powerpc/slb: Add documentation on runtime patching of SLB encoding

static inline void patch_slb_encoding(unsigned int *insn_addr,

				      unsigned int immed)

{

	int insn = (*insn_addr & 0xffff0000) | immed;

	/*

	 * This function patches either an li or a cmpldi instruction with

	 * a new immediate value. This relies on the fact that both li

	 * (which is actually addi) and cmpldi both take a 16-bit immediate

	 * value, and it is situated in the same location in the instruction,

	 * ie. bits 16-31 (Big endian bit order) or the lower 16 bits.

	 * The signedness of the immediate operand differs between the two

	 * instructions however this code is only ever patching a small value,

	 * much less than 1 << 15, so we can get away with it.

	 * To patch the value we read the existing instruction, clear the

	 * immediate value, and or in our new value, then write the instruction

	 * back.

	 */

	unsigned int insn = (*insn_addr & 0xffff0000) | immed;

	patch_instruction(insn_addr, insn);

}