linux-kernel - Re: [PATCH 0/2] PM: QoS/pmdomains: support resume latencies for system-wide PM

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Message-ID: <CAPDyKFr+OjWLnzxBOhfJaMZb1GY7axgyeUAPinQTXMB6Ngpz8w@mail.gmail.com>
Date: Wed, 28 Jan 2026 11:33:11 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: "Kevin Hilman (TI)" <khilman@...libre.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org, 
	Dhruva Gole <d-gole@...com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] PM: QoS/pmdomains: support resume latencies for
 system-wide PM

On Wed, 21 Jan 2026 at 02:54, Kevin Hilman (TI) <khilman@...libre.com> wrote:
>
> Currently QoS resume latencies are only considered for runtime PM
> transitions of pmdomains, which remains the default.
>
> In order to also support QoS resume latencies during system-wide PM,
> add a new flag to indicate a resume latency should be used for
> system-wide PM *instead of* runtime PM.
>
> For example, by doing this:
>
>    # echo 500000 > /sys/devices/.../<dev0>/power/pm_qos_resume_latency_us
>
> dev0 now has a resume latency of 500000 usec for runtime PM
> transitions.
>
> Then, if the new flag is also set:
>
>    # echo 1 > /sys/devices/.../<dev0>/power/pm_qos_latency_sys
>
> That 500000 usec delay now applies to system-wide PM (and not to
> runtime PM).
>
> If a user requires a different latency value for system-wide PM
> compared to runtime PM, then the runtime PM value can be set for
> normal operations, and the system-wide value (and flag) can be set by
> userspace before suspend, and the runtime PM value can be restored
> after resume.

That's sounds complicated for user-space to manage - and causes churns
during every suspend/resume cycle. Why don't we just add a new latency
value instead, that applies both to runtime PM and system-wide PM,
similar and consistent to what we did for CPU QoS?

Kind regards
Uffe

>
> To: Rafael J. Wysocki <rafael@...nel.org>
> To: Ulf Hansson <ulf.hansson@...aro.org>
> To: linux-pm@...r.kernel.org
>
> Signed-off-by: Kevin Hilman (TI) <khilman@...libre.com>
> ---
> Kevin Hilman (TI) (2):
>       PM / QoS: add flag to indicate latency applies system-wide
>       pmdommain: add support system-wide resume latency constraints
>
>  drivers/base/power/sysfs.c  | 27 +++++++++++++++++++++++++++
>  drivers/pmdomain/governor.c | 42 +++++++++++++++++++++++++++++++++++++++++-
>  include/linux/pm_qos.h      |  2 ++
>  3 files changed, 70 insertions(+), 1 deletion(-)
> ---
> base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
> change-id: 20260120-topic-lpm-pmdomain-device-constraints-e5e78ce48502
>
> Best regards,
> --
> Kevin Hilman (TI) <khilman@...libre.com>
>