > +                       count =
> > > readl(residency_info_mem.iomem_addr);
> > > +                       break;
> > > +               case 64:
> > > +                       count =
> > > readq(residency_info_mem.iomem_addr);
> > > +                       break;
> > > +               default:
> > > +                       return -EINVAL;
> > > +               }
> > 
> > I saw something very similar already under drivers/acpi. Can we
> > utilize it (split a helper out of it and re-use)?
> This functionality is probably not only for ACPI, but may be other
> parts of the kernel too. So if there is a common function then it can
> be more generic outside of ACPI.

If the value of the field is a GAS, we can use the ACPICA's library
routine for reading from there I suppose.

Thanks,
Rafael