Documentation: boards: stm32mp: document eMMC boot configuration

In order to boot from eMMC on the STM32MP15x, the boot_ack bit in the
ext_csd register must be set. Document this and while at it, fix a
misformatted code block in the same section.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Bastian Krause <bst@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 12 insertions, 1 deletions

diff --git a/Documentation/boards/stm32mp.rst b/Documentation/boards/stm32mp.rst
index 7c657eb990..b235c39927 100644
--- a/Documentation/boards/stm32mp.rst
+++ b/Documentation/boards/stm32mp.rst
@@ -64,7 +64,7 @@ An appropriate image for a SD-Card can be generated with following
   }
 
 For eMMC, the boot partitions are used as the FSBL partitions and so the user
-partitions may look like this:
+partitions may look like this::
 
   image @STM32MP_BOARD@.img {
       partition ssbl {
@@ -80,6 +80,17 @@ partitions may look like this:
 The fsbl1 and fsbl2 can be flashed by writing to barebox ``/dev/mmcX.boot0`` and
 ``/dev/mmcX.boot1`` respectively or from a booted operating system.
 
+Additionally, the eMMC's ``ext_csd`` register must be modified to activate the
+boot acknowledge signal (``BOOT_ACK``) and to select a boot partition.
+
+Assuming ``CONFIG_CMD_MMC_EXTCSD`` is enabled and the board shall boot from
+``/dev/mmc1.boot1``::
+
+  mmc_extcsd /dev/mmc1 -i 179 -v 0x50
+
+The STM32MP1 BootROM does *not* support booting from eMMC without fast boot
+acknowledge.
+
 Boot source selection
 ---------------------