[PATCH] x86_64: Display more intutive error message if kernel is not 2MB aligned

[Vivek Goyal <vgoyal@in.ibm.com>]

o x86_64 kernel needs to be compiled for 2MB aligned addresses. Currently
  we are using BUILD_BUG_ON() to warn the user if he has not done so. But
  looks like folks are not finding message very intutive and don't open
  the respective c file to find problem source. (Bug 8439)

arch/x86_64/kernel/head64.c: In function 'x86_64_start_kernel':
arch/x86_64/kernel/head64.c:70: error: size of array 'type name' is negative

o Using preprocessor directive #error to print a better message if
  CONFIG_PHYSICAL_START is not aligned to 2MB boundary. 

Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
---

 linux-2.6.21-rc7-mm2-root/arch/x86_64/kernel/head64.c |    7 -------
 linux-2.6.21-rc7-mm2-root/include/asm-x86_64/page.h   |    9 +++++++++
 2 files changed, 9 insertions(+), 7 deletions(-)

diff -puN arch/x86_64/kernel/head64.c~x86_64-warn-if-kernel-is-not-2MB-aligned arch/x86_64/kernel/head64.c
--- linux-2.6.21-rc7-mm2/arch/x86_64/kernel/head64.c~x86_64-warn-if-kernel-is-not-2MB-aligned	2007-05-07 16:17:21.000000000 +0530
+++ linux-2.6.21-rc7-mm2-root/arch/x86_64/kernel/head64.c	2007-05-07 16:25:42.000000000 +0530
@@ -62,13 +62,6 @@ void __init x86_64_start_kernel(char * r
 {
 	int i;
 
-	/*
-	 * Make sure kernel is aligned to 2MB address. Catching it at compile
-	 * time is better. Change your config file and compile the kernel
-	 * for a 2MB aligned address (CONFIG_PHYSICAL_START)
-	 */
-	BUILD_BUG_ON(CONFIG_PHYSICAL_START & (__KERNEL_ALIGN - 1));
-
 	/* clear bss before set_intr_gate with early_idt_handler */
 	clear_bss();
 
diff -puN include/asm-x86_64/page.h~x86_64-warn-if-kernel-is-not-2MB-aligned include/asm-x86_64/page.h
--- linux-2.6.21-rc7-mm2/include/asm-x86_64/page.h~x86_64-warn-if-kernel-is-not-2MB-aligned	2007-05-07 16:24:16.000000000 +0530
+++ linux-2.6.21-rc7-mm2-root/include/asm-x86_64/page.h	2007-05-07 16:25:29.000000000 +0530
@@ -80,6 +80,15 @@ extern unsigned long phys_base;
 
 #define __PHYSICAL_START	CONFIG_PHYSICAL_START
 #define __KERNEL_ALIGN		0x200000
+/*
+ * Make sure kernel is aligned to 2MB address. Catching it at compile
+ * time is better. Change your config file and compile the kernel
+ * for a 2MB aligned address (CONFIG_PHYSICAL_START)
+ */
+#if (CONFIG_PHYSICAL_START % __KERNEL_ALIGN) != 0
+#error "CONFIG_PHYSICAL_START must be a multiple of 2MB"
+#endif
+
 #define __START_KERNEL		(__START_KERNEL_map + __PHYSICAL_START)
 #define __START_KERNEL_map	0xffffffff80000000
 #define __PAGE_OFFSET           0xffff810000000000
_
-
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