On Thu, 18 Oct 2018 13:42:46 -0400 Alan Stern <st...@rowland.harvard.edu> 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 > > > returns. This is true regardless of the reason for returning: normal > > > termination, timeout, signal, whatever. Nothing else would be safe. > > > > > > > Will below steps work safely (sometimes pseudo-code/snippets help to > > align)? - > > > > "new" ioctl - > > > > timeout is parameter to ioctl. > > > > /* attempt suspend of device */ > > usb_autosuspend_device(dev); > > > > usb_unlock_device(dev); > > r = wait_event_interruptible_timeout(ps->resume_wait, > > (ps->resume_done == true), timeout * HZ); > > Not exactly. The condition to test here is whether the device has been > suspended, so it's more like this: > > r = wait_event_interruptible_timeout(ps->suspend_wait, > (ps->suspend_done == true), timeout * HZ); > > where ps->suspend_done is set by the runtime_suspend callback. After > this we will do: > > if (r > 0) /* Device suspended before the timeout expired */ > r = wait_event_interruptible(ps->resume_wait, > (ps->resume_done == true)); > > > usb_lock_device(dev); > > > > /* > > * 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); > > > > ps->resume_done = false; > > > > "ps->resume_done = true;" will be done by the runtime resume call-back. > > Exactly. You got it. Will that let you accomplish what you want? > We spend time internally to go over the "new" ioctl proposal. Overall it looks promising. However, we still have an issue as below - Consider a use-case where, user-space calls "new" ioctl, but suspend never happen (for various reasons) && async event happens on USB device side by the end-user. In such a case, since user-space is waiting in "new" ioctl, it is not in position to queue a request to read-out the async event info. It will be able to queue a request when the "new" ioctl returns which will be "time-out" later (in this case). Due to auto-suspend time-out's default's value of 2 sec, the user-space has to choose the time-out to "new" ioctl > 2 sec. Does this use-case make sense? If yes, this probably is not a good UX since user did something and it took > 2 sec for the system to respond and take the appropriate action. Looking into finer details of this use-case, now. > Alan Stern
