arch/x86: Adjust size of postcar stack

With VBOOT=y && VBOOT_MEASURED_BOOT=y message
digest will be allocated from the stack and
1 KiB reserve used with the recent platforms
was no longer sufficient.

The comment of LZMA scratchpad consuming stack
was obsolete for postcar, so these can be reduced
to same 4 KiB.

Change-Id: Iba1fb5bfad6946f316feac2d8c998a782142a56a
Signed-off-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33775
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

src/arch/x86/include/arch/cpu.h

@@ -309,8 +309,9 @@ struct postcar_frame {
 };
 
 /*
- * Initialize postcar_frame object allocating stack size in cbmem
- * with the provided size. Returns 0 on success, < 0 on error.
+ * Initialize postcar_frame object allocating stack from cbmem,
+ * with stack_size == 0, default 4 KiB is allocated.
+ * Returns 0 on success, < 0 on error.
  */
 int postcar_frame_init(struct postcar_frame *pcf, size_t stack_size);
 

src/arch/x86/postcar_loader.c

@@ -48,6 +48,15 @@ int postcar_frame_init(struct postcar_frame *pcf, size_t stack_size)
 {
 	void *stack;
 
+	/*
+	 * Use default postcar stack size of 4 KiB. This value should
+	 * not be decreased, because if mainboards use vboot, 1 KiB will
+	 * not be enough anymore.
+	 */
+
+	if (stack_size == 0)
+		stack_size = 4 * KiB;
+
 	stack = cbmem_add(CBMEM_ID_ROMSTAGE_RAM_STACK, stack_size);
 	if (stack == NULL) {
 		printk(BIOS_ERR, "Couldn't add %zd byte stack in cbmem.\n",