[uart] Add support for MMIO-accessible 16550 UARTs

16550 UARTs exist on non-x86 platforms but will be accessible via MMIO
rather than port I/O.  It is possible to encounter MMIO-mapped 16550
UARTs on x86 platforms, but there is no real requirement to support
them in iPXE since the standard COM1, COM2, etc ports have been
present on every PC-compatible machine since 1981.

Assume for now that accessing 16550 UART registers requires
inb()/outb() on x86 and readb()/writeb() on other architectures.

Allow for the existence of a register shift on MMIO-mapped 16550
UARTs, since modern SoCs tend to treat register addresses as being
aligned to either 32-bit or 64-bit boundaries.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/bits/ns16550.h

@@ -3,13 +3,40 @@
 
 /** @file
  *
- * Dummy architecture-specific 16550-compatible UART
- *
- * This file is included only if the architecture does not provide its
- * own version of this file.
+ * 16550-compatible UART
  *
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
+#include <stdint.h>
+#include <ipxe/io.h>
+
+/**
+ * Write to UART register
+ *
+ * @v ns16550		16550 UART
+ * @v address		Register address
+ * @v data		Data
+ */
+static inline __attribute__ (( always_inline )) void
+ns16550_write ( struct ns16550_uart *ns16550, unsigned int address,
+		uint8_t data ) {
+
+	writeb ( data, ( ns16550->base + ( address << ns16550->shift ) ) );
+}
+
+/**
+ * Read from UART register
+ *
+ * @v ns16550		16550 UART
+ * @v address		Register address
+ * @ret data		Data
+ */
+static inline __attribute__ (( always_inline )) uint8_t
+ns16550_read ( struct ns16550_uart *ns16550, unsigned int address ) {
+
+	return readb ( ns16550->base + ( address << ns16550->shift ) );
+}
+
 #endif /* _BITS_NS16550_H */

src/include/ipxe/ns16550.h

@@ -82,6 +82,8 @@ struct ns16550_uart {
 	struct uart uart;
 	/** Register base address */
 	void *base;
+	/** Register shift */
+	unsigned int shift;
 	/** Baud rate divisor */
 	uint16_t divisor;
 };