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

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Date:   Fri, 16 Sep 2016 16:54:31 +0100
From:   One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        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: Re: Should drivers like nvme let userspace control their latency
 via dev_pm_qos?

On Fri, 16 Sep 2016 08:26:03 -0700
Andy Lutomirski <luto@...nel.org> wrote:

> 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.

Why is this in the driver. Surely the latency is a property of the
request queue and the requests being made. Now it may well be that its
implement as min(list-of-queues) but a device sysfs node seems a strange
place to stick it.

Alan