[PATCH] Recognise a 64-bit Linux kernel on a 32-bit EFI firmware

[Steve McIntyre <steve@einval.com>]

Some x86 systems might be capable of running a 64-bit Linux kernel but
only use a 32-bit EFI (e.g. Intel Bay Trail systems). It's useful for
grub-install to be able to recognise such systems, to set the default
x86 platform correctly.

To allow grub-install to know the size of the firmware rather than
just the size of the kernel, there is now an extra EFI sysfs file to
describe the underlying firmware. Read that if possible, otherwise
fall back to the kernel type as before.

Signed-off-by: Steve McIntyre <steve@einval.com>
---
 grub-core/osdep/linux/platform.c |   39 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/grub-core/osdep/linux/platform.c b/grub-core/osdep/linux/platform.c
index 4b9f6ef..775b6c0 100644
--- a/grub-core/osdep/linux/platform.c
+++ b/grub-core/osdep/linux/platform.c
@@ -60,6 +60,43 @@ is_64_kernel (void)
   return strcmp (un.machine, "x86_64") == 0;
 }
 
+static int
+read_platform_size (void)
+{
+  FILE *fp;
+  char *buf = NULL;
+  size_t len = 0;
+  int ret = 0;
+
+  /* Newer kernels can tell us directly about the size of the
+   * underlying firmware - let's see if that interface is there. */
+  fp = grub_util_fopen ("/sys/firmware/efi/fw_platform_size", "r");
+  if (fp != NULL)
+  {
+    if (getline (&buf, &len, fp) >= 3) /* 2 digits plus newline */
+      {
+	if (strncmp (buf, "32", 2) == 0)
+	  ret = 32;
+	else if (strncmp (buf, "64", 2) == 0)
+	  ret = 64;
+      }
+    free (buf);
+    fclose (fp);
+  }
+
+  if (ret == 0)
+    {
+      /* Unrecognised - fall back to matching the kernel size
+       * instead */
+      if (is_64_kernel ())
+	ret = 64;
+      else
+	ret = 32;
+    }
+
+  return ret;
+}
+
 const char *
 grub_install_get_default_x86_platform (void)
 { 
@@ -77,7 +114,7 @@ grub_install_get_default_x86_platform (void)
   if (is_not_empty_directory ("/sys/firmware/efi"))
     {
       grub_util_info ("...found");
-      if (is_64_kernel ())
+      if (read_platform_size() == 64)
 	return "x86_64-efi";
       else
 	return "i386-efi";
-- 
1.7.10.4