[RFC PATCH 2/2] firmware: Add support for Linux kernel image header

[Vivian Wang]

The firmwares fw_payload and fw_jump support the same entrypoint ABI as
M-mode Linux. To support booting theses firmware images from bootloaders
already supporting M-mode Linux, add a compatible header to the firmware
image.

The header starts with a jump to skip to the actual code, so including
it does not affect operation without a bootloader.

Signed-off-by: Vivian Wang <dramforever at live.com>
---
 firmware/Kconfig                   | 17 +++++++++++++++
 firmware/fw_base.ldS               |  5 +++++
 firmware/fw_header_linux.S         | 34 ++++++++++++++++++++++++++++++
 firmware/fw_jump.S                 |  1 +
 firmware/fw_payload.S              |  1 +
 platform/generic/configs/defconfig |  1 +
 6 files changed, 59 insertions(+)
 create mode 100644 firmware/fw_header_linux.S

diff --git a/firmware/Kconfig b/firmware/Kconfig
index d6e0506..50ec340 100644
--- a/firmware/Kconfig
+++ b/firmware/Kconfig
@@ -1 +1,18 @@
 # SPDX-License-Identifier: BSD-2-Clause
+
+menu "Firmware Configuration"
+
+choice FW_HEADER
+	bool "Include Firmware Header"
+	optional
+
+config FW_HEADER_LINUX
+	bool "Linux Kernel Image"
+	help
+	  Compatibility with Linux kernel image header, for fw_jump and fw_payload
+
+	  See: https://www.kernel.org/doc/html/v6.8/arch/riscv/boot-image-header.html
+
+endchoice
+
+endmenu
diff --git a/firmware/fw_base.ldS b/firmware/fw_base.ldS
index fb47984..6a3b21b 100644
--- a/firmware/fw_base.ldS
+++ b/firmware/fw_base.ldS
@@ -15,6 +15,11 @@
 
 	/* Beginning of the code section */
 
+	.header :
+	{
+		*(.header)
+	}
+
 	.text :
  	{
 		PROVIDE(_text_start = .);
diff --git a/firmware/fw_header_linux.S b/firmware/fw_header_linux.S
new file mode 100644
index 0000000..fbd0cf9
--- /dev/null
+++ b/firmware/fw_header_linux.S
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+
+#include <sbi/sbi_byteorder.h>
+
+#ifdef CONFIG_FW_HEADER_LINUX
+
+	/*
+	 * Compatibility with Linux kernel image header, for fw_jump and fw_payload
+	 *
+	 * See: https://www.kernel.org/doc/html/v6.8/arch/riscv/boot-image-header.html
+	 */
+
+	.section .header, "a", %progbits
+
+	/* 8 bytes at beginning reserved for code */
+	j _start
+	.align 3
+
+	/* We want to be loaded at start of RAM, so text_offset is 0 */
+	.dword 0					/* text_offset */
+	.dword cpu_to_le64(_fw_reloc_end - _fw_start)	/* image_size */
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	.dword 0					/* flags: Not big-endian */
+#else
+	.dword 1					/* flags: Big-endian */
+#endif
+	.word cpu_to_le16(0x0002)			/* version (0.2) */
+	.word 0						/* res1 */
+	.dword 0					/* res2 */
+	.ascii "RISCV\0\0\0"				/* magic */
+	.ascii "RSC\x05"				/* magic2 */
+	.word 0						/* res3 */
+
+#endif /* CONFIG_FW_HEADER_LINUX */
diff --git a/firmware/fw_jump.S b/firmware/fw_jump.S
index ebf297f..9b2ddda 100644
--- a/firmware/fw_jump.S
+++ b/firmware/fw_jump.S
@@ -8,6 +8,7 @@
  */
 
 #include "fw_base.S"
+#include "fw_header_linux.S"
 
 	.section .entry, "ax", %progbits
 	.align 3
diff --git a/firmware/fw_payload.S b/firmware/fw_payload.S
index 3c8433e..e324f6d 100644
--- a/firmware/fw_payload.S
+++ b/firmware/fw_payload.S
@@ -8,6 +8,7 @@
  */
 
 #include "fw_base.S"
+#include "fw_header_linux.S"
 
 	.section .entry, "ax", %progbits
 	.align 3
diff --git a/platform/generic/configs/defconfig b/platform/generic/configs/defconfig
index 1ce6a12..4f7217b 100644
--- a/platform/generic/configs/defconfig
+++ b/platform/generic/configs/defconfig
@@ -42,3 +42,4 @@ CONFIG_SERIAL_SEMIHOSTING=y
 CONFIG_FDT_TIMER=y
 CONFIG_FDT_TIMER_MTIMER=y
 CONFIG_FDT_TIMER_PLMT=y
+CONFIG_FW_HEADER_LINUX=y
-- 
2.42.0