autoboot: Improve docs for CONFIG_AUTOBOOT_ENCRYPTION

This option is not documented properly at present. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>

README

@@ -3421,6 +3421,8 @@ List of environment variables (most likely not complete):
 		  allowed for use by the bootm command. See also "bootm_low"
 		  environment variable.
 
+  bootstopkeysha256, bootdelaykey, bootstopkey	- See README.autoboot
+
   updatefile	- Location of the software update file on a TFTP server, used
 		  by the automatic software update feature. Please refer to
 		  documentation in doc/README.update for more details.

cmd/Kconfig

@@ -101,7 +101,14 @@ config AUTOBOOT_PROMPT
 config AUTOBOOT_ENCRYPTION
 	bool "Enable encryption in autoboot stopping"
 	depends on AUTOBOOT_KEYED
-	default n
+	help
+	  This option allows a string to be entered into U-Boot to stop the
+	  autoboot. The string itself is hashed and compared against the hash
+	  in the environment variable 'bootstopkeysha256'. If it matches then
+	  boot stops and a command-line prompt is presented.
+
+	  This provides a way to ship a secure production device which can also
+	  be accessed at the U-Boot command line.
 
 config AUTOBOOT_DELAY_STR
 	string "Delay autobooting via specific input key / string"

common/autoboot.c

@@ -54,6 +54,14 @@ static int slow_equals(u8 *a, u8 *b, int len)
 	return diff == 0;
 }
 
+/**
+ * passwd_abort_sha256() - check for a hashed key sequence to abort booting
+ *
+ * This checks for the user entering a SHA256 hash within a given time.
+ *
+ * @etime: Timeout value ticks (stop when get_ticks() reachs this)
+ * @return 0 if autoboot should continue, 1 if it should stop
+ */
 static int passwd_abort_sha256(uint64_t etime)
 {
 	const char *sha_env_str = env_get("bootstopkeysha256");
@@ -106,6 +114,14 @@ static int passwd_abort_sha256(uint64_t etime)
 	return abort;
 }
 
+/**
+ * passwd_abort_key() - check for a key sequence to aborted booting
+ *
+ * This checks for the user entering a string within a given time.
+ *
+ * @etime: Timeout value ticks (stop when get_ticks() reachs this)
+ * @return 0 if autoboot should continue, 1 if it should stop
+ */
 static int passwd_abort_key(uint64_t etime)
 {
 	int abort = 0;

doc/README.autoboot

@@ -132,6 +132,21 @@ What they do
 	provides an escape sequence from the limited "password"
 	strings.
 
+  CONFIG_AUTOBOOT_ENCRYPTION
+
+  "bootstopkeysha256"	environment variable
+
+	- Hash value of the input which unlocks the device and
+	  stops autoboot.
+
+	This option allows a string to be entered into U-Boot to stop the
+	autoboot. The string itself is hashed and compared against the hash
+	in the environment variable 'bootstopkeysha256'. If it matches then
+	boot stops and a command-line prompt is presented.
+
+	This provides a way to ship a secure production device which can also
+	be accessed at the U-Boot command line.
+
   CONFIG_RESET_TO_RETRY
 
 	(Only effective when CONFIG_BOOT_RETRY_TIME is also set)