lsusb: detach the kernel driver before claiming the interface

[jazzguitar81]

To display a report descriptor for HID device, we have to success claim
the interface. So, we need to detach the kernel driver before claming the
interface.

The libusb_set_auto_detach_kernel_driver() in libusb automatically do detach
the kernel driver on an interface when claiming the interface, and attach it
when releasing the interface.

Signed-off-by: Jaejoong Kim climbbb.kim@gmail.com

[gregkh]

Sorry for the delay.

What type of side-affect does this cause? I'm leery of changing the way the interaction with the kernel works here, as unbind/bind could cause issues for some devices, right?

[jazzguitar81]

With original source(without my patch), the return value of libusb_claim_interface is -6 which means LIBUSB_ERROR_BUSY if another program or driver has claimed the interface.

To display the report descriptor for HID device, we need to detach the kernel driver or another program. The libusb_set_auto_detach_kernel_driver() makes 'auto_detach_kernel_driver' field from struct libusb_device_handle set as enable.

There is a comment about the fail case about libusb_claim_interface:

static void dump_hid_device()
.. snip..

3195 } else {
3196 /* recent Linuxes require claim() for RECIP_INTERFACE,
3197 * so "rmmod hid" will often make these available.
3198 */
3199 printf(" Report Descriptors: \n"
3200 " ** UNAVAILABLE **\n");
3201 }

I don't think this makes the side-effect the HID devices already claimed interface. The only problem is the shortly mouse pointer is stop while detached the HID kernel driver and re-attach the kernel driver if the user run the 'lsusb' command.

[gregkh]

Let me think about this for maybe the next release...

[jazzguitar81]

Still here..
I'm OK if you drop this patch :)

Anyway, I checked usbhid-dump(dump hid report descriptor) source code and it also detach a hid interface before claim interface. Proceed in the following order:
detach --> claim --> releae --> attach interface back

So, the usbhid-dump program also causes the mouse pointer to be briefly interrupted.

[gregkh]

On Thu, Feb 14, 2019 at 04:00:52PM +0000, Valerii Zapodovnikov wrote: @jazzguitar81 @gregkh Maybe I don't understand smth, but is "detach/unbind the kernel driver" really nesessary? One could just use > cat /sys/kernel/debug/hid/*/rdesc You can read about it there http://www.slashdev.ca/2010/05/08/get-usb-report-descriptor-with-linux/ (first comment).

Only works with HID devices. And as usbhid-dump comes with usbutils, there's not much more I can do about that :)

[jazzguitar81]

@ValZapod
Sorry about late response.

@jazzguitar81 @gregkh Maybe I don't understand smth, but is "detach/unbind the kernel driver" really necessary? One could just use

No really necessary. I just want to show a report descriptor from lsusb without any other actions.

cat /sys/kernel/debug/hid/*/rdesc

You can read about it there http://www.slashdev.ca/2010/05/08/get-usb-report-descriptor-with-linux/ (first comment).

@gregkh
I still OK if you close this PR.

[gregkh]

There's nothing to comment on it. debugfs is accessible only by root, as it is for debugging information only.

I really don't like the unbind/bind method here, as I have stated a number of times already.

[gregkh]

And about debug descriptors?

I have no idea what you are referring to with when you say this, sorry, please be more specific.

[gregkh]

I really do not like the unbind/bind stuff here, it can cause problems with userspace so I'm going to just close this out, thanks.