ARM: 9480/1: entry: expand comment in __switch_tografted

As per discussion between the developers in the mail thread
linked, expand the comment in __switch_to so that readers
of the code understand what is going on.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 31 insertions, 0 deletions

diff --git a/Documentation/userspace-api/ioctl/ioctl-decoding.rst b/Documentation/userspace-api/ioctl/ioctl-decoding.rst
new file mode 100644
index 000000000..380d6bb3e
--- /dev/null
+++ b/Documentation/userspace-api/ioctl/ioctl-decoding.rst
@@ -0,0 +1,31 @@
+==============================
+Decoding an IOCTL Magic Number
+==============================
+
+To decode a hex IOCTL code:
+
+Most architectures use this generic format, but check
+include/ARCH/ioctl.h for specifics, e.g. powerpc
+uses 3 bits to encode read/write and 13 bits for size.
+
+ ====== ==================================
+ bits   meaning
+ ====== ==================================
+ 31-30	00 - no parameters: uses _IO macro
+	10 - read: _IOR
+	01 - write: _IOW
+	11 - read/write: _IOWR
+
+ 29-16	size of arguments
+
+ 15-8	ascii character supposedly
+	unique to each driver
+
+ 7-0	function #
+ ====== ==================================
+
+
+So for example 0x82187201 is a read with arg length of 0x218,
+character 'r' function 1. Grepping the source reveals this is::
+
+	#define VFAT_IOCTL_READDIR_BOTH         _IOR('r', 1, struct dirent [2])