# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1540570924 -7200
# Node ID bb0c949a94e1b4c1783321aea34968e2c95270a1
# Parent  45a99a68c2ed4abc30a2fffbb6f6b7b58a43968b
Attempted to initialise writable data by relocating the .data section.
Demonstrate initialisation by setting UART echo in the demo example.

diff -r 45a99a68c2ed -r bb0c949a94e1 examples/demo/main.c
--- a/examples/demo/main.c	Thu Oct 25 21:19:01 2018 +0200
+++ b/examples/demo/main.c	Fri Oct 26 18:22:04 2018 +0200
@@ -43,7 +43,7 @@
 #define CELLSIZE 4
 #endif
 
-static int uart_echo;
+static int uart_echo = 1;
 
 
 
@@ -75,8 +75,6 @@
 
 void main(void)
 {
-    uart_echo = 0;
-
     init_memory();
     init_pins();
     init_outputs();
diff -r 45a99a68c2ed -r bb0c949a94e1 lib/payload.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/payload.c	Fri Oct 26 18:22:04 2018 +0200
@@ -0,0 +1,36 @@
+/*
+ * Payload initialisation functions.
+ *
+ * Copyright (C) 2018 Paul Boddie <paul@boddie.org.uk>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdint.h>
+
+/* Relocate the .data section's contents to initialise mutable structures. */
+
+void relocate_data(uint32_t *original, uint32_t *current, uint32_t size)
+{
+    /* Reference stored content as values. */
+
+    uint32_t *source, *target;
+
+    /* Copy data from program memory to the relocated data area in RAM. */
+
+    for (source = original, target = current; source < original + size;
+         source++, target++)
+
+        *target = *source;
+}
diff -r 45a99a68c2ed -r bb0c949a94e1 lib/payload.ld
--- a/lib/payload.ld	Thu Oct 25 21:19:01 2018 +0200
+++ b/lib/payload.ld	Fri Oct 26 18:22:04 2018 +0200
@@ -43,7 +43,7 @@
 
   .irqstack : {
                 . += IRQ_STACK_SIZE;
-              } > kseg0_data_mem    AT > physical_data_mem
+              }                 > kseg0_data_mem    AT > physical_data_mem
 
   /* Add other data after the IRQ stack. */
 
@@ -51,13 +51,24 @@
 
   /* Store constant data in program memory. */
 
-  .data     : { *(.data*) }     > kseg0_program_mem AT > physical_program_mem
   .rodata   : { *(.rodata*) }   > kseg0_program_mem AT > physical_program_mem
   .got      : {
                 _gp = ALIGN(16);
                 *(.got*)
               }                 > kseg0_program_mem AT > physical_program_mem
 
+  /* Store initialised non-constant data in program memory for relocation.
+     Employ data memory addresses for generated content. */
+
+  .data     : {
+                _data = .;
+                *(.data*)
+                _data_end = .;
+              }                 > kseg0_data_mem    AT > physical_program_mem
+
+  _data_original = LOADADDR(.data) - ORIGIN(physical_program_mem) + ORIGIN(kseg0_program_mem);
+  _data_size = SIZEOF(.data);
+
   /* Device configuration registers to be flashed. */
 
   .devcfg0  : { *(.devcfg0) }   > config0           AT > physical_config0
diff -r 45a99a68c2ed -r bb0c949a94e1 lib/start.S
--- a/lib/start.S	Thu Oct 25 21:19:01 2018 +0200
+++ b/lib/start.S	Fri Oct 26 18:22:04 2018 +0200
@@ -44,6 +44,10 @@
 .section .boot, "a"
 
 .globl _start
+.extern relocate_data
+.extern _data_original
+.extern _data
+.extern _data_size
 .extern main
 
 _start:
@@ -72,11 +76,29 @@
         ori $gp, $gp, %lo(_GLOBAL_OFFSET_TABLE_)
 
 	/*
-	Jump to the main program. Since the boot code is separate from the
-	other code, the address cannot be obtained via the GOT.
-	("relocation truncated to fit: R_MIPS_PC16 against `main'")
+	Relocate mutable data and then jump to the main program.
+
+	Since the boot code is separate from the other code, addresses cannot be
+	obtained via the GOT:
+
+	"relocation truncated to fit: R_MIPS_PC16 against `main'"
 	*/
 
+	lui $a0, %hi(_data_original)
+	ori $a0, $a0, %lo(_data_original)
+
+	lui $a1, %hi(_data)
+	ori $a1, $a1, %lo(_data)
+
+	lui $a2, %hi(_data_size)
+	ori $a2, $a2, %lo(_data_size)
+	lw $a2, 0($a2)
+
+	lui $t9, %hi(relocate_data)
+	ori $t9, $t9, %lo(relocate_data)
+	jal $t9
+	nop
+
         lui $t9, %hi(main)
         ori $t9, $t9, %lo(main)
 	jr $t9
diff -r 45a99a68c2ed -r bb0c949a94e1 mk/common.mk
--- a/mk/common.mk	Thu Oct 25 21:19:01 2018 +0200
+++ b/mk/common.mk	Fri Oct 26 18:22:04 2018 +0200
@@ -48,8 +48,8 @@
 # Application-specific files appear after the above but before those below in
 # the application Makefiles.
 
-COMMON_SRC = $(LIBDIR)/init.c $(LIBDIR)/debug.c $(LIBDIR)/cpu.S
-COMMON_OBJ = $(LIBDIR)/init.o $(LIBDIR)/debug.o $(LIBDIR)/cpu.o
+COMMON_SRC = $(LIBDIR)/payload.c $(LIBDIR)/init.c $(LIBDIR)/debug.c $(LIBDIR)/cpu.S
+COMMON_OBJ = $(LIBDIR)/payload.o $(LIBDIR)/init.o $(LIBDIR)/debug.o $(LIBDIR)/cpu.o
 
 DISPLAY_SRC = $(LIBDIR)/display.c $(LIBDIR)/vga_display.c
 DISPLAY_OBJ = $(LIBDIR)/display.o $(LIBDIR)/vga_display.o