[PATCH] ARM: boot: Obtain start of physical memory from DTB

[Geert Uytterhoeven <geert+renesas@glider.be>]

Currently, the start address of physical memory is obtained by masking
the program counter with a fixed mask of 0xf8000000.  This mask value
was chosen as a balance between the requirements of different platforms.
However, this does require that the start address of physical memory is
a multiple of 128 MiB, precluding booting Linux on platforms where this
requirement is not fulfilled.

Fix this limitation by obtaining the start address from the passed DTB
instead, if available.  Note that for now this is limited to DTBs passed
explicitly by the boot loader.  DTBs appended to a zImage or uImage are
not inspected.  Fall back to the traditional method when needed.

This allows to boot Linux on r7s9210/rza2mevb using the 64 MiB of SDRAM
on the RZA2MEVB sub board, which is located at 0x0C000000 (CS3 space),
i.e. not at a multiple of 128 MiB.

Suggested-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
Against arm/for-next.

Tested with the following configurations:
  - zImage + DTB (r8a7791/koelsch): physical memory start address
    obtained from DT,
  - uImage + DTB (r8a73a4/ape6evm, r7s72100/rskrza1, r7s9210/rza2mevb):
    physical memory start address obtained from DT,
  - zImage with appended DTB (r8a7740/armadillo, sh73a0/kzm9g): physical
    memory start address obtained by masking, as before.

An appended DTB is currently processed after the start of physical
memory is obtained.  Hence obtaining that address from an appended DTB
requires moving/copying that copy.  Given the complexity w.r.t. the
"restart" label, and the lack of a need for me to support this, I didn't
implement that part.
---
 arch/arm/boot/compressed/Makefile            |  6 ++-
 arch/arm/boot/compressed/fdt_get_mem_start.c | 52 ++++++++++++++++++++
 arch/arm/boot/compressed/head.S              | 16 +++++-
 3 files changed, 72 insertions(+), 2 deletions(-)
 create mode 100644 arch/arm/boot/compressed/fdt_get_mem_start.c

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index da599c3a11934332..bbfecd648a1a3b57 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -86,12 +86,15 @@ libfdt_objs	:= $(addsuffix .o, $(basename $(libfdt)))
 $(addprefix $(obj)/,$(libfdt) $(libfdt_hdrs)): $(obj)/%: $(srctree)/scripts/dtc/libfdt/%
 	$(call cmd,shipped)
 
-$(addprefix $(obj)/,$(libfdt_objs) atags_to_fdt.o): \
+$(addprefix $(obj)/,$(libfdt_objs) atags_to_fdt.o fdt_get_mem_start.o): \
 	$(addprefix $(obj)/,$(libfdt_hdrs))
 
 ifeq ($(CONFIG_ARM_ATAG_DTB_COMPAT),y)
 OBJS	+= $(libfdt_objs) atags_to_fdt.o
 endif
+ifeq ($(CONFIG_USE_OF),y)
+OBJS	+= $(libfdt_objs) fdt_get_mem_start.o
+endif
 
 targets       := vmlinux vmlinux.lds piggy_data piggy.o \
 		 lib1funcs.o ashldi3.o bswapsdi2.o \
@@ -116,6 +119,7 @@ CFLAGS_fdt.o := $(nossp-flags-y)
 CFLAGS_fdt_ro.o := $(nossp-flags-y)
 CFLAGS_fdt_rw.o := $(nossp-flags-y)
 CFLAGS_fdt_wip.o := $(nossp-flags-y)
+CFLAGS_fdt_get_mem_start.o := $(nossp-flags-y)
 
 ccflags-y := -fpic $(call cc-option,-mno-single-pic-base,) -fno-builtin -I$(obj)
 asflags-y := -DZIMAGE
diff --git a/arch/arm/boot/compressed/fdt_get_mem_start.c b/arch/arm/boot/compressed/fdt_get_mem_start.c
new file mode 100644
index 0000000000000000..2c5ac47f656317ee
--- /dev/null
+++ b/arch/arm/boot/compressed/fdt_get_mem_start.c
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <libfdt.h>
+
+static const void *getprop(const void *fdt, const char *node_path,
+			   const char *property)
+{
+	int offset = fdt_path_offset(fdt, node_path);
+
+	if (offset == -FDT_ERR_NOTFOUND)
+		return NULL;
+
+	return fdt_getprop(fdt, offset, property, NULL);
+}
+
+static uint32_t get_addr_size(const void *fdt)
+{
+	const __be32 *addr_len = getprop(fdt, "/", "#address-cells");
+
+	if (!addr_len) {
+		/* default */
+		return 1;
+	}
+
+	return fdt32_to_cpu(*addr_len);
+}
+
+/*
+ * Get the start of physical memory
+ */
+
+unsigned long fdt_get_mem_start(const void *fdt)
+{
+	const __be32 *memory;
+	uint32_t addr_size;
+
+	if (!fdt)
+		return -1;
+
+	if (*(__be32 *)fdt != cpu_to_fdt32(FDT_MAGIC))
+		return -1;
+
+	/* Find the first memory node */
+	memory = getprop(fdt, "/memory", "reg");
+	if (!memory)
+		return -1;
+
+	/* There may be multiple cells on LPAE platforms */
+	addr_size = get_addr_size(fdt);
+
+	return fdt32_to_cpu(memory[addr_size - 1]);
+}
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index 927f5dc413d7dff2..cb4e6a84b156c204 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -235,8 +235,20 @@ not_angel:
 		.text
 
 #ifdef CONFIG_AUTO_ZRELADDR
+#ifdef CONFIG_USE_OF
 		/*
-		 * Find the start of physical memory.  As we are executing
+		 * Find the start of physical memory.
+		 * Try the passed DTB first, if available.
+		 */
+		mov	r0, r8
+		bl	fdt_get_mem_start
+		mov	r4, r0
+		cmn	r0, #1
+		bne	1f
+#endif
+
+		/*
+		 * Fall back to the traditional method.  As we are executing
 		 * without the MMU on, we are in the physical address space.
 		 * We just need to get rid of any offset by aligning the
 		 * address.
@@ -254,6 +266,8 @@ not_angel:
 		 */
 		mov	r4, pc
 		and	r4, r4, #0xf8000000
+
+1:
 		/* Determine final kernel image address. */
 		add	r4, r4, #TEXT_OFFSET
 #else
-- 
2.17.1