linux-kernel - Should drivers like nvme let userspace control their latency via dev_pm

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Message-ID: <CALCETrUsgA2H=TuwZMuwW2XpXZ7KPrEYu6xckmTbhU8oZwr6Nw@mail.gmail.com>
Date:   Fri, 16 Sep 2016 08:26:03 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        linux-nvme@...ts.infradead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        J Freyensee <james_p_freyensee@...ux.intel.com>,
        Christoph Hellwig <hch@....de>
Subject: Should drivers like nvme let userspace control their latency via dev_pm_qos?

I'm adding power management to the nvme driver, and I'm exposing
exactly one knob via sysfs: the maximum permissible latency.  This
isn't a power domain issue, and it has no dependencies -- it's
literally just the maximum latency that the driver may impose on I/O
for power saving purposes.

ISTM userspace should be able to specify its own latency tolerance in
a uniform way, and dev_pm_qos seems like the natural interface for
this, except that I cannot find a single instance in the tree of *any*
driver using it via the notifier mechanism.  I can find two drivers
that do it using dev_pm_qos_expose_latency_tolerance(), and both are
LPSS drivers?

So: should I be exposing .set_latency_tolerance() or should I just use
a custom sysfs attribute?  Or both?