drivers/serial/serial.c: adapt to the iovec-based api

This would fix readv/writev issues mentioned in
#12674.
(only for this specific driver though. with this approach,
we basically have to fix every single drivers and
filesystems.)

Lightly tested on the serial console, using micropython REPL
on toywasm with esp32s3-devkit:toywasm, which used to be
suffered by the readv issue.

drivers/serial/serial.c

@@ -99,6 +99,8 @@ static int     uart_putxmitchar(FAR uart_dev_t *dev, int ch,
 static inline ssize_t uart_irqwrite(FAR uart_dev_t *dev,
                                     FAR const char *buffer,
                                     size_t buflen);
+static inline ssize_t uart_irqwritev(FAR uart_dev_t *dev,
+                                     FAR struct uio *uio);
 static int     uart_tcdrain(FAR uart_dev_t *dev,
                             bool cancelable, clock_t timeout);
 
@@ -110,11 +112,8 @@ static int     uart_tcsendbreak(FAR uart_dev_t *dev,
 
 static int     uart_open(FAR struct file *filep);
 static int     uart_close(FAR struct file *filep);
-static ssize_t uart_read(FAR struct file *filep,
-                         FAR char *buffer, size_t buflen);
-static ssize_t uart_write(FAR struct file *filep,
-                          FAR const char *buffer,
-                          size_t buflen);
+static ssize_t uart_readv(FAR struct file *filep, FAR struct uio *uio);
+static ssize_t uart_writev(FAR struct file *filep, FAR struct uio *uio);
 static int     uart_ioctl(FAR struct file *filep,
                           int cmd, unsigned long arg);
 static int     uart_poll(FAR struct file *filep,
@@ -141,15 +140,15 @@ static const struct file_operations g_serialops =
 {
   uart_open,    /* open */
   uart_close,   /* close */
-  uart_read,    /* read */
-  uart_write,   /* write */
+  NULL,         /* read */
+  NULL,         /* write */
   NULL,         /* seek */
   uart_ioctl,   /* ioctl */
   NULL,         /* mmap */
   NULL,         /* truncate */
   uart_poll,    /* poll */
-  NULL,         /* readv */
-  NULL          /* writev */
+  uart_readv,   /* readv */
+  uart_writev   /* writev */
 #ifndef CONFIG_DISABLE_PSEUDOFS_OPERATIONS
   , uart_unlink /* unlink */
 #endif
@@ -441,6 +440,50 @@ static inline ssize_t uart_irqwrite(FAR uart_dev_t *dev,
   return buflen;
 }
 
+/****************************************************************************
+ * Name: uart_irqwritev
+ ****************************************************************************/
+
+static inline ssize_t uart_irqwritev(FAR uart_dev_t *dev,
+                                     FAR struct uio *uio)
+{
+  ssize_t error = 0;
+  ssize_t total = 0;
+  int iovcnt = uio->uio_iovcnt;
+  int i;
+
+  for (i = 0; i < iovcnt; i++)
+    {
+      const struct iovec *iov = &uio->uio_iov[i];
+      if (iov->iov_len == 0)
+        {
+          continue;
+        }
+
+      ssize_t written = uart_irqwrite(dev, iov->iov_base, iov->iov_len);
+      if (written < 0)
+        {
+          error = written;
+          break;
+        }
+
+      if (SSIZE_MAX - total < written)
+        {
+          error = -EOVERFLOW;
+          break;
+        }
+
+      total += written;
+    }
+
+  if (error != 0 && total == 0)
+    {
+      return error;
+    }
+
+  return total;
+}
+
 /****************************************************************************
  * Name: uart_tcdrain
  *
@@ -847,11 +890,10 @@ static int uart_close(FAR struct file *filep)
 }
 
 /****************************************************************************
- * Name: uart_read
+ * Name: uart_readv
  ****************************************************************************/
 
-static ssize_t uart_read(FAR struct file *filep,
-                         FAR char *buffer, size_t buflen)
+static ssize_t uart_readv(FAR struct file *filep, FAR struct uio *uio)
 {
   FAR struct inode *inode = filep->f_inode;
   FAR uart_dev_t *dev = inode->i_private;
@@ -862,11 +904,18 @@ static ssize_t uart_read(FAR struct file *filep,
 #endif
   irqstate_t flags;
   ssize_t recvd = 0;
+  ssize_t buflen;
   bool echoed = false;
   int16_t tail;
   char ch;
   int ret;
 
+  buflen = uio_resid(uio);
+  if (buflen < 0)
+    {
+      return buflen;
+    }
+
   /* Only one user can access rxbuf->tail at a time */
 
   ret = nxmutex_lock(&dev->recv.lock);
@@ -885,7 +934,7 @@ static ssize_t uart_read(FAR struct file *filep,
    * data from the end of the buffer.
    */
 
-  while ((size_t)recvd < buflen)
+  while (recvd < buflen)
     {
 #ifdef CONFIG_SERIAL_REMOVABLE
       /* If the removable device is no longer connected, refuse to read any
@@ -961,7 +1010,8 @@ static ssize_t uart_read(FAR struct file *filep,
             {
               if (recvd > 0)
                 {
-                  *buffer-- = '\0';
+                  static const char zero = '\0';
+                  uio_copyfrom(uio, recvd, &zero, 1);
                   recvd--;
                   if (dev->tc_lflag & ECHO)
                     {
@@ -990,7 +1040,7 @@ static ssize_t uart_read(FAR struct file *filep,
 
           /* Store the received character */
 
-          *buffer++ = ch;
+          uio_copyfrom(uio, recvd, &ch, 1);
           recvd++;
 
           if (dev->tc_lflag & ECHO)
@@ -1325,19 +1375,24 @@ static ssize_t uart_read(FAR struct file *filep,
 #endif
 
   nxmutex_unlock(&dev->recv.lock);
+  if (recvd >= 0)
+    {
+      uio_advance(uio, recvd);
+    }
+
   return recvd;
 }
 
 /****************************************************************************
- * Name: uart_write
+ * Name: uart_writev
  ****************************************************************************/
 
-static ssize_t uart_write(FAR struct file *filep, FAR const char *buffer,
-                          size_t buflen)
+static ssize_t uart_writev(FAR struct file *filep, FAR struct uio *uio)
 {
   FAR struct inode *inode    = filep->f_inode;
   FAR uart_dev_t   *dev      = inode->i_private;
-  ssize_t           nwritten = buflen;
+  ssize_t           nwritten;
+  ssize_t           buflen;
   bool              oktoblock;
   int               ret;
   char              ch;
@@ -1363,12 +1418,18 @@ static ssize_t uart_write(FAR struct file *filep, FAR const char *buffer,
 #endif
 
       flags = enter_critical_section();
-      ret = uart_irqwrite(dev, buffer, buflen);
+      ret = uart_irqwritev(dev, uio);
       leave_critical_section(flags);
 
       return ret;
     }
 
+  buflen = nwritten = uio_resid(uio);
+  if (nwritten < 0)
+    {
+      return nwritten;
+    }
+
   /* Only one user can access dev->xmit.head at a time */
 
   ret = nxmutex_lock(&dev->xmit.lock);
@@ -1408,9 +1469,9 @@ static ssize_t uart_write(FAR struct file *filep, FAR const char *buffer,
    */
 
   uart_disabletxint(dev);
-  for (; buflen; buflen--)
+  for (; buflen; uio_advance(uio, 1), buflen--)
     {
-      ch  = *buffer++;
+      uio_copyto(uio, 0, &ch, 1);
       ret = OK;
 
       /* Do output post-processing */