src/kernel/linux/v4.19/Documentation/arm/firmware.txt

Interface for registering and calling firmware-specific operations for ARM.
----
Written by Tomasz Figa <t.figa@samsung.com>

Some boards are running with secure firmware running in TrustZone secure
world, which changes the way some things have to be initialized. This makes
a need to provide an interface for such platforms to specify available firmware
operations and call them when needed.

Firmware operations can be specified by filling in a struct firmware_ops
with appropriate callbacks and then registering it with register_firmware_ops()
function.

	void register_firmware_ops(const struct firmware_ops *ops)

The ops pointer must be non-NULL. More information about struct firmware_ops
and its members can be found in arch/arm/include/asm/firmware.h header.

There is a default, empty set of operations provided, so there is no need to
set anything if platform does not require firmware operations.

To call a firmware operation, a helper macro is provided

	#define call_firmware_op(op, ...)				\
		((firmware_ops->op) ? firmware_ops->op(__VA_ARGS__) : (-ENOSYS))

the macro checks if the operation is provided and calls it or otherwise returns
-ENOSYS to signal that given operation is not available (for example, to allow
fallback to legacy operation).

Example of registering firmware operations:

	/* board file */

	static int platformX_do_idle(void)
	{
		/* tell platformX firmware to enter idle */
		return 0;
	}

	static int platformX_cpu_boot(int i)
	{
		/* tell platformX firmware to boot CPU i */
		return 0;
	}

	static const struct firmware_ops platformX_firmware_ops = {
		.do_idle        = exynos_do_idle,
		.cpu_boot       = exynos_cpu_boot,
		/* other operations not available on platformX */
	};

	/* init_early callback of machine descriptor */
	static void __init board_init_early(void)
	{
		register_firmware_ops(&platformX_firmware_ops);
	}

Example of using a firmware operation:

	/* some platform code, e.g. SMP initialization */

	__raw_writel(__pa_symbol(exynos4_secondary_startup),
		CPU1_BOOT_REG);

	/* Call Exynos specific smc call */
	if (call_firmware_op(cpu_boot, cpu) == -ENOSYS)
		cpu_boot_legacy(...); /* Try legacy way */

	gic_raise_softirq(cpumask_of(cpu), 1);