RISC-V: Add an Image header that boot loader can parse.

Currently, the last stage boot loaders such as U-Boot can accept only
uImage which is an unnecessary additional step in automating boot
process.

Add an image header that boot loader understands and boot Linux from
flat Image directly.

This header is based on ARM64 boot image header and provides an
opportunity to combine both ARM64 & RISC-V image headers in future.

Also make sure that PE/COFF header can co-exist in the same image so
that EFI stub can be supported for RISC-V in future. EFI specification
needs PE/COFF image header in the beginning of the kernel image in order
to load it as an EFI application. In order to support EFI stub, code0
should be replaced with "MZ" magic string and res4(at offset 0x3c)
should point to the rest of the PE/COFF header (which will be added
during EFI support).

Tested on both QEMU and HiFive Unleashed using OpenSBI + U-Boot + Linux.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Karsten Merker <merker@debian.org>
Tested-by: Karsten Merker <merker@debian.org> (QEMU+OpenSBI+U-Boot)
Tested-by: Kevin Hilman <khilman@baylibre.com> (OpenSBI + U-Boot + Linux)
[paul.walmsley@sifive.com: fixed whitespace in boot-image-header.txt;
 converted structure comment to kernel-doc format and added some detail]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

Documentation/riscv/boot-image-header.txt

@@ -0,0 +1,50 @@
+				Boot image header in RISC-V Linux
+			=============================================
+
+Author: Atish Patra <atish.patra@wdc.com>
+Date  : 20 May 2019
+
+This document only describes the boot image header details for RISC-V Linux.
+The complete booting guide will be available at Documentation/riscv/booting.txt.
+
+The following 64-byte header is present in decompressed Linux kernel image.
+
+	u32 code0;		  /* Executable code */
+	u32 code1; 		  /* Executable code */
+	u64 text_offset;	  /* Image load offset, little endian */
+	u64 image_size;		  /* Effective Image size, little endian */
+	u64 flags;		  /* kernel flags, little endian */
+	u32 version;		  /* Version of this header */
+	u32 res1  = 0;		  /* Reserved */
+	u64 res2  = 0;    	  /* Reserved */
+	u64 magic = 0x5643534952; /* Magic number, little endian, "RISCV" */
+	u32 res3;		  /* Reserved for additional RISC-V specific header */
+	u32 res4;		  /* Reserved for PE COFF offset */
+
+This header format is compliant with PE/COFF header and largely inspired from
+ARM64 header. Thus, both ARM64 & RISC-V header can be combined into one common
+header in future.
+
+Notes:
+- This header can also be reused to support EFI stub for RISC-V in future. EFI
+  specification needs PE/COFF image header in the beginning of the kernel image
+  in order to load it as an EFI application. In order to support EFI stub,
+  code0 should be replaced with "MZ" magic string and res5(at offset 0x3c) should
+  point to the rest of the PE/COFF header.
+
+- version field indicate header version number.
+	Bits 0:15  - Minor version
+	Bits 16:31 - Major version
+
+  This preserves compatibility across newer and older version of the header.
+  The current version is defined as 0.1.
+
+- res3 is reserved for offset to any other additional fields. This makes the
+  header extendible in future. One example would be to accommodate ISA
+  extension for RISC-V in future. For current version, it is set to be zero.
+
+- In current header, the flag field has only one field.
+	Bit 0: Kernel endianness. 1 if BE, 0 if LE.
+
+- Image size is mandatory for boot loader to load kernel image. Booting will
+  fail otherwise.

arch/riscv/include/asm/image.h

@@ -0,0 +1,65 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __ASM_IMAGE_H
+#define __ASM_IMAGE_H
+
+#define RISCV_IMAGE_MAGIC	"RISCV"
+
+#define RISCV_IMAGE_FLAG_BE_SHIFT	0
+#define RISCV_IMAGE_FLAG_BE_MASK	0x1
+
+#define RISCV_IMAGE_FLAG_LE		0
+#define RISCV_IMAGE_FLAG_BE		1
+
+#ifdef CONFIG_CPU_BIG_ENDIAN
+#error conversion of header fields to LE not yet implemented
+#else
+#define __HEAD_FLAG_BE		RISCV_IMAGE_FLAG_LE
+#endif
+
+#define __HEAD_FLAG(field)	(__HEAD_FLAG_##field << \
+				RISCV_IMAGE_FLAG_##field##_SHIFT)
+
+#define __HEAD_FLAGS		(__HEAD_FLAG(BE))
+
+#define RISCV_HEADER_VERSION_MAJOR 0
+#define RISCV_HEADER_VERSION_MINOR 1
+
+#define RISCV_HEADER_VERSION (RISCV_HEADER_VERSION_MAJOR << 16 | \
+			      RISCV_HEADER_VERSION_MINOR)
+
+#ifndef __ASSEMBLY__
+/**
+ * struct riscv_image_header - riscv kernel image header
+ * @code0:		Executable code
+ * @code1:		Executable code
+ * @text_offset:	Image load offset (little endian)
+ * @image_size:		Effective Image size (little endian)
+ * @flags:		kernel flags (little endian)
+ * @version:		version
+ * @res1:		reserved
+ * @res2:		reserved
+ * @magic:		Magic number
+ * @res3:		reserved (will be used for additional RISC-V specific
+ *			header)
+ * @res4:		reserved (will be used for PE COFF offset)
+ *
+ * The intention is for this header format to be shared between multiple
+ * architectures to avoid a proliferation of image header formats.
+ */
+
+struct riscv_image_header {
+	u32 code0;
+	u32 code1;
+	u64 text_offset;
+	u64 image_size;
+	u64 flags;
+	u32 version;
+	u32 res1;
+	u64 res2;
+	u64 magic;
+	u32 res3;
+	u32 res4;
+};
+#endif /* __ASSEMBLY__ */
+#endif /* __ASM_IMAGE_H */

arch/riscv/kernel/head.S

@@ -11,9 +11,41 @@
 #include <asm/thread_info.h>
 #include <asm/page.h>
 #include <asm/csr.h>
+#include <asm/image.h>
 
 __INIT
 ENTRY(_start)
+	/*
+	 * Image header expected by Linux boot-loaders. The image header data
+	 * structure is described in asm/image.h.
+	 * Do not modify it without modifying the structure and all bootloaders
+	 * that expects this header format!!
+	 */
+	/* jump to start kernel */
+	j _start_kernel
+	/* reserved */
+	.word 0
+	.balign 8
+#if __riscv_xlen == 64
+	/* Image load offset(2MB) from start of RAM */
+	.dword 0x200000
+#else
+	/* Image load offset(4MB) from start of RAM */
+	.dword 0x400000
+#endif
+	/* Effective size of kernel image */
+	.dword _end - _start
+	.dword __HEAD_FLAGS
+	.word RISCV_HEADER_VERSION
+	.word 0
+	.dword 0
+	.asciz RISCV_IMAGE_MAGIC
+	.word 0
+	.balign 4
+	.word 0
+
+.global _start_kernel
+_start_kernel:
 	/* Mask all interrupts */
 	csrw CSR_SIE, zero
 	csrw CSR_SIP, zero