doc: Add a description for bootmeth_qfw

Add documentation for the qfw bootmeth.

Fix up the compatible string to drop the 'extlinux' part, which is not
relevant to this bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

boot/bootmeth_qfw.c

@@ -88,7 +88,7 @@ static struct bootmeth_ops qfw_bootmeth_ops = {
 };
 
 static const struct udevice_id qfw_bootmeth_ids[] = {
-	{ .compatible = "u-boot,qfw-extlinux" },
+	{ .compatible = "u-boot,qfw-bootmeth" },
 	{ }
 };
 

doc/develop/bootstd/index.rst

@@ -9,3 +9,4 @@ Standard Boot
    overview
    extlinux
    pxelinux
+   qfw

doc/develop/bootstd/overview.rst

@@ -423,6 +423,7 @@ Bootmeth drivers are provided for booting from various media:
    - VBE
    - EFI boot using boot manager
    - Android bootflow (boot image v4)
+   - :doc:`QFW <qfw>`: QEMU firmware interface
 
 Each driver is controlled by a Kconfig option. If no bootmeth driver is
 selected by a compatible string in the devicetree, all available bootmeth

doc/develop/bootstd/qfw.rst

@@ -0,0 +1,20 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+QFW Bootmeth
+============
+
+`QEMU <hhttps://www.qemu.org/>`_ is a system emulator which is able to boot
+Operating Systems. QEMU provides specific support for booting an OS image
+provided on the QEMU command line.
+
+When invoked on a bootdev for UCLASS_QFW, this bootmeth reads the kernel
+provided by the QEMU `-kernel` argument, the initial ramdisk provided by
+`-initrd` and the boot arguments (command line) provided by `-append` into
+memory ready for booting.
+
+When the bootflow is booted, the bootmeth tries the `booti` command first, then
+falls back to the `bootz` command. U-Boot's 'control' devicetree is passed
+through to the kernel.
+
+The compatible string "u-boot,qfw-bootmeth" is used for the driver. It is
+present if `CONFIG_QFW` is enabled.