Add support for the UART interface on the LiteX SoC (#230)

Tested using the RocketChip CPU option.
(see https://github.com/enjoy-digital/litex)

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>

machine/machine.mk.in

@@ -15,6 +15,7 @@ machine_hdrs = \
   mtrap.h \
   uart.h \
   uart16550.h \
+  uart_litex.h \
   finisher.h \
   unprivileged_memory.h \
   vm.h \
@@ -30,6 +31,7 @@ machine_c_srcs = \
   fp_ldst.c \
   uart.c \
   uart16550.c \
+  uart_litex.c \
   finisher.c \
   misaligned_ldst.c \
   flush_icache.c \
@@ -43,4 +45,4 @@ mentry.o: custom.dtb
 
 custom.dtb: $(CUSTOM_DTS)
 	dtc -O dtb $^ -o $@
-endif
+endif

machine/minit.c

@@ -7,6 +7,7 @@
 #include "fdt.h"
 #include "uart.h"
 #include "uart16550.h"
+#include "uart_litex.h"
 #include "finisher.h"
 #include "disabled_hart_mask.h"
 #include "htif.h"
@@ -173,6 +174,7 @@ void init_first_hart(uintptr_t hartid, uintptr_t dtb)
   // Confirm console as early as possible
   query_uart(dtb);
   query_uart16550(dtb);
+  query_uart_litex(dtb);
   query_htif(dtb);
   printm("bbl loader\r\n");
 

machine/mtrap.c

@@ -8,6 +8,7 @@
 #include "vm.h"
 #include "uart.h"
 #include "uart16550.h"
+#include "uart_litex.h"
 #include "finisher.h"
 #include "fdt.h"
 #include "unprivileged_memory.h"
@@ -27,6 +28,8 @@ static uintptr_t mcall_console_putchar(uint8_t ch)
     uart_putchar(ch);
   } else if (uart16550) {
     uart16550_putchar(ch);
+  } else if (uart_litex) {
+    uart_litex_putchar(ch);
   } else if (htif) {
     htif_console_putchar(ch);
   }
@@ -69,6 +72,8 @@ static uintptr_t mcall_console_getchar()
     return uart_getchar();
   } else if (uart16550) {
     return uart16550_getchar();
+  } else if (uart_litex) {
+    return uart_litex_getchar();
   } else if (htif) {
     return htif_console_getchar();
   } else {

machine/uart_litex.c

@@ -0,0 +1,77 @@
+// See LICENSE for license details.
+
+#include <string.h>
+#include "uart_litex.h"
+#include "fdt.h"
+
+volatile unsigned int *uart_litex;
+
+#define UART_REG_RXTX       0
+#define UART_REG_TXFULL     1
+#define UART_REG_RXEMPTY    2
+#define UART_REG_EV_STATUS  3
+#define UART_REG_EV_PENDING 4
+#define UART_REG_EV_ENABLE  5
+
+void uart_litex_putchar(uint8_t c)
+{
+    while ((uart_litex[UART_REG_TXFULL] & 0x01)); // wait while tx-buffer full
+    uart_litex[UART_REG_RXTX] = c;
+}
+
+int uart_litex_getchar()
+{
+    int c = -1;
+    if (!(uart_litex[UART_REG_RXEMPTY] & 0x01)) { // if rx-buffer not empty
+        c = uart_litex[UART_REG_RXTX];
+        uart_litex[UART_REG_EV_PENDING] = 0x02; // ack (UART_EV_RX)
+    }
+    return c;
+}
+
+struct uart_litex_scan
+{
+    int compat;
+    uint64_t reg;
+};
+
+static void uart_litex_open(const struct fdt_scan_node *node, void *extra)
+{
+    struct uart_litex_scan *scan = (struct uart_litex_scan *)extra;
+    memset(scan, 0, sizeof(*scan));
+}
+
+static void uart_litex_prop(const struct fdt_scan_prop *prop, void *extra)
+{
+    struct uart_litex_scan *scan = (struct uart_litex_scan *)extra;
+    if (!strcmp(prop->name, "compatible") &&
+        !strcmp((const char *)prop->value, "litex,uart0")) {
+        scan->compat = 1;
+    } else if (!strcmp(prop->name, "reg")) {
+        fdt_get_address(prop->node->parent, prop->value, &scan->reg);
+    }
+}
+
+static void uart_litex_done(const struct fdt_scan_node *node, void *extra)
+{
+    struct uart_litex_scan *scan = (struct uart_litex_scan *)extra;
+    if (!scan->compat || !scan->reg || uart_litex)
+        return;
+
+    // Initialize LiteX UART
+    uart_litex = (void *)(uintptr_t)scan->reg;
+}
+
+void query_uart_litex(uintptr_t fdt)
+{
+    struct fdt_cb cb;
+    struct uart_litex_scan scan;
+
+    memset(&cb, 0, sizeof(cb));
+    cb.open = uart_litex_open;
+    cb.prop = uart_litex_prop;
+    cb.done = uart_litex_done;
+    cb.extra = &scan;
+
+    fdt_scan(fdt, &cb);
+}

machine/uart_litex.h

@@ -0,0 +1,14 @@
+// See LICENSE for license details.
+
+#ifndef _RISCV_UARTLR_H
+#define _RISCV_UARTLR_H
+
+#include <stdint.h>
+
+extern volatile unsigned int *uart_litex;
+
+void uart_litex_putchar(uint8_t ch);
+int uart_litex_getchar();
+void query_uart_litex(uintptr_t dtb);
+
+#endif