From: Oliver Neukum <oneukum@suse.com>
To: Alan Stern <stern@rowland.harvard.edu>,
Mayuresh Kulkarni <mkulkarni@opensource.cirrus.com>
Cc: Greg KH <gregkh@linuxfoundation.org>,
patches@opensource.cirrus.com,
USB list <linux-usb@vger.kernel.org>
Subject: Re: [PATCH] usb: core: devio: add ioctls for suspend and resume
Date: Wed, 26 Jun 2019 09:42:40 +0200 [thread overview]
Message-ID: <1561534960.23604.6.camel@suse.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1906250945410.1493-100000@iolanthe.rowland.org>
Am Dienstag, den 25.06.2019, 10:08 -0400 schrieb Alan Stern:
> On Tue, 25 Jun 2019, Mayuresh Kulkarni wrote:
> > Right, so user space should do the following when it determines the
> > device is idle from its point of view -
> >
> > 1. Call ALLOW_SUSPEND ioctl
That is a race in principle. You should reverse steps 1 and 2
> > 2. Queue an URB and wait for its REAP. When the wait returns -EFAIL (or
> > something similar), that is the indication that the device is no longer
> > active (or suspended)
> > 3. Call WAIT_FOR_RESUME ioctl
It seems to me that there ought to be one API for that. Either an
ioctl or poll.
> > 4. When WAIT_FOR_RESUME ioctl returns, it is guaranteed that device is
> > active.
> > 5. Call FORBID_SUSPEND ioctl and read the cause of resume.
> > 6. Go to (1) when appropriate
> >
> > Have I summarized this approach correctly from user-space point of view?
>
> Yes, except for one thing: In step 4, it is _not_ guaranteed that the
> device is active when WAIT_FOR_RESUME returns. The only guarantee is
> that a resume did occur sometime after step 1, but the device might
> have gone back into suspend after that occurred.
Now is that a good API? Shouldn't we rather have an API for either
* resume the device now and bump the counter
* bump the counter when te device resumes
I don't see a value in not having a defined power state after resume.
> > 3. Extend usbdev_poll() to wait for udev->state == USB_STATE_SUSPENDED
> > when events == POLLPRI. Return POLLPRI when state = USB_STATE_SUSPENDED.
> > 4. As before, any ioctl != (ALLOW_SUSPEND or WAIT_FOR_RESUME)
> > calls usb_autoresume_device().
>
> 3 sounds reasonable at first, but I'm not sure it would work.
> Consider what would happen if the device is suspended very briefly and
> then wakes up. The usbdev_poll() call might not return, because by the
> time it checks udev->state, the state has already changed back to
> USB_STATE_CONFIGURED.
Indeed. It seems to me that any power transition should be reported
back.
> In any case, we shouldn't do 4. It would prevent the device from ever
> going into suspend, because the program would want to continue making
> usbfs ioctl calls while waiting for the suspend to occur.
Exactly.
> > The corresponding user-space calls would be -
> > A. When determined device is idle, call ALLOW_SUSPEND ioctl.
> > B. Call poll(device_fd, POLLPRI). When poll returns check revents
> > == POLLPRI.
>
> What if the device never does go into suspend? The poll() call
> wouldn't return and the program would just stop working.
Well, that is why you can poll for multiple events at the same
time and the syscall has a timeout.
> > 2. Is it safe to wait for udev->state to be of a particular value?
>
> No, not really, because the state can change.
That is a general issue with poll. You cannot be sure that there
is still data to be read when you are ready to read.
Regards
Oliver
next prev parent reply other threads:[~2019-06-26 7:56 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 [this message]
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 ` [PATCH] usb: core: devio: add ioctls for " Mayuresh Kulkarni
2019-06-20 14:43 ` Alan Stern
2019-06-13 13:32 ` Mayuresh Kulkarni
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