From: Mayuresh Kulkarni <mkulkarni@opensource.cirrus.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Greg KH <gregkh@linuxfoundation.org>, <linux-usb@vger.kernel.org>,
<patches@opensource.cirrus.com>, <oneukum@suse.com>
Subject: Re: [PATCH] usb: core: devio: add ioctls for suspend and resume
Date: Tue, 18 Jun 2019 15:00:41 +0100 [thread overview]
Message-ID: <1560866441.8425.8.camel@opensource.cirrus.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1906171119130.1738-100000@iolanthe.rowland.org>
On Mon, 2019-06-17 at 11:55 -0400, Alan Stern wrote:
> On Mon, 17 Jun 2019, Mayuresh Kulkarni wrote:
>
> >
> > >
> > > (Note: I imagine you might run into trouble because devices
> > > generally
> > > do not get put into runtime suspend immediately. So if you call
> > > the
> > > USBDEVFS_SUSPEND ioctl and then the USBDEVFS_WAIT_FOR_RESUME
> > > ioctl,
> > > the
> > > wait will return immediately because the device hasn't yet been
> > > suspended.)
> > >
> > Hi Alan,
> >
> > For this particular comment, how about we add suspend-waiter similar
> > to
> > resume-waiter?
> > As per the below changes, usbfs_notify_suspend() can wake the
> > suspend-
> > waiter, if generic_suspend() is called. So, the suspend ioctl will
> > be
> > partial blocking i.e.: it will wait till suspend happens and will
> > return
> > when it is safe to do resume.
> >
> > Will this work?
> Probably not. Just think about it: Your program stops communicating
> with the device and tells the kernel that it's ready for the device to
> be suspended. But the device doesn't go into suspend for several
> seconds (or longer!) and during that time your program has no idea
> what's happening to it. I'm pretty sure that's not what you want.
>
Right, sounds good. Thanks.
> 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.
>
> >
> > The reason for asking this is - I think the suspend ioctl should
> > return
> > appropriate status to user-space indicating weather to wait-for-
> > resume
> > or not.
> >
> > Or are you suggesting to always have a delay in suspend/resume in
> > user-
> > space?
> >
> > Please do review my comment below in this context too.
> >
> > In a typical SoC based system (XHCI compliant USB host controller
> > with
> > one port exposed out on PCB), wouldn't this
> > call usbfs_notify_suspend()
> > twice - first for udev of connected device and then for udev of
> > root-
> > hub?
> Yes, it would. This wouldn't make any difference to your program,
> since your program would have an open file reference only for the
> connected device, not for the root hub.
>
> >
> > If yes, how about we call usbfs_notify_*() for root-hubs only? That
> > would be a good indication of suspend/resume since root-hubs will be
> > suspended last while resumed first.
> >
> > Will that work?
> No. Remember, this mechanism has to work on non-SoC systems
> too. And
> even on an SoC, it's possible that your device is just one of several
> plugged into an external hub. So your device might be suspended
> while
> the others remain active; then the root hub would not be suspended.
>
I think my point is - usbfs driver is actually doing nothing w.r.t USB-
2.0 L2 state, right? The root-hub's suspend will invoke the USB-2.0 L2
transitions. This will happen when all the USB devices on that port
report idle to USB-core.
I agree that usually driver's suspend/resume call-back will put the
device in its low power state. But that is not applicable to udev of
usbfs driver, right?
So, doesn't it makes sense to tell user-space about actual USB-2.0 L2
state transitions rather than suspend/resume entry call-backs of device-
driver model of kernel (which are stub in this context)?
> Alan Stern
>
next prev parent reply other threads:[~2019-06-18 14:00 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 [this message]
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 ` [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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