On Mon, 2019-08-05 at 10:07 -0400, Alan Stern wrote: > On Mon, 5 Aug 2019, Mayuresh Kulkarni wrote: > > > > > I think I found what is happening here: > > - Looks like, the USB audio class driver is > > calling usb_enable_autosuspend(). > The audio maintainers must have some good reason for doing that, but > I > don't know what it is. Normally kernel drivers are not supposed to > enable autosuspend. > Thanks, please see my comment below. > > > > - 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. > > Apologies, as I send the above response, without checking the latest stable kernel tree. In the kernel used on the platform I use (and also in Pixel-2 kernel), the .probe() of USB-audio class driver calls usb_enable_autosuspend(). usb_enable_autosuspend() call is NOT present in latest stable kernel's USB-audio class driver, which is in-sync with your comment. > > 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? > Generally it is _not_ okay. Especially not for the HID class driver > -- > there are far too many HID devices (like mice and keyboards) that > don't > work correctly when they go to low power. > > > > > OR > > 2. Is it recommended to call usb_enable_autosuspend() from user- > > space by > > writing "auto" to "cat /sys/bus/usb/devices/.../power/control"? > That is the recommended approach. > This all makes sense now. Thanks for all the comments and explainations. > > > > In my opinion, both should be fine. > Alan Stern >
