Re: Query on usb/core/devio.c

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On Mon, 8 Oct 2018, Oliver Neukum wrote:

> 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.

In theory we could add ioctls to perform the runtime PM put and get 
operations.

Alan Stern




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