On Mon, 22 Oct 2018 10:24:46 -0400
Alan Stern <[email protected]> wrote:
Apologies for late response on this thread, had been on PTOs and also
checking internally about the pros/cons of suggested approach.
> On Mon, 22 Oct 2018, Oliver Neukum wrote:
>
> > On Do, 2018-10-18 at 13:42 -0400, Alan Stern wrote:
> > > On Thu, 18 Oct 2018, Mayuresh Kulkarni wrote:
> > >
> > > > > The only way to make the ioctl work properly is to have it do a
> > > > > runtime-PM put at the start and then a runtime-PM get before it
> >
> > If and only if you want to do this with one ioctl()
> > If you separate the runtime-PM put and the get, you can do it without
> > the waiting part.
>
> Sure, but you still need a runtime-PM put at the start and a runtime-PM
> get at the end. In fact, if you separate this into two calls then you
> run the risk that the user may never perform the second call, and you
> would end up with an unbalanced put.
>
A suggestion - shall we mandate the user-space to close the device before
calling the new ioctl?
As I understand, if that is done then, we don't need to worry about pm runtime
APIs in the new ioctl handler.
The new ioctl handler shall do the following steps:
- Check if open-count.
If !0 then return error.
If 0 then proceed to next step.
- Follow the sequence you mentioned in previous response, except no pm runtime
calls.
Sounds reasonable, in case, the user-space forgets to close the device and call
new ioctl, then we would bail-out with error i.e. user-space cannot expect the
device to suspend/resume unless it knows it is not going to use the device.
> > > > /*
> > > > * There are 3 possibilities here:
> > > > * 1. Device did suspend and resume (success)
> > > > * 2. Signal was received (failed suspend)
> > > > * 3. Time-out happened (failed suspend)
> > >
> > > 4. Device did suspend but a signal was received before the device
> > > resumed.
> > >
> > > > * In any of above cases, we need to resume device.
> > > > */
> > > > usb_autoresume_device(dev);
> >
> > Yes and that is the problem. Why do you want to wait for the result
> > of runtime-PM put ? If we need a channel for notifying user space
> > about resume of a device, why wait for the result of suspend instead
> > of using the same channel?
>
> This is not meant to be a general-purpose channel for notifying
> userspace when a device resumes. Such a channel should be defined in
> the runtime-PM layer, not in the USB layer.
>
> This is instead meant to be a special-purpose mechanism for adding a
> runtime-suspend/resume interface to usbfs.
>
> > > >
> > > > ps->resume_done = false;
> > > >
> > > > "ps->resume_done = true;" will be done by the runtime resume call-back.
> >
> > No. You cannot do that in this way. It needs to be a unified device
> > state or a sequence of multiple suspends and resumes will have strange
> > results.
>
> They won't, because such a sequence cannot occur. The ioctl thread
> will wake up when the first resume occurs and it will do a runtime-PM
> get, thus preventing any further suspends.
>
> Alan Stern
