[PATCH v2 09/10] docs/system/arm: Document MCIMX6UL-EVK Buildroot boot

[Bin Meng <bin.meng@processmission.com>]

Provide verified instructions for U-Boot and direct Linux boot using
Buildroot images.

Signed-off-by: Bin Meng <bin.meng@processmission.com>
---

(no changes since v1)

 docs/system/arm/mcimx6ul-evk.rst | 67 ++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/docs/system/arm/mcimx6ul-evk.rst b/docs/system/arm/mcimx6ul-evk.rst
index 8871138ab3..84331b734d 100644
--- a/docs/system/arm/mcimx6ul-evk.rst
+++ b/docs/system/arm/mcimx6ul-evk.rst
@@ -3,3 +3,70 @@ NXP MCIMX6UL-EVK (``mcimx6ul-evk``)
 
 The ``mcimx6ul-evk`` machine models the NXP i.MX6UltraLite Evaluation Kit
 MCIMX6UL-EVK development board. It has a single Cortex-A7 CPU.
+
+Booting Buildroot images
+------------------------
+
+Build the SD card image with the tested Buildroot version:
+
+.. code-block:: bash
+
+  $ git clone https://gitlab.com/buildroot.org/buildroot.git
+  $ cd buildroot
+  $ git checkout 2026.05
+  $ make imx6ulevk_defconfig
+  $ make
+
+The following examples use images generated by Buildroot::
+
+  IMAGES=/path/to/buildroot/output/images
+  QEMU_BUILD=/path/to/qemu/build
+
+The generated raw ``sdcard.img`` is about 84 MiB, which is not a capacity
+supported by QEMU's SD card model. Resize the image in place to 128 MiB::
+
+  "$QEMU_BUILD/qemu-img" resize -f raw "$IMAGES/sdcard.img" 128M
+
+This modifies the Buildroot output image; rebuilding the image restores its
+original size.
+
+The SD card must be attached to USDHC2 with ``index=1``. It is consequently
+named ``mmc1`` by U-Boot and ``mmcblk1`` by Linux. The first partition contains
+the kernel and device tree, while the second partition contains the root file
+system.
+
+U-Boot boot
+~~~~~~~~~~~
+
+The machine does not model the i.MX6UL Boot ROM or its SPL loading sequence.
+Load U-Boot proper at its linked address::
+
+.. code-block:: bash
+
+  "$QEMU_BUILD/qemu-system-arm" \
+      -M mcimx6ul-evk -m 512M \
+      -device loader,file="$IMAGES/u-boot.bin",addr=0x87800000,cpu-num=0 \
+      -drive file="$IMAGES/sdcard.img",if=sd,index=1,format=raw \
+      -nographic
+
+The default Buildroot U-Boot environment selects ``mmc1``, loads ``zImage``
+and ``imx6ul-14x14-evk.dtb`` from the first partition, and starts Linux with
+``/dev/mmcblk1p2`` as the root file system. A ``bad CRC`` warning while loading
+an uninitialized persistent U-Boot environment is expected; U-Boot continues
+with its compiled-in defaults.
+
+Direct Linux boot
+~~~~~~~~~~~~~~~~~
+
+QEMU can load the Buildroot kernel and device tree directly. The kernel then
+mounts the second SD card partition as its root file system::
+
+.. code-block:: bash
+
+  "$QEMU_BUILD/qemu-system-arm" \
+      -M mcimx6ul-evk -m 512M \
+      -kernel "$IMAGES/zImage" \
+      -dtb "$IMAGES/imx6ul-14x14-evk.dtb" \
+      -append "console=ttymxc0,115200 root=/dev/mmcblk1p2 rootwait rw" \
+      -drive file="$IMAGES/sdcard.img",if=sd,index=1,format=raw \
+      -nographic
-- 
2.53.0