RE: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

Gross, Mark
2010-06-03T15:48:39Z

>-----Original Message-----
>From: Kevin Hilman [mailto:khilman@deeprootsystems.com]
>Sent: Thursday, June 03, 2010 7:43 AM
>To: Peter Zijlstra
>Cc: Alan Cox; Gross, Mark; Florian Mickler; James Bottomley; Arve
>Hjønnevåg; Neil Brown; tytso@mit.edu; LKML; Thomas Gleixner; Linux OMAP
>Mailing List; Linux PM; felipe.balbi@nokia.com
>Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
>
>Peter Zijlstra <peterz@infradead.org> writes:
>
>> On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote:
>>> > [mtg: ] This has been a pain point for the PM_QOS implementation.
>>> They change the constrain back and forth at the transaction level of
>>> the i2c driver. The pm_qos code really wasn't made to deal with such
>>> hot path use, as each such change triggers a re-computation of what
>>> the aggregate qos request is.
>>>
>>> That should be trivial in the usual case because 99% of the time you can
>>> hot path
>>>
>>> the QoS entry changing is the latest one
>>> there have been no other changes
>>> If it is valid I can use the cached previous aggregate I cunningly
>>> saved in the top QoS entry when I computed the new one
>>>
>>> (ie most of the time from the kernel side you have a QoS stack)
>>
>> Why would the kernel change the QoS state of a task? Why not have two
>> interacting QoS variables, one for the task, one for the subsystem in
>> question, and the action depends on their relative value?
>
>Yes, having a QoS parameter per-subsystem (or even per-device) is very
>important for SoCs that have independently controlled powerdomains.
>If all devices/subsystems in a particular powerdomain have QoS
>parameters that permit, the power state of that powerdomain can be
>lowered independently from system-wide power state and power states of
>other power domains.
>
This seems similar to that pm_qos generalization into bus drivers we where
waving our hands at during the collab summit in April? We never did get
into meaningful detail at that time.

--mgross