On Mon, Aug 22, 2016 at 8:19 AM, Jay Aurabind wrote:
> Hi Tormod and Steve,
>
> Thanks for your responses. I tried running the pyocd-gdbserver with
> LIBUSB_DEBUG=9, and got the following[snipped]:
>
> libusb: debug [libusb_open] open 2.1
> libusb: error [_get_usbfs_fd] libusb couldn't open USB device
> /dev/bus/usb/002/001: Permission denied
> libusb: error [_get_usbfs_fd] libusb requires write access to USB device
> nodes.
> libusb: debug [libusb_open] open 2.1 returns -3
> uncaught exception: The device has no langid
> Traceback (most recent call last):
>
> The device 1 on bus 2, is actually the root hub - the output of lsusb
> -v -s2:1 is:
I am a bit confused here, did you really want to manipulate the root
hub with pyusb, or did something go wrong here?
>
> Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
> Device Descriptor:
> bLength 18
> bDescriptorType 1
> bcdUSB 2.00
> bDeviceClass 9 Hub
> bDeviceSubClass 0
> bDeviceProtocol 0 Full speed (or root) hub
> bMaxPacketSize0 64
> idVendor 0x1d6b Linux Foundation
> idProduct 0x0002 2.0 root hub
> bcdDevice 4.06
> iManufacturer 3 Linux 4.6.6-300.fc24.x86_64 ehci_hcd
> iProduct 2 EHCI Host Controller
> iSerial 1 0000:00:1d.0
> bNumConfigurations 1
(Obviously the underlying hardware was not made by the Linux
Foundation, the kernel usb stack just presents the root hub like it
would be yet another USB device.)
A "device" like this will be grabbed by the kernel (ehci_hcd driver).
More generally it can happen per USB interface of the device and each
can be claimed by its own driver.
The same would be the case for e.g. a USB mass storage device. The
relevant kernel driver will grab it exclusively, and you won't get
permissions to write to it, whatever the file permissions on the
corresponding USB device node.
Some devices can be released from the kernel, and then be available
for you to manipulate it through the device node (and thus via e.g
libusb and pyusb). See for instance the libusb_detach_kernel_driver()
function in libusb. However, in most cases people use libusb and pyusb
to access devices that have no kernel driver.
> But even after adding the following rule to udev, I am still getting
> the permission error:
>
> ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="1d6b",
> ATTRS{idProduct}=="0003", MODE="0666", GROUP="plugdev"
> ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="0d28",
> ATTRS{idProduct}=="0204", MODE="0666", GROUP="plugdev"
>
> I tried with both with and without GROUP="plugdev" because the group
> didnt really exist in my (Fedora 24) distro. Although I created the
> group and added myself to it, and rebooted.
(You can of course use any group that you are member of.)
Regards,
Tormod
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