libchip/serial: Fix NS16550 for pppstart()

Call rtems_termios_dequeue_characters() only in case all characters of
the last write handler invocation have been transmitted.  This avoids
problems with pppstart() since the start line discipline does not know
how many characters have been dequeued and assumes the maximum.

2 files changed, 40 insertions, 18 deletions

diff --git a/c/src/libchip/serial/ns16550-context.c b/c/src/libchip/serial/ns16550-context.c
index a812271037..0b5d1b0595 100644
--- a/c/src/libchip/serial/ns16550-context.c
+++ b/c/src/libchip/serial/ns16550-context.c
@@ -162,6 +162,24 @@ bool ns16550_probe(rtems_termios_device_context *base)
   return true;
 }
 
+static size_t ns16550_write_to_fifo(
+  const ns16550_context *ctx,
+  const char *buf,
+  size_t len
+)
+{
+  uint32_t port = ctx->port;
+  ns16550_set_reg set = ctx->set_reg;
+  size_t out = len > SP_FIFO_SIZE ? SP_FIFO_SIZE : len;
+  size_t i;
+
+  for (i = 0; i < out; ++i) {
+    (*set)(port, NS16550_TRANSMIT_BUFFER, buf[i]);
+  }
+
+  return out;
+}
+
 #if defined(BSP_FEATURE_IRQ_EXTENSION) || defined(BSP_FEATURE_IRQ_LEGACY)
 
 /**
@@ -190,15 +208,20 @@ static void ns16550_isr(void *arg)
     /* Enqueue fetched characters */
     rtems_termios_enqueue_raw_characters(tty, buf, i);
 
-    /* Check if we can dequeue transmitted characters */
-    if (ctx->transmit_fifo_chars > 0
-        && (get( port, NS16550_LINE_STATUS) & SP_LSR_THOLD) != 0) {
+    /* Do transmit */
+    if (ctx->out_total > 0
+        && (get(port, NS16550_LINE_STATUS) & SP_LSR_THOLD) != 0) {
+      size_t current = ctx->out_current;
 
-      /* Dequeue transmitted characters */
-      rtems_termios_dequeue_characters(
-        tty,
-        ctx->transmit_fifo_chars
-      );
+      ctx->out_buf += current;
+      ctx->out_remaining -= current;
+
+      if (ctx->out_remaining > 0) {
+        ctx->out_current =
+          ns16550_write_to_fifo(ctx, ctx->out_buf, ctx->out_remaining);
+      } else {
+        rtems_termios_dequeue_characters(tty, ctx->out_total);
+      }
     }
   } while ((get( port, NS16550_INTERRUPT_ID) & SP_IID_0) == 0);
 }
@@ -577,18 +600,14 @@ static void ns16550_write_support_int(
 )
 {
   ns16550_context *ctx = (ns16550_context *) base;
-  uint32_t port = ctx->port;
-  ns16550_set_reg set = ctx->set_reg;
-  int i = 0;
-  int out = len > SP_FIFO_SIZE ? SP_FIFO_SIZE : len;
 
-  for (i = 0; i < out; ++i) {
-    set( port, NS16550_TRANSMIT_BUFFER, buf [i]);
-  }
+  ctx->out_total = len;
 
-  ctx->transmit_fifo_chars = out;
+  if (len > 0) {
+    ctx->out_remaining = len;
+    ctx->out_buf = buf;
+    ctx->out_current = ns16550_write_to_fifo(ctx, buf, len);
 
-  if (out > 0) {
     ns16550_enable_interrupts(ctx, NS16550_ENABLE_ALL_INTR);
   } else {
     ns16550_enable_interrupts(ctx, NS16550_ENABLE_ALL_INTR_EXCEPT_TX);
diff --git a/c/src/libchip/serial/ns16550.h b/c/src/libchip/serial/ns16550.h
index ef1f9f9d95..0b05fe3df3 100644
--- a/c/src/libchip/serial/ns16550.h
+++ b/c/src/libchip/serial/ns16550.h
@@ -70,7 +70,10 @@ typedef struct {
   uint32_t initial_baud;
   bool has_fractional_divider_register;
   uint8_t modem_control;
-  size_t transmit_fifo_chars;
+  size_t out_total;
+  size_t out_remaining;
+  size_t out_current;
+  const char *out_buf;
 } ns16550_context;
 
 extern const rtems_termios_device_handler ns16550_handler_interrupt;