docs: aspeed: Reorganize the "Boot options" section

Add subsubsections for possible boot methods and introduce a new
section on eMMC boot support for the ast2600-evb and rainier-emmc
machines, boot partitions assumptions and limitations.

Acked-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Jan Luebbe <jlu@pengutronix.de>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

docs/system/arm/aspeed.rst

@@ -105,6 +105,9 @@ or directly from the ASPEED Forked OpenBMC GitHub release repository :
 
    https://github.com/AspeedTech-BMC/openbmc/releases
 
+Booting from a kernel image
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 To boot a kernel directly from a Linux build tree:
 
 .. code-block:: bash
@@ -114,16 +117,10 @@ To boot a kernel directly from a Linux build tree:
         -dtb arch/arm/boot/dts/aspeed-ast2600-evb.dtb \
         -initrd rootfs.cpio
 
-To boot the machine from the flash image, use an MTD drive :
+Booting from a flash image
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-.. code-block:: bash
+The machine options specific to Aspeed to boot from a flash image are :
-
-  $ qemu-system-arm -M romulus-bmc -nic user \
-	-drive file=obmc-phosphor-image-romulus.static.mtd,format=raw,if=mtd -nographic
-
-Options specific to Aspeed machines are :
-
- * ``boot-emmc`` to set or unset boot from eMMC (AST2600).
 
  * ``execute-in-place`` which emulates the boot from the CE0 flash
    device by using the FMC controller to load the instructions, and
@@ -134,10 +131,12 @@ Options specific to Aspeed machines are :
 
  * ``spi-model`` to change the default SPI Flash model.
 
- * ``bmc-console`` to change the default console device. Most of the
+To boot the machine from the flash image, use an MTD drive :
-   machines use the ``UART5`` device for a boot console, which is
+
-   mapped on ``/dev/ttyS4`` under Linux, but it is not always the
+.. code-block:: bash
-   case.
+
+  $ qemu-system-arm -M romulus-bmc -nic user \
+	-drive file=obmc-phosphor-image-romulus.static.mtd,format=raw,if=mtd -nographic
 
 To use other flash models, for instance a different FMC chip and a
 bigger (64M) SPI for the ``ast2500-evb`` machine, run :
@@ -169,6 +168,78 @@ In that case, the machine boots fetching instructions from the FMC0
 device. It is slower to start but closer to what HW does. Using the
 machine option ``execute-in-place`` has a similar effect.
 
+Booting from an eMMC image
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The machine options specific to Aspeed machines to boot from an eMMC
+image are :
+
+ * ``boot-emmc`` to set or unset boot from eMMC (AST2600).
+
+Only the ``ast2600-evb`` and ``rainier-emmc`` machines have support to
+boot from an eMMC device. In this case, the machine assumes that the
+eMMC image includes special boot partitions. Such an image can be
+built this way :
+
+.. code-block:: bash
+
+   $ dd if=/dev/zero of=mmc-bootarea.img count=2 bs=1M
+   $ dd if=u-boot-spl.bin of=mmc-bootarea.img conv=notrunc
+   $ dd if=u-boot.bin of=mmc-bootarea.img conv=notrunc count=64 bs=1K
+   $ cat mmc-bootarea.img obmc-phosphor-image.wic > mmc.img
+   $ truncate --size 16GB mmc.img
+
+Boot the machine ``rainier-emmc`` with :
+
+.. code-block:: bash
+
+   $ qemu-system-arm -M rainier-bmc \
+         -drive file=mmc.img,format=raw,if=sd,index=2 \
+         -nographic
+
+The ``boot-emmc`` option can be set or unset, to change the default
+boot mode of machine: SPI or eMMC. This can be useful to boot the
+``ast2600-evb`` machine from an eMMC device (default being SPI) or to
+boot the ``rainier-bmc`` machine from a flash device (default being
+eMMC).
+
+As an example, here is how to to boot the ``rainier-bmc`` machine from
+the flash device with ``boot-emmc=false`` and let the machine use an
+eMMC image :
+
+.. code-block:: bash
+
+   $ qemu-system-arm -M rainier-bmc,boot-emmc=false \
+        -drive file=flash.img,format=raw,if=mtd \
+        -drive file=mmc.img,format=raw,if=sd,index=2 \
+        -nographic
+
+It should be noted that in this case the eMMC device must not have
+boot partitions, otherwise the contents will not be accessible to the
+machine.  This limitation is due to the use of the ``-drive``
+interface.
+
+Ideally, one should be able to define the eMMC device and the
+associated backend directly on the command line, such as :
+
+.. code-block:: bash
+
+   -blockdev node-name=emmc0,driver=file,filename=mmc.img \
+   -device emmc,bus=sdhci-bus.2,drive=emmc0,boot-partition-size=1048576,boot-config=8
+
+This is not yet supported (as of QEMU-10.0). Work is needed to
+refactor the sdhci bus model.
+
+Other booting options
+^^^^^^^^^^^^^^^^^^^^^
+
+Other machine options specific to Aspeed machines are :
+
+ * ``bmc-console`` to change the default console device. Most of the
+   machines use the ``UART5`` device for a boot console, which is
+   mapped on ``/dev/ttyS4`` under Linux, but it is not always the
+   case.
+
 To change the boot console and use device ``UART3`` (``/dev/ttyS2``
 under Linux), use :
 
@@ -176,6 +247,8 @@ under Linux), use :
 
   -M ast2500-evb,bmc-console=uart3
 
+Booting the ast2700-evb machine
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Boot the AST2700 machine from the flash image, use an MTD drive :