sysfs.txt: add note on available attribute macros

The common cases of attributes wrappers should probably be using the
__ATTR_XXX macros to make code more concise and readable but the current
sysfs.txt does not point developers to those convenience macros. Further
there is no note in sysfs.txt currently explaining why trying to set a
sysfs file to mode 0666 will fail respectively revert to 0664.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/sysfs.txt

@@ -116,6 +116,27 @@ static struct device_attribute dev_attr_foo = {
 	.store = store_foo,
 };
 
+Note as stated in include/linux/kernel.h "OTHER_WRITABLE?  Generally
+considered a bad idea." so trying to set a sysfs file writable for
+everyone will fail reverting to RO mode for "Others".
+
+For the common cases sysfs.h provides convenience macros to make
+defining attributes easier as well as making code more concise and
+readable. The above case could be shortened to:
+
+static struct device_attribute dev_attr_foo = __ATTR_RW(foo);
+
+the list of helpers available to define your wrapper function is:
+__ATTR_RO(name): assumes default name_show and mode 0444
+__ATTR_WO(name): assumes a name_store only and is restricted to mode
+                 0200 that is root write access only.
+__ATTR_RO_MODE(name, mode): fore more restrictive RO access currently
+                 only use case is the EFI System Resource Table
+                 (see drivers/firmware/efi/esrt.c)
+__ATTR_RW(name): assumes default name_show, name_store and setting
+                 mode to 0644.
+__ATTR_NULL: which sets the name to NULL and is used as end of list
+                 indicator (see: kernel/workqueue.c)
 
 Subsystem-Specific Callbacks
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~