linux-kernel - Re: [RFC/RFT][PATCH v2 0/6] PM / QoS: Introduce latency tolerance device PM QoS type

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Date:	Fri, 24 Jan 2014 16:19:23 +0200
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/RFT][PATCH v2 0/6] PM / QoS: Introduce latency tolerance
 device PM QoS type

On Thu, Jan 23, 2014 at 02:34:59PM +0100, Rafael J. Wysocki wrote:
> On Friday, January 17, 2014 03:42:13 PM Rafael J. Wysocki wrote:
> > Hi All,
> > 
> > On some platforms hardware may switch to an energy-saving mode on the fly
> > on the basis of certain utilization metrics used by it.  That usually is
> > desirable from the energy conservation standpoint, but it generally causes
> > latencies to increase which may adversely affect some operations.  For this
> > reason, the platforms in question usually provide some interfaces for software
> > to indicate its latency tolerance and possibly to prevent the energy-saving
> > modes from being selected too aggressively.
> > 
> > The following series of patches introduces a device PM QoS type allowing
> > those interfaces to be used by kernel code and user space.  It is designed
> > in analogy with the existing resume latency device PM QoS type, which allows
> > some pieces of the existing device PM QoS code to be re-used and makes the
> > new user space interface fit into the existing framework.
> > 
> > Patch [1/5] modifies the names of symbols, variables, functions and structure
> > fields associated with the existing resume latency device PM QoS type to
> > avoid any confusion with the new one introduced by the subsequent patches.
> > 
> > Patch [2/5] introduces a new field in struct pm_qos_constraints for specifying
> > a special value to be returned as the effective requirement when the given list
> > of PM QoS requirements is empty.  That field is necessary for the new latency
> > tolerance device PM QoS type.
> > 
> > Patch [3/5] introduces the latency tolerance device PM QoS type along with
> > documentation.
> > 
> > Patch [4/5] modifies the ACPI LPSS (Low-Power Subsystem) driver to hook up
> > LPSS devices to the new latency tolerance device PM QoS interface.
> > 
> > Patch [5/5] modifies the dev_pm_qos_add_ancestor_request() routine so that it
> > can be used by drivers of devices without hardware latency tolerance support
> > for specifying their requirements via the ancestors of those devies.
> 
> As usual, testing uncovered some issues, so an updated series follows.

I tested this series on HSWULT and adding/removing QoS LTR requests from
both userspace and from a modified i2c-hid driver (calling
dev_pm_qos_add_ancestor_request()) and seems to work nicely -- When I read
back the HW LTR values they are what is expected.

Tested-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
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