The random_harvest function is used by device drivers and other kernel processes to pass data that is considered (at least partially) stochastic to the entropy device.
The caller should pass a pointer (to no more than 16 bytes) of the "random" data in entropy. The argument size contains the number of bytes pointed to. The caller should "very conservatively" estimate the number of random bits in the sample, and pass this in bits or frac. If the estimated number of bits per sample is an integer, then bits is used, and frac is 0. Otherwise, for low-entropy samples, "fractional" entropy can be supplied in frac. (This is considered to be frac / 1024 bits of entropy.) The source is chosen from RANDOM_WRITE, RANDOM_KEYBOARD, RANDOM_MOUSE, RANDOM_NET and RANDOM_INTERRUPT, and is used to indicate the source of the entropy.
Interrupt harvesting has been simplified for the kernel programmer. If a device driver registers an interrupt handler with BUS_SETUP_INTR(9) or bus_setup_intr(9), then it is only necessary to include the INTR_ENTROPY bit in the flags argument to have that interrupt source be used for entropy harvesting.