Re: About usb_new_device() API

[Mayuresh Kulkarni <mkulkarni@opensource.cirrus.com>]

On Fri, 2019-08-02 at 12:27 -0400, Alan Stern wrote:
> On Fri, 2 Aug 2019, Mayuresh Kulkarni wrote:
> 
> > 
> > Hi Alan,
> > 
> > Thanks a lot for clearing out the confusion.
> > 
> > Our USB device can operate in 2 mutually exclusive modes: one is
> > normal
> > composite USB audio mode and other is vendor specific HID device
> > mode.
> > 
> > On the same platform (Android based):
> > - When the device is in normal composite USB audio mode,
> > "cat /sys/bus/usb/devices/.../power/control" show "auto".
> > - When the device is in vendor specific HID device mode,
> > "cat /sys/bus/usb/devices/.../power/control" show "on".
> > 
> > And hence as per your comment, I am unable to see USB-2.0 L2 for
> > vendor
> > specific HID device mode.
> > 
> > I guess I need to find out "who" is setting the /power/control =
> > "auto"
> > when composite USB audio device is detected. And explore if it could
> > be
> > moved to a more generic place.
> I'm pretty sure this is done by some program, not automatically done
> by 
> the kernel.

I think I found what is happening here:
- Looks like, the USB audio class driver is
calling usb_enable_autosuspend().
- Please check $KERNEL_SRC/sound/usb, card.c, usb_audio_probe().
- It is using interface_to_usbdev() to get the parent of its interface
device, which is the USB device allocated by the hub driver.
And hence, in above use-case, I can see L2 when our device is in
composite USB-audio mode.

Moreover, the HID-class driver doesn't seem to call
usb_enable_autosuspend() on its parent and hence I don't see L2 when our
device operates as a vendor specific HID device.
So a simple fix would be to call usb_enable_autosuspend() from HID class
driver as well.

With that said, what would be your recommendation here, Alan -
1. Is it OK for USB-class drivers to call usb_enable_autosuspend() on
their parent device to ensure low power state is entered?
OR
2. Is it recommended to call usb_enable_autosuspend() from user-space by
writing "auto" to "cat /sys/bus/usb/devices/.../power/control"?

In my opinion, both should be fine.

Thanks,

> 
> > 
> > Is there any module parameter (or some other means) by which,
> > power/control (or deprecated power/level) will always be "auto", by
> > default?
> No, there isn't.  The power/control (or power/level) attribute has to 
> be set by the user or a program.
> 
> Alan Stern
>