Re: [PATCH] usb: core: devio: add ioctls for suspend and resume

[Oliver Neukum <oneukum@suse.com>]

Am Mittwoch, den 19.06.2019, 10:40 -0400 schrieb Alan Stern:
> On Wed, 19 Jun 2019, Oliver Neukum wrote:

> > It would be easiest to export the usb_autopm_* API to user space.
> 
> But ->suspend and ->resume callbacks are part of that API, and as you 
> say, it is not feasible to have these callbacks run in userspace.

We can notify user space about resume(). There will be a delay, but
there will always be a delay, even in kernel space.

> The only solution I can think of is for the userspace program to first
> set the device's autosuspend delay to 0.  Then whenever the
> WAIT_FOR_RESUME ioctl returns -- even if it returns immediately -- the
> program should assume the suspend is over or is not going to happen.  
> Then the program can run normally for a short while (10 seconds,
> perhaps) before making another attempt to suspend.

That is ugly. Quite ugly actually.

Why actually do we care about suspend()? Would the user space
task do anything else but cease IO? We can do that in usbfs,
assuming we do not care about the order of killing.
User space will learn from an appropriate error code the device
went to power save. And if it does not do IO, why care?
I have never seen a driver that actually saves device state in
suspend().

	Regards
		Oliver