doc: U-Boot boot phases

Add more detail to the description of U-Boot boot phases:

* describe which steps are optional
* mentions alternative boot flows

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

diff --git a/doc/develop/spl.rst b/doc/develop/spl.rst
index a1515a7b43b5fb7ebb1ce27e0c6838e2f0c10042..76e87f07c720acced896c5519a0819adec5e0323 100644 (file)
--- a/doc/develop/spl.rst
+++ b/doc/develop/spl.rst
@@ -77,10 +77,11 @@ To check whether a feature is enabled, use CONFIG_IS_ENABLED()::
 This checks CONFIG_CLK for the main build, CONFIG_SPL_CLK for the SPL build,
 CONFIG_TPL_CLK for the TPL build, etc.
 
-U-Boot Phases
--------------
+U-Boot Boot Phases
+------------------
 
-U-Boot boots through the following phases:
+U-Boot goes through the following boot phases where TPL, VPL, SPL are optional.
+While many boards use SPL, less use TPL.
 
 TPL
    Very early init, as tiny as possible. This loads SPL (or VPL if enabled).
@@ -97,6 +98,12 @@ SPL
 U-Boot
    U-Boot proper, containing the command line and boot logic.
 
+Further usages of U-Boot SPL comprise:
+
+* Launching BL31 of ARM Trusted Firmware which invokes main U-Boot as BL33
+* launching EDK II
+* launching Linux kernel
+* launching RISC-V OpenSBI which invokes main U-Boot
 
 Checking the boot phase
 -----------------------