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I'm trying to modify the libcomposite module in a way that it prints the functions of which are loaded to the log file.

I'm using an Raspberry Pi Zero W with the 4.9.20+ Kernel. I updated everything with the rpi-source then edited the composite.c file in drivers/usb/gadget folder to print with printk(..).

And recompiled everything with make modules SUBDIR=drivers/usb/gadget and the installed everything with make modules_install SUBDIR=drivers/usb/gadget.

The libcomposite is loaded with modprobe libcomposite in a systemd service and then the functions are appended.

But the problem is that the messages of printk(..) are neither shown in dmesg nor any log files.

Thanks for your help!

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This works for me, but there are many pitfalls that you could have stumbled on.

  • Correct install path: I cross-compiled from a PC so had to make sure INSTALL_MOD_PATH was specified properly. It sounds like you're building on the Pi itself so that this may not be an issue, but to double-check, confirm that the timestamp on /lib/modules/4.9.56+/kernel/drivers/usb/gadget/libcomposite.ko (or newer kernel version) has been updated.

  • Correct kernel.img: Make sure kernel.img is updated as well. If you run the old kernel it'll pull libcomposite.ko out of /lib/modules/4.9.35+/ (or whatever your original kernel version is). You could nuke the old modules directory to be certain.

  • kernel message level: KERN_INFO (6) is too high for messages to land in dmesg by default. Try printk(KERN_WARNING ...) for level 4 initially. pr_info() may be preferable for modules and does not have this complication.

  • Which part of composite.c are you putting your printout in? If usb_add_function(), note that this won't be called until you've fully configured a gadget and do the ls /sys/class/udc > UDC step.

By the way, I find the SUBDIR variable having no effect, but it doesn't hurt.

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