m68k: Increase initial mapping to 8 or 16 MiB if possible

If the size of the first memory chunk is at least 8 or 16 MiB increase the
initial mapping to 8 resp. 16 MiB instead of 4 MiB.
This makes it possible to
  1. Map more memory in the first node without running out of space for the
     page tables,
  2. Boot kernels that don't fit in 4 MiB (e.g. multi_defconfig).
Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>

  - Add support for 8 MiB,
  - Store initial mapping size in head.S for later reuse,
  - Add comment about large kernels.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/kernel/head.S

@@ -292,6 +292,7 @@
 
 .globl kernel_pg_dir
 .globl availmem
+.globl m68k_init_mapped_size
 .globl m68k_pgtable_cachemode
 .globl m68k_supervisor_cachemode
 #ifdef CONFIG_MVME16x
@@ -907,10 +908,21 @@ L(nocon):
  *
  *	This block of code does what's necessary to map in the various kinds
  *	of machines for execution of Linux.
- *	First map the first 4 MB of kernel code & data
+ *	First map the first 4, 8, or 16 MB of kernel code & data
  */
 
-	mmu_map	#PAGE_OFFSET,%pc@(L(phys_kernel_start)),#4*1024*1024,\
+	get_bi_record BI_MEMCHUNK
+	movel	%a0@(4),%d0
+	movel	#16*1024*1024,%d1
+	cmpl	%d0,%d1
+	jls	1f
+	lsrl	#1,%d1
+	cmpl	%d0,%d1
+	jls	1f
+	lsrl	#1,%d1
+1:
+	movel	%d1,m68k_init_mapped_size
+	mmu_map	#PAGE_OFFSET,%pc@(L(phys_kernel_start)),%d1,\
 		%pc@(m68k_supervisor_cachemode)
 
 	putc	'C'
@@ -3730,6 +3742,9 @@ func_return	console_plot_pixel
 __INITDATA
 	.align	4
 
+m68k_init_mapped_size:
+	.long	0
+
 #if defined(CONFIG_ATARI) || defined(CONFIG_AMIGA) || \
     defined(CONFIG_HP300) || defined(CONFIG_APOLLO)
 L(custom):

arch/m68k/mm/motorola.c

@@ -45,7 +45,7 @@ EXPORT_SYMBOL(mm_cachebits);
 #endif
 
 /* size of memory already mapped in head.S */
-#define INIT_MAPPED_SIZE	(4UL<<20)
+extern __initdata unsigned long m68k_init_mapped_size;
 
 extern unsigned long availmem;
 
@@ -271,10 +271,12 @@ void __init paging_init(void)
 	 */
 	addr = m68k_memory[0].addr;
 	size = m68k_memory[0].size;
-	free_bootmem_node(NODE_DATA(0), availmem, min(INIT_MAPPED_SIZE, size) - (availmem - addr));
+	free_bootmem_node(NODE_DATA(0), availmem,
+			  min(m68k_init_mapped_size, size) - (availmem - addr));
 	map_node(0);
-	if (size > INIT_MAPPED_SIZE)
-		free_bootmem_node(NODE_DATA(0), addr + INIT_MAPPED_SIZE, size - INIT_MAPPED_SIZE);
+	if (size > m68k_init_mapped_size)
+		free_bootmem_node(NODE_DATA(0), addr + m68k_init_mapped_size,
+				  size - m68k_init_mapped_size);
 
 	for (i = 1; i < m68k_num_memory; i++)
 		map_node(i);