From: Mayuresh Kulkarni <mkulkarni@opensource.cirrus.com>
To: Oliver Neukum <oneukum@suse.com>, Alan Stern <stern@rowland.harvard.edu>
Cc: Greg KH <gregkh@linuxfoundation.org>,
<patches@opensource.cirrus.com>, <linux-usb@vger.kernel.org>
Subject: Re: [PATCH] usb: core: devio: add ioctls for suspend and resume
Date: Thu, 20 Jun 2019 15:34:55 +0100 [thread overview]
Message-ID: <1561041295.20348.4.camel@opensource.cirrus.com> (raw)
In-Reply-To: <1560935981.4587.10.camel@suse.com>
On Wed, 2019-06-19 at 11:19 +0200, Oliver Neukum wrote:
> Am Dienstag, den 18.06.2019, 11:50 -0400 schrieb Alan Stern:
> >
> > On Tue, 18 Jun 2019, Mayuresh Kulkarni wrote:
> >
> > >
> > > >
> > > > You're right that the program needs to know when the device is
> > > > about
> > > > to
> > > > be suspended. But waiting for an ioctl to return isn't a good
> > > > way
> > > > to do it; this needs to be a callback of some sort. That is,
> > > > the
> > > > kernel also needs to know when the program is ready for the
> > > > suspend.
> > > >
> > > > I don't know what is the best approach.
> > > This is becoming tricky now.
> > Yes. There probably are mechanisms already in use in other parts
> > of
> > the kernel that would be suitable here, but I don't know what they
> > are.
> > We could try asking some other people for advice.
> Waiting for an ioctl() is horrible. If you really want to do this
> poll() would be the obvious API. It is made for waiting for changes
> of states.
>
Understood and agreed.
> [..]
> >
> > The suspend callback is _not_ responsible for actually suspending
> > the
> > device; that is handled by the USB subsystem core.
> >
> > These ideas are indeed applicable to programs using usbfs. The
> > kernel
> Not fully. Usbfs has the same issue as FUSE here. Drivers are per
> interface but power management is per device. Hence every driver
> is in the block IO path for these issues. You cannot do block IO
> in user space. The best you can do is notify of state changes,
> but you cannot wait for them.
>
> >
> > needs to have a way to inform the program that the device is about
> > enter (or has just left) a low-power state, so that the program can
> > stop (or start) trying to communicate with the device. And the
> > kernel
> > needs to know when the program is ready for the state change.
> That has difficulties based in fundamental issues. We can let user
> space block power transitions. We can notify it. But we cannot
> block on it.
>
> It would be easiest to export the usb_autopm_* API to user space.
AFAIU, usb_autopm_* API operate on interface rather than on device.
Due to this, they are *indirectly* exposed by appropriate class drivers
to the user-space class drivers cater to. E.g.: USB audio class driver
calls usb_autopm_* APIs when user space calls pcm_open(playback_stream).
>
> Regards
> Oliver
>
next prev parent reply other threads:[~2019-06-20 14:35 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-10 10:01 [PATCH] usb: core: devio: add ioctls for suspend and resume Mayuresh Kulkarni
2019-06-05 9:41 ` Greg KH
2019-06-05 21:02 ` Alan Stern
2019-06-13 14:00 ` Mayuresh Kulkarni
2019-06-13 20:54 ` Alan Stern
2019-06-17 11:38 ` Mayuresh Kulkarni
2019-06-17 15:55 ` Alan Stern
2019-06-18 14:00 ` Mayuresh Kulkarni
2019-06-18 15:50 ` Alan Stern
2019-06-19 9:19 ` Oliver Neukum
2019-06-19 14:40 ` Alan Stern
2019-06-19 15:12 ` Oliver Neukum
2019-06-19 16:07 ` Alan Stern
2019-06-20 15:11 ` Mayuresh Kulkarni
2019-06-20 15:49 ` Alan Stern
2019-06-21 16:16 ` Mayuresh Kulkarni
2019-06-21 19:28 ` Alan Stern
2019-06-24 16:02 ` Mayuresh Kulkarni
2019-06-24 17:48 ` Alan Stern
2019-06-25 10:41 ` Mayuresh Kulkarni
2019-06-25 14:08 ` Alan Stern
2019-06-26 7:42 ` Oliver Neukum
2019-06-26 14:35 ` Alan Stern
2019-06-26 14:15 ` Mayuresh Kulkarni
2019-06-26 15:07 ` Alan Stern
2019-06-27 13:20 ` Mayuresh Kulkarni
2019-06-27 13:52 ` Alan Stern
2019-07-01 9:18 ` Oliver Neukum
2019-07-01 14:17 ` Alan Stern
2019-07-02 9:21 ` Oliver Neukum
2019-07-02 14:29 ` Alan Stern
2019-07-03 14:44 ` Mayuresh Kulkarni
2019-07-05 18:51 ` [RFC] usbfs: Add ioctls for runtime " Alan Stern
2019-07-11 9:16 ` Mayuresh Kulkarni
2019-07-11 14:20 ` Alan Stern
2019-07-11 14:36 ` Greg KH
2019-07-25 9:10 ` Mayuresh Kulkarni
2019-07-25 9:18 ` Greg KH
2019-07-25 15:18 ` Alan Stern
2019-07-25 16:05 ` Greg KH
2019-06-20 14:34 ` Mayuresh Kulkarni [this message]
2019-06-20 14:43 ` [PATCH] usb: core: devio: add ioctls for " Alan Stern
2019-06-13 13:32 ` Mayuresh Kulkarni
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