[a20] Send a null command to the KBC after changing A20

Send a null command, specifically "pulse outputs" with no outputs
selected, to the KBC after changing A20.  This was apparently done by DOS,
presumably as a synchronization hack, and the authors of the UHCI spec
thought it was inherent.  Therefore, there are systems out there (e.g. HP
DL360 G5) which will stop responsing to "legacy USB" unless they see the
null command, 0xFF, written to port 0x64 at the end of the A20 toggling
sequence.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

src/arch/i386/firmware/pcbios/gateA20.c

@@ -12,6 +12,7 @@
 
 #define KC_CMD_WIN	0xd0		/* read  output port */
 #define KC_CMD_WOUT	0xd1		/* write output port */
+#define KC_CMD_NULL	0xff		/* null command ("pulse nothing") */
 #define KB_SET_A20	0xdf		/* enable A20,
 					   enable output buffer full interrupt
 					   enable data line
@@ -130,6 +131,8 @@ void gateA20_set ( void ) {
 			empty_8042();
 			outb ( KB_SET_A20, K_RDWR );
 			empty_8042();
+			outb ( KC_CMD_NULL, K_CMD );
+			empty_8042();
 			if ( gateA20_is_set ( A20_KBC_RETRIES ) ) {
 				DBG ( "Enabled gate A20 using "
 				      "keyboard controller\n" );