firmware loader: document firmware cache mechanism

This patch documents the firmware cache mechanism so that
users of request_firmware() know that it can be called
safely inside device's suspend and resume callback, and
the device's firmware needn't be cached any more by individual
driver itself to deal with firmware loss during system resume.
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/firmware_class/README

@@ -119,3 +119,10 @@
    on the setup, so I think that the choice on what firmware to make
    persistent should be left to userspace.
 
+ about firmware cache:
+ --------------------
+ After firmware cache mechanism is introduced during system sleep,
+ request_firmware can be called safely inside device's suspend and
+ resume callback, and callers need't cache the firmware by
+ themselves any more for dealing with firmware loss during system
+ resume.

drivers/base/firmware_class.c

@@ -988,6 +988,9 @@ static int _request_firmware_load(struct firmware_priv *fw_priv, bool uevent,
  *      firmware image for this or any other device.
  *
  *	Caller must hold the reference count of @device.
+ *
+ *	The function can be called safely inside device's suspend and
+ *	resume callback.
  **/
 int
 request_firmware(const struct firmware **firmware_p, const char *name,