x86/earlyprintk: Clean up pciserial

While working on a GRUB patch to support PCI-serial, a number of
cleanups were suggested that apply to the code I took inspiration from.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>   # pci_ids.h
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lkml.kernel.org/r/YwdeyCEtW+wa+QhH@worktop.programming.kicks-ass.net

arch/x86/kernel/early_printk.c

@@ -264,11 +264,11 @@ static __init void early_pci_serial_init(char *s)
 	bar0 = read_pci_config(bus, slot, func, PCI_BASE_ADDRESS_0);
 
 	/*
-	 * Verify it is a UART type device
+	 * Verify it is a 16550-UART type device
 	 */
 	if (((classcode >> 16 != PCI_CLASS_COMMUNICATION_MODEM) &&
 	     (classcode >> 16 != PCI_CLASS_COMMUNICATION_SERIAL)) ||
-	   (((classcode >> 8) & 0xff) != 0x02)) /* 16550 I/F at BAR0 */ {
+	    (((classcode >> 8) & 0xff) != PCI_SERIAL_16550_COMPATIBLE)) {
 		if (!force)
 			return;
 	}
@@ -276,11 +276,11 @@ static __init void early_pci_serial_init(char *s)
 	/*
 	 * Determine if it is IO or memory mapped
 	 */
-	if (bar0 & 0x01) {
+	if ((bar0 & PCI_BASE_ADDRESS_SPACE) == PCI_BASE_ADDRESS_SPACE_IO) {
 		/* it is IO mapped */
 		serial_in = io_serial_in;
 		serial_out = io_serial_out;
-		early_serial_base = bar0&0xfffffffc;
+		early_serial_base = bar0 & PCI_BASE_ADDRESS_IO_MASK;
 		write_pci_config(bus, slot, func, PCI_COMMAND,
 				 cmdreg|PCI_COMMAND_IO);
 	} else {
@@ -289,7 +289,7 @@ static __init void early_pci_serial_init(char *s)
 		serial_out = mem32_serial_out;
 		/* WARNING! assuming the address is always in the first 4G */
 		early_serial_base =
-			(unsigned long)early_ioremap(bar0 & 0xfffffff0, 0x10);
+			(unsigned long)early_ioremap(bar0 & PCI_BASE_ADDRESS_MEM_MASK, 0x10);
 		write_pci_config(bus, slot, func, PCI_COMMAND,
 				 cmdreg|PCI_COMMAND_MEMORY);
 	}

include/linux/pci_ids.h

@@ -75,6 +75,9 @@
 #define PCI_CLASS_COMMUNICATION_MODEM	0x0703
 #define PCI_CLASS_COMMUNICATION_OTHER	0x0780
 
+/* Interface for SERIAL/MODEM */
+#define PCI_SERIAL_16550_COMPATIBLE	0x02
+
 #define PCI_BASE_CLASS_SYSTEM		0x08
 #define PCI_CLASS_SYSTEM_PIC		0x0800
 #define PCI_CLASS_SYSTEM_PIC_IOAPIC	0x080010