[App_rpt-users] [PATCH] chan_usbradio fixes for newer kernels

[James Le Cuirot]

Hello all,

As mentioned in my previous post, I needed to make changes to
chan_usbradio as well as Zaptel for Linux 3.1.0. The crux of the issue
here was that the structure of /sys had changed.

The first clue was when Asterisk started reporting that the USB device
".." had been assigned to a radio channel. The code previously looked
up a path like:

/sys/class/sound/dsp1/device

On older kernels, this path would be a symlink to something like:

../../../devices/pci0000:00/0000:00:04.2/usb1/1-1/1-1:1.0

The second last component of this was extracted, resulting in 1-1.
However, the above symlink now simply points to ../../card1, resulting
in .. as I mentioned before. Now everything under /sys/class/sound is
also a symlink and it is these that need to be queried instead. For
dsp1, the new target is:

../../devices/pci0000:00/0000:00:04.2/usb1/1-1/1-1:1.0/sound/card1/dsp1

This is similar to before but the code that previously handled the path
extraction was long winded. basename and dirname do the job for us much
more efficiently.

This new code will not work on older kernels. I was going to address
that issue when it occurred to me that this whole process is very
convoluted and over-complicated. Surely libusb can simply tell us the
"devstr" value without having to iterate over all the sound cards and
then go digging around in /sys? As it turns out, it can't, at least not
in this version. The old "legacy" libusb API is being used here. The
newer API includes a function called libusb_get_device_address but I'm
not sure whether this returns the right information. The following
discussion from August has led me to believe that the function we need
may actually be introduced in the next version.

http://libusb.6.n5.nabble.com/Find-specific-USB-device-based-on-udev-info-td4539379.html

Despite this, I was still able to simplify the confusingly named
usb_get_usbdev function. Instead of iterating over all the sound cards,
looking for the one that matches the given devstr, we simply look up
the device directly under /sys. For the 1-1 example:

/sys/bus/usb/devices/1-1/1-1:1.0/sound

This is a directory containing just one other directory, card1 in this
case. We just look for a directory starting with "card" and take the
number from the end. Job done.

So to sum up, the attached patch works but only with newer kernels. I
don't know exactly which kernel version introduced the change. It may
have been as far back as 2.6.25. This could be fixed up to work with
older kernels too but moving to the newer libusb and vastly simplifying
this code is well worth considering. Of course, so is ditching
Asterisk, but that's covered in my other post. ;) This code would look
much the same under any system anyway.

Regards,
James
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