allow additional ns16550a config via device tree

This commit makes bbl read some additional fields from
the device tree if it detects an ns16550a:

- reg-shift
- reg-offset
- clock-frequency

For explanation of these check out the Linux Kernel doc:
https://www.kernel.org/doc/Documentation/devicetree/bindings/serial/8250.txt

In particular this allows the Xilinx AXI UART 16550 to act
as serial console with bbl and the Linux early boot console.

This also fixes a bug in which bbl will ignore any other than the first
"compatible" string when iterating over the nodes.
Previously this line would not have worked:

compatible = "xlnx,xps-uart16550-2.00.a", "ns16550a";

Before bbl would have just checked the first field instead of checking
all strings in the list.

machine/fdt.c

@@ -126,6 +126,11 @@ const uint32_t *fdt_get_size(const struct fdt_scan_node *node, const uint32_t *v
   return value;
 }
 
+uint32_t fdt_get_value(const struct fdt_scan_prop *prop, uint32_t index)
+{
+  return bswap(prop->value[index]);
+}
+
 int fdt_string_list_index(const struct fdt_scan_prop *prop, const char *str)
 {
   const char *list = (const char *)prop->value;

machine/fdt.h

@@ -54,6 +54,7 @@ uint32_t fdt_size(uintptr_t fdt);
 // Extract fields
 const uint32_t *fdt_get_address(const struct fdt_scan_node *node, const uint32_t *base, uint64_t *value);
 const uint32_t *fdt_get_size(const struct fdt_scan_node *node, const uint32_t *base, uint64_t *value);
+uint32_t fdt_get_value(const struct fdt_scan_prop *prop, uint32_t index);
 int fdt_string_list_index(const struct fdt_scan_prop *prop, const char *str); // -1 if not found
 
 // Setup memory+clint+plic

machine/uart16550.c

@@ -1,26 +1,42 @@
 // See LICENSE for license details.
 
 #include <string.h>
+#include <stdarg.h>
+#include <assert.h>
 #include "uart16550.h"
 #include "fdt.h"
 
 volatile uint8_t* uart16550;
+// some devices require a shifted register index
+// (e.g. 32 bit registers instead of 8 bit registers)
+static uint32_t uart16550_reg_shift;
+static uint32_t uart16550_clock = 1843200;   // a "common" base clock
 
-#define UART_REG_QUEUE     0
-#define UART_REG_LINESTAT  5
+#define UART_REG_QUEUE     0    // rx/tx fifo data
+#define UART_REG_DLL       0    // divisor latch (LSB)
+#define UART_REG_IER       1    // interrupt enable register
+#define UART_REG_DLM       1    // divisor latch (MSB) 
+#define UART_REG_FCR       2    // fifo control register
+#define UART_REG_LCR       3    // line control register
+#define UART_REG_MCR       4    // modem control register
+#define UART_REG_LSR       5    // line status register
+#define UART_REG_MSR       6    // modem status register
+#define UART_REG_SCR       7    // scratch register
 #define UART_REG_STATUS_RX 0x01
 #define UART_REG_STATUS_TX 0x20
 
+#define UART_DEFAULT_BAUD  38400
+
 void uart16550_putchar(uint8_t ch)
 {
-  while ((uart16550[UART_REG_LINESTAT] & UART_REG_STATUS_TX) == 0);
-  uart16550[UART_REG_QUEUE] = ch;
+  while ((uart16550[UART_REG_LSR << uart16550_reg_shift] & UART_REG_STATUS_TX) == 0);
+  uart16550[UART_REG_QUEUE << uart16550_reg_shift] = ch;
 }
 
 int uart16550_getchar()
 {
-  if (uart16550[UART_REG_LINESTAT] & UART_REG_STATUS_RX)
-    return uart16550[UART_REG_QUEUE];
+  if (uart16550[UART_REG_LSR << uart16550_reg_shift] & UART_REG_STATUS_RX)
+    return uart16550[UART_REG_QUEUE << uart16550_reg_shift];
   return -1;
 }
 
@@ -28,6 +44,9 @@ struct uart16550_scan
 {
   int compat;
   uint64_t reg;
+  uint32_t reg_offset;
+  uint32_t reg_shift;
+  uint32_t clock_freq;
 };
 
 static void uart16550_open(const struct fdt_scan_node *node, void *extra)
@@ -39,26 +58,41 @@ static void uart16550_open(const struct fdt_scan_node *node, void *extra)
 static void uart16550_prop(const struct fdt_scan_prop *prop, void *extra)
 {
   struct uart16550_scan *scan = (struct uart16550_scan *)extra;
-  if (!strcmp(prop->name, "compatible") && !strcmp((const char*)prop->value, "ns16550a")) {
+  if (!strcmp(prop->name, "compatible") && fdt_string_list_index(prop, "ns16550a") != -1) {
     scan->compat = 1;
   } else if (!strcmp(prop->name, "reg")) {
     fdt_get_address(prop->node->parent, prop->value, &scan->reg);
+  } else if (!strcmp(prop->name, "reg-shift")) {
+    scan->reg_shift = fdt_get_value(prop, 0);
+  } else if (!strcmp(prop->name, "reg-offset")) {
+    scan->reg_offset = fdt_get_value(prop, 0);
+  } else if (!strcmp(prop->name, "clock-frequency")) {
+    scan->clock_freq = fdt_get_value(prop, 0);
   }
 }
 
 static void uart16550_done(const struct fdt_scan_node *node, void *extra)
 {
+  uint32_t clock_freq;
   struct uart16550_scan *scan = (struct uart16550_scan *)extra;
   if (!scan->compat || !scan->reg || uart16550) return;
 
-  uart16550 = (void*)(uintptr_t)scan->reg;
+  if (scan->clock_freq != 0)
+    uart16550_clock = scan->clock_freq;
+  // if device tree doesn't supply a clock, fallback to default clock of 1843200
+
+  uint32_t divisor = uart16550_clock / (16 * UART_DEFAULT_BAUD);
+  assert (divisor < 0x10000u);
+
+  uart16550 = (void*)((uintptr_t)scan->reg + scan->reg_offset);
+  uart16550_reg_shift = scan->reg_shift;
   // http://wiki.osdev.org/Serial_Ports
-  uart16550[1] = 0x00;    // Disable all interrupts
-  uart16550[3] = 0x80;    // Enable DLAB (set baud rate divisor)
-  uart16550[0] = 0x03;    // Set divisor to 3 (lo byte) 38400 baud
-  uart16550[1] = 0x00;    //                  (hi byte)
-  uart16550[3] = 0x03;    // 8 bits, no parity, one stop bit
-  uart16550[2] = 0xC7;    // Enable FIFO, clear them, with 14-byte threshold
+  uart16550[UART_REG_IER << uart16550_reg_shift] = 0x00;                // Disable all interrupts
+  uart16550[UART_REG_LCR << uart16550_reg_shift] = 0x80;                // Enable DLAB (set baud rate divisor)
+  uart16550[UART_REG_DLL << uart16550_reg_shift] = (uint8_t)divisor;    // Set divisor (lo byte)
+  uart16550[UART_REG_DLM << uart16550_reg_shift] = (uint8_t)(divisor >> 8);     //     (hi byte)
+  uart16550[UART_REG_LCR << uart16550_reg_shift] = 0x03;                // 8 bits, no parity, one stop bit
+  uart16550[UART_REG_FCR << uart16550_reg_shift] = 0xC7;                // Enable FIFO, clear them, with 14-byte threshold
 }
 
 void query_uart16550(uintptr_t fdt)