On Mo, 2018-10-08 at 10:50 +0100, Mayuresh Kulkarni wrote:
>
> As a result of this, the USB suspend (L2) does not seem to happen, even if
> all the interface drivers of a composite USB device report "idle" to USB core
> driver. The USB suspend seem to happen only when the caller in user-space (in
> our case) closes the device file.
>
> Is this correct understanding?
Yes, it is.
> If yes, could you please help understand -
> 1. Any specific reason to choose this design approach? Apologies, but "git
> blame" does not reveal much information (or maybe I did not do git blame on
> correct kernel version).
We cannot assume that a device is done executing a command as soon as
communication is done. Think of a scanner moving its sensor bar
or a printer printing a page. Or a display displaying something.
> 2. Is it possible to modify this driver to take PM ref-count on USB device,
> only when user-space is actively interacting with the USB device (so in
> open/close and appropriate ioctl calls, with special handling for async URB
> submission)?
Technically this is possible, but it is a bad idea.
> 3. Will (2) break any known existing device(s)?
Yes, it would and that makes it a bad idea.
Regards
Oliver
