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long unlocked_ioctl(struct file *f,
unsigned int cmd, unsigned long arg)
Associated to the ioctl() system call
Called unlocked because it doesn't hold the Big Kernel Lock.
Allows to extend the driver capabilities beyond the limited read/write API
For example: changing the speed of a serial port, setting video output format, querying a device serial number...
cmd is a number identifying the operation to perform
arg is the optional argument passed as third argument of the ioctl() system call. Can be an integer, an address, etc.
The semantic of cmd and arg is driver-specific.