I have serial-usb adaptor from Sunplus, it is using module spcp8x5 from kernel. When I try to connect it using usb_claim_interface from usb.h it returns code -16 (it should be "device is busy").
Do you have any idea where can be the problem?

dmesg writes:
usb 1-1: new full speed USB device using uhci_hcd and address 3
usb 1-1: configuration #1 chosen from 1 choice
spcp8x5 1-1:1.0: SPCP8x5 converter detected
usb 1-1: SPCP8x5 converter now attached to ttyUSB0

I use kernel 2.6.28, gentoo.

PS: the problem shouldn't be in the program, I have downloaded it from http://multimeter.schewe.com/, and it looks that it works, only the function usb_claim_interface fails.

Welcome to LinuxQuestions.
Are you using a USB serial port adapter or are you connecting the multimeter in the posted link to your computer. I have not downloaded and examined program but my first guess is that it is not going to work as you expected. I might be reading to much into the what was posted in the link but it appears that the module is actually part of the multimeter program. In either case the device is being recognized, the module loading and the device is assigned ttyUSB0.

If this is a USB serial port adapter what type of device do you plan on connecting to the cable?