[PATCH 4/4] doc: README.socfpga: Add official boot flow support info

[Jit Loon Lim <jit.loon.lim@intel.com>]

From: Yau Wai Gan <yau.wai.gan@intel.com>

The official boot-up flow for SoC FPGA 64-bit is with
Arm Trusted Firmware, ATF BL31. The non-ATF flow is
legacy and is not supported officially moving forward.
Update the README to include the information.

Signed-off-by: Yau Wai Gan <yau.wai.gan@intel.com>
Signed-off-by: Jit Loon Lim <jit.loon.lim@intel.com>
---
 doc/README.socfpga | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/doc/README.socfpga b/doc/README.socfpga
index 362361e014..3f0ce8ce7b 100644
--- a/doc/README.socfpga
+++ b/doc/README.socfpga
@@ -17,6 +17,7 @@ Table of Contents
 	6. mkimage for Cyclone V, Arria V and Arria 10
 	7. SDRAM secure region in U-boot ATF flow
 	8. binman for U-boot ATF flow
+	9. Official Boot-up Flow Support
 
 
 1. Device Family Support vs Tested Intel Quartus
@@ -304,4 +305,31 @@ Table of Contents
 			This command generate u-boot.itb only.
 
 		$ <U-boot path>/tools/binman/binman build -u -d u-boot.dtb -O . -i kernel
-			This command generate kernel.itb only.
\ No newline at end of file
+			This command generate kernel.itb only.
+
+9. Official Boot-up Flow Support
+---------------------------------------------------------------------
+	U-boot with Arm Trusted Firmware (ATF, TF-A) boot-up is the official supported
+	boot flow for Intel SoC FPGA Arm 64-bit architecture devices including
+	Stratix 10, Agilex and N5X. ATF is the secure runtime firmware running at EL3
+	which handles secure accesses from U-boot proper and Linux running at EL2 and EL1.
+
+	Official boot flow:
+		U-boot -> ATF BL31 -> U-boot proper -> Linux
+
+	Legacy (aka non ATF) boot flow:
+		U-boot -> U-boot proper -> Linux
+
+	U-boot version socfpga_v2021.07 and onwards change the defconfig name to match
+	the official boot flow. The non ATF flow defconfigs are renamed with _legacy_
+	appended and is not officially supported moving forward. See details as follow.
+
+	Legacy boot flow:
+		socfpga_agilex_defconfig    -> socfpga_agilex_legacy_defconfig
+		socfpga_n5x_defconfig       -> socfpga_n5x_legacy_defconfig
+		socfpga_stratix10_defconfig -> socfpga_stratix10_legacy_defconfig
+
+	Official boot flow:
+		socfpga_agilex_atf_defconfig    -> socfpga_agilex_defconfig
+		socfpga_n5x_atf_defconfig       -> socfpga_n5x_defconfig
+		socfpga_stratix10_atf_defconfig -> socfpga_stratix10_defconfig
\ No newline at end of file
-- 
2.26.2