2 files changed, 76 insertions, 22 deletions

diff --git a/arch/i386/boot/edd.S b/arch/i386/boot/edd.S
index 4b84ea216f2b..34321368011a 100644
--- a/arch/i386/boot/edd.S
+++ b/arch/i386/boot/edd.S
@@ -15,42 +15,95 @@
 #include <asm/setup.h>
 
 #if defined(CONFIG_EDD) || defined(CONFIG_EDD_MODULE)
+
+# It is assumed that %ds == INITSEG here
+
         movb    $0, (EDD_MBR_SIG_NR_BUF)
         movb    $0, (EDDNR)
 
-# Check the command line for two options:
+# Check the command line for options:
 # edd=of  disables EDD completely  (edd=off)
 # edd=sk  skips the MBR test    (edd=skipmbr)
+# edd=on  re-enables EDD (edd=on)
+
         pushl   %esi
-        cmpl    $0, %cs:cmd_line_ptr
-        jz      done_cl
+        movw    $edd_mbr_sig_start, %di # Default to edd=on
+
         movl    %cs:(cmd_line_ptr), %esi
-# ds:esi has the pointer to the command line now
-        movl    $(COMMAND_LINE_SIZE-7), %ecx
-# loop through kernel command line one byte at a time
-cl_loop:
-        cmpl    $EDD_CL_EQUALS, (%si)
+        andl    %esi, %esi
+        jz      old_cl                  # Old boot protocol?
+
+# Convert to a real-mode pointer in fs:si
+        movl    %esi, %eax
+        shrl    $4, %eax
+        movw    %ax, %fs
+        andw    $0xf, %si
+        jmp     have_cl_pointer
+
+# Old-style boot protocol?
+old_cl:
+        push    %ds                     # aka INITSEG
+        pop     %fs
+
+        cmpw    $0xa33f, (0x20)
+        jne     done_cl                 # No command line at all?
+        movw    (0x22), %si             # Pointer relative to INITSEG
+
+# fs:si has the pointer to the command line now
+have_cl_pointer:
+
+# Loop through kernel command line one byte at a time.  Just in
+# case the loader is buggy and failed to null-terminate the command line
+# terminate if we get close enough to the end of the segment that we
+# cannot fit "edd=XX"...
+cl_atspace:
+        cmpw    $-5, %si                # Watch for segment wraparound
+        jae     done_cl
+        movl    %fs:(%si), %eax
+        andb    %al, %al                # End of line?
+        jz      done_cl
+        cmpl    $EDD_CL_EQUALS, %eax
         jz      found_edd_equals
-        incl    %esi
-        loop    cl_loop
-        jmp     done_cl
+        cmpb    $0x20, %al              # <= space consider whitespace
+        ja      cl_skipword
+        incw    %si
+        jmp     cl_atspace
+
+cl_skipword:
+        cmpw    $-5, %si                # Watch for segment wraparound
+        jae     done_cl
+        movb    %fs:(%si), %al          # End of string?
+        andb    %al, %al
+        jz      done_cl
+        cmpb    $0x20, %al
+        jbe     cl_atspace
+        incw    %si
+        jmp     cl_skipword
+
 found_edd_equals:
 # only looking at first two characters after equals
-        addl    $4, %esi
-        cmpw    $EDD_CL_OFF, (%si)      # edd=of
-        jz      do_edd_off
-        cmpw    $EDD_CL_SKIP, (%si)     # edd=sk
-        jz      do_edd_skipmbr
-        jmp     done_cl
+# late overrides early on the command line, so keep going after finding something
+        movw    %fs:4(%si), %ax
+        cmpw    $EDD_CL_OFF, %ax        # edd=of
+        je      do_edd_off
+        cmpw    $EDD_CL_SKIP, %ax       # edd=sk
+        je      do_edd_skipmbr
+        cmpw    $EDD_CL_ON, %ax         # edd=on
+        je      do_edd_on
+        jmp     cl_skipword
 do_edd_skipmbr:
-        popl    %esi
-        jmp     edd_start
+        movw    $edd_start, %di
+        jmp     cl_skipword
 do_edd_off:
-        popl    %esi
-        jmp     edd_done
+        movw    $edd_done, %di
+        jmp     cl_skipword
+do_edd_on:
+        movw    $edd_mbr_sig_start, %di
+        jmp     cl_skipword
+
 done_cl:
         popl    %esi
-
+        jmpw    *%di
 
 # Read the first sector of each BIOS disk device and store the 4-byte signature
 edd_mbr_sig_start:
diff --git a/include/linux/edd.h b/include/linux/edd.h
index 162512b886f7..b2b3e68aa512 100644
--- a/include/linux/edd.h
+++ b/include/linux/edd.h
@@ -52,6 +52,7 @@
 #define EDD_CL_EQUALS   0x3d646465     /* "edd=" */
 #define EDD_CL_OFF      0x666f         /* "of" for off  */
 #define EDD_CL_SKIP     0x6b73         /* "sk" for skipmbr */
+#define EDD_CL_ON       0x6e6f         /* "on" for on */
 
 #ifndef __ASSEMBLY__