MIPS: JZ4770: Workaround for corrupted DMA transfers

We have seen MMC DMA transfers read corrupted data from SDRAM when
a burst interval ends at physical address 0x10000000. To avoid this
problem, we remove the final page of low memory from the memory map.

arch/mips/jz4770/setup.c

@@ -40,6 +40,7 @@
 #include <asm/reboot.h>
 #include <asm/pgtable.h>
 #include <asm/time.h>
+#include <asm/page.h>
 
 #include <asm/mach-jz4770/jz4770cpm.h>
 #include <asm/mach-jz4770/jz4770dmac.h>
@@ -190,3 +191,24 @@ void __init plat_mem_setup(void)
 #endif
 }
 
+/*
+ * We have seen MMC DMA transfers read corrupted data from SDRAM when a burst
+ * interval ends at physical address 0x10000000. To avoid this problem, we
+ * remove the final page of low memory from the memory map.
+ */
+void __init jz4770_reserve_unsafe_for_dma(void)
+{
+	int i;
+	for (i = 0; i < boot_mem_map.nr_map; i++) {
+		struct boot_mem_map_entry *entry = boot_mem_map.map + i;
+
+		if (entry->type != BOOT_MEM_RAM)
+			continue;
+
+		if (entry->addr + entry->size != 0x10000000)
+			continue;
+
+		entry->size -= PAGE_SIZE;
+		break;
+	}
+}

arch/mips/kernel/setup.c

@@ -640,6 +640,13 @@ static void __init arch_mem_init(char **cmdline_p)
 
 	parse_early_param();
 
+#ifdef CONFIG_MACH_JZ4770
+	{
+		extern void __init jz4770_reserve_unsafe_for_dma(void);
+		jz4770_reserve_unsafe_for_dma();
+	}
+#endif
+
 	if (usermem) {
 		pr_info("User-defined physical RAM map:\n");
 		print_memory_map();