Re: [RFT][PATCH 0/3] cpufreq / PM: QoS: Introduce frequency QoS and use it in cpufreq

[Leonard Crestez <leonard.crestez@nxp.com>]

Hello,

I've been working on a series which add DEV_PM_QOS support to devfreq, 
now at v9:

	https://patchwork.kernel.org/cover/11171807/

Your third patch removes DEV_PM_QOS_FREQUENCY_MIN/MAX that my series 
depends upon. I found the email on patchwork, hopefully the in-reply-to 
header is OK?

As far as I can tell the replacement ("frequency qos") needs constraints 
to be managed outside the device infrastructure and it's not obviously 
usable a generic mechanism for making "min_freq/max_freq" requests to a 
specific device.

I've read a bit through your emails and it seems the problem is that 
you're dealing with dev_pm_qos on per-policy basis but each "struct 
cpufreq_policy" can cover multiple CPU devices.

An alternative solution which follows dev_pm_qos would be to add 
notifiers for each CPU inside cpufreq_online and cpufreq_offline. This 
makes quite a bit of sense because each CPU is a separate "device" with 
a possibly distinct list of qos requests.

If cpufreq needs a group of CPUs to run at the same frequency then it 
should deal with this by doing dev_pm_qos_read_frequency on each CPU 
device and picking a frequency that attempts to satisfy all constraints.

Handling sysfs min/max_freq through dev_pm_qos would be of dubious 
value, though I guess you could register identical requests for each CPU.

I'm not familiar with what you're trying to accomplish with PM_QOS other 
than replace the sysfs min_freq/max_freq files: What I want to do is add 
a driver using the interconnect driver which translates requests for 
"bandwidth-on-a-path" into "frequency-on-a-device". More specifically a 
display driver could request bandwidth to RAM and this would be 
translated into min frequency for NoC and the DDR controller, both of 
which implement scaling via devfreq:

	https://patchwork.kernel.org/cover/11104113/
	https://patchwork.kernel.org/cover/11111865/

This is part of an effort to upstream an out-of-tree "busfreq" feature 
which allows device device to make "min frequency requests" through an 
entirely out-of-tree mechanism. It would also allow finer-grained 
scaling that what IMX tree currently support.

If you're making cpufreq qos constrains be "per-cpufreq-policy" then 
it's not clear how you would handle in-kernel constraints from other 
subsystems. Would users have to get a pointer to struct cpufreq_policy 
and struct freq_constraints? That would make object lifetime a 
nightmare! But dev_pm_qos solves this by tying to struct device.

And if you don't care about in-kernel requests then what's the purpose 
of involving PM QoS? The old min/max_freq sysfs implementation worked.

--
Regards,
Leonard