ref-manual: Add documentation for kernel-fitimage

This documents kernel-fitimage.bbclass as well as
the variables that are required to use the class
to create a FIT image.

(From yocto-docs rev: 3aeca3b342e5be92be562f6dea6b71390c31b636)

Signed-off-by: Usama Arif <usama.arif@arm.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 75 insertions, 1 deletions

diff --git a/documentation/ref-manual/ref-classes.xml b/documentation/ref-manual/ref-classes.xml
index ab12373ae0..1dcd5fdd03 100644
--- a/documentation/ref-manual/ref-classes.xml
+++ b/documentation/ref-manual/ref-classes.xml
@@ -1880,8 +1880,82 @@ This check was removed for YP 2.3 release
 
     <para>
         The <filename>kernel-fitimage</filename> class provides support to
-        pack zImages.
+        pack a kernel Image, device trees and a RAM disk into a single
+        FIT image. In theory, a FIT image can support any number of kernels,
+        RAM disks and device-trees.
+        However, <filename>kernel-fitimage</filename> currently only supports
+        limited usescases: just one kernel image, an optional RAM disk, and
+        any number of device tree.
     </para>
+
+    <para>
+        To create a FIT image, it is required that
+        <filename><link linkend='var-KERNEL_CLASSES'>KERNEL_CLASSES</link></filename>
+        is set to "kernel-fitimage" and
+        <filename><link linkend='var-KERNEL_IMAGETYPE'>KERNEL_IMAGETYPE</link></filename>
+        is set to "fitImage".
+    </para>
+
+    <para>
+        The options for the device tree compiler passed to mkimage -D feature
+        when creating the FIT image are specified using the
+        <filename><link linkend='var-UBOOT_MKIMAGE_DTCOPTS'>UBOOT_MKIMAGE_DTCOPTS</link></filename>
+        variable.
+    </para>
+
+    <para>
+        Only a single kernel can be added to the FIT image created by
+        <filename>kernel-fitimage</filename> and the kernel image in FIT is
+        mandatory.
+        The address where the kernel image is to be loaded by U-boot is
+        specified by
+        <filename><link linkend='var-UBOOT_LOADADDRESS'>UBOOT_LOADADDRESS</link></filename>
+        and the entrypoint by
+        <filename><link linkend='var-UBOOT_ENTRYPOINT'>UBOOT_ENTRYPOINT</link></filename>.
+    </para>
+
+    <para>
+        Multiple device trees can be added to the FIT image created by
+        <filename>kernel-fitimage</filename> and the device tree is optional.
+        The address where the device tree is to be loaded by U-boot is
+        specified by
+        <filename><link linkend='var-UBOOT_DTBO_LOADADDRESS'>UBOOT_DTBO_LOADADDRESS</link></filename>
+        for device tree overlays and by
+        <filename><link linkend='var-UBOOT_DTB_LOADADDRESS'>UBOOT_DTB_LOADADDRESS</link></filename>
+        for device tree binaries.
+    </para>
+
+    <para>
+        Only a single RAM disk can be added to the FIT image created by
+        <filename>kernel-fitimage</filename> and the RAM disk in FIT is
+        optional.
+        The address where the RAM disk image is to be loaded by U-boot
+        is specified by
+        <filename><link linkend='var-UBOOT_RD_LOADADDRESS'>UBOOT_RD_LOADADDRESS</link></filename>
+        and the entrypoint by
+        <filename><link linkend='var-UBOOT_RD_ENTRYPOINT'>UBOOT_RD_ENTRYPOINT</link></filename>.
+        The ramdisk is added to FIT image when
+        <filename><link linkend='var-INITRAMFS_IMAGE'>INITRAMFS_IMAGE</link></filename>
+        is specified.
+    </para>
+
+    <para>
+        The FIT image generated by <filename>kernel-fitimage</filename> class
+        is signed when the variables
+        <filename><link linkend='var-UBOOT_SIGN_ENABLE'>UBOOT_SIGN_ENABLE</link></filename>,
+        <filename><link linkend='var-UBOOT_MKIMAGE_DTCOPTS'>UBOOT_MKIMAGE_DTCOPTS</link></filename>,
+        <filename><link linkend='var-UBOOT_SIGN_KEYDIR'>UBOOT_SIGN_KEYDIR</link></filename>
+        and
+        <filename><link linkend='var-UBOOT_SIGN_KEYNAME'>UBOOT_SIGN_KEYNAME</link></filename>
+        are set appropriately.
+        The default values used for
+        <filename><link linkend='var-FIT_HASH_ALG'>FIT_HASH_ALG</link></filename>
+        and
+        <filename><link linkend='var-FIT_SIGN_ALG'>FIT_SIGN_ALG</link></filename>
+        in <filename>kernel-fitimage</filename> are "sha256" and "rsa2048"
+        respectively.
+    </para>
+
 </section>
 
 <section id='ref-classes-kernel-grub'>