uio api tweaks

* Make readv/writev implementations update struct uio
  This can simplify partial result handling.

* change the error number on the overflow from EOVERFLOW to EINVAL
  to match NetBSD

* add a commented out uio_offset field. I used "#if 0" here as
  C comments can't nest.

* add a few helper functions

Note on uio_copyfrom/uio_copyto:
although i'm not quite happy with the "offset" functionality,
it's necessary to simplify the adaptation of some drivers like
drivers/serial/serial.c, which (ab)uses the user-supplied buffer
as a line-buffer.

fs/vfs/fs_read.c

@@ -50,8 +50,7 @@
  *
  ****************************************************************************/
 
-static ssize_t file_readv_compat(FAR struct file *filep,
-                                 FAR const struct uio *uio)
+static ssize_t file_readv_compat(FAR struct file *filep, FAR struct uio *uio)
 {
   FAR const struct iovec *iov = uio->uio_iov;
   int iovcnt = uio->uio_iovcnt;
@@ -102,6 +101,11 @@ static ssize_t file_readv_compat(FAR struct file *filep,
       remaining -= nread;
     }
 
+  if (ntotal >= 0)
+    {
+      uio_advance(uio, ntotal);
+    }
+
   return ntotal;
 }
 
@@ -130,7 +134,7 @@ static ssize_t file_readv_compat(FAR struct file *filep,
  *
  ****************************************************************************/
 
-ssize_t file_readv(FAR struct file *filep, FAR const struct uio *uio)
+ssize_t file_readv(FAR struct file *filep, FAR struct uio *uio)
 {
   FAR struct inode *inode;
   ssize_t ret = -EBADF;
@@ -204,11 +208,16 @@ ssize_t file_read(FAR struct file *filep, FAR void *buf, size_t nbytes)
 {
   struct iovec iov;
   struct uio uio;
+  ssize_t ret;
 
   iov.iov_base = buf;
   iov.iov_len = nbytes;
-  uio.uio_iov = &iov;
-  uio.uio_iovcnt = 1;
+  ret = uio_init(&uio, &iov, 1);
+  if (ret != 0)
+    {
+      return ret;
+    }
+
   return file_readv(filep, &uio);
 }
 
@@ -251,9 +260,12 @@ ssize_t nx_readv(int fd, FAR const struct iovec *iov, int iovcnt)
 
   /* Then let file_readv do all of the work. */
 
-  uio.uio_iov = iov;
-  uio.uio_iovcnt = iovcnt;
-  ret = file_readv(filep, &uio);
+  ret = uio_init(&uio, iov, iovcnt);
+  if (ret == 0)
+    {
+      ret = file_readv(filep, &uio);
+    }
+
   fs_putfilep(filep);
   return ret;
 }

fs/vfs/fs_uio.c

@@ -35,19 +35,19 @@
 #include <errno.h>
 
 /****************************************************************************
- * Public Functions
+ * Private Functions
  ****************************************************************************/
 
 /****************************************************************************
- * Name: uio_total_len
+ * Name: uio_calc_resid
  *
  * Description:
- *   Return the total length of data in bytes.
- *   Or -EOVERFLOW.
+ *   Return the remaining length of data in bytes.
+ *   Or -EINVAL.
  *
  ****************************************************************************/
 
-ssize_t uio_total_len(FAR const struct uio *uio)
+ssize_t uio_calc_resid(FAR const struct uio *uio)
 {
   const struct iovec *iov = uio->uio_iov;
   int iovcnt = uio->uio_iovcnt;
@@ -58,7 +58,7 @@ ssize_t uio_total_len(FAR const struct uio *uio)
     {
       if (SSIZE_MAX - len < iov[i].iov_len)
         {
-          return -EOVERFLOW;
+          return -EINVAL;
         }
 
       len += iov[i].iov_len;
@@ -66,3 +66,164 @@ ssize_t uio_total_len(FAR const struct uio *uio)
 
   return len;
 }
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: uio_advance
+ *
+ * Description:
+ *   Advance the pointer/offset in uio by the specified amount.
+ *
+ ****************************************************************************/
+
+void uio_advance(FAR struct uio *uio, size_t sz)
+{
+  FAR const struct iovec *iov = uio->uio_iov;
+  int iovcnt = uio->uio_iovcnt;
+  size_t offset_in_iov = uio->uio_offset_in_iov;
+
+  DEBUGASSERT(sz <= SSIZE_MAX);
+  DEBUGASSERT(uio->uio_resid <= SSIZE_MAX);
+  DEBUGASSERT(sz <= uio->uio_resid);
+  DEBUGASSERT(uio->uio_offset_in_iov + uio->uio_resid ==
+              uio_calc_resid(uio));
+  uio->uio_resid -= sz;
+  while (iovcnt > 0)
+    {
+      DEBUGASSERT(offset_in_iov <= iov->iov_len);
+      if (sz < iov->iov_len - offset_in_iov)
+        {
+          offset_in_iov += sz;
+          break;
+        }
+
+      sz -= iov->iov_len - offset_in_iov;
+      iov++;
+      iovcnt--;
+      offset_in_iov = 0;
+    }
+
+  uio->uio_iov = iov;
+  uio->uio_iovcnt = iovcnt;
+  uio->uio_offset_in_iov = offset_in_iov;
+  DEBUGASSERT(uio->uio_offset_in_iov + uio->uio_resid ==
+              uio_calc_resid(uio));
+}
+
+/****************************************************************************
+ * Name: uio_init
+ *
+ * Description:
+ *   Initialize the uio structure with reasonable default values.
+ *
+ ****************************************************************************/
+
+int uio_init(FAR struct uio *uio, FAR const struct iovec *iov, int iovcnt)
+{
+  ssize_t resid;
+
+  memset(uio, 0, sizeof(*uio));
+  uio->uio_iov = iov;
+  uio->uio_iovcnt = iovcnt;
+  resid = uio_calc_resid(uio);
+  if (resid < 0)
+    {
+      return -EINVAL;
+    }
+
+  uio->uio_resid = resid;
+  return 0;
+}
+
+/****************************************************************************
+ * Name: uio_copyfrom
+ *
+ * Description:
+ *   Copy data from the linear buffer to uio.
+ *
+ ****************************************************************************/
+
+void uio_copyfrom(FAR struct uio *uio, size_t offset, FAR const void *buf,
+                  size_t len)
+{
+  FAR const struct iovec *iov = uio->uio_iov;
+
+  DEBUGASSERT(uio->uio_resid >= 0);
+  DEBUGASSERT(uio->uio_resid <= SSIZE_MAX);
+  DEBUGASSERT(len <= uio->uio_resid);
+  DEBUGASSERT(offset <= uio->uio_resid - len);
+  DEBUGASSERT(SSIZE_MAX - offset >= uio->uio_offset_in_iov);
+  DEBUGASSERT(uio->uio_offset_in_iov + uio->uio_resid ==
+              uio_calc_resid(uio));
+
+  offset += uio->uio_offset_in_iov;
+  while (offset > iov->iov_len)
+    {
+      offset -= iov->iov_len;
+      iov++;
+    }
+
+  while (len > 0)
+    {
+      size_t blen = len;
+      if (blen > iov->iov_len - offset)
+        {
+          blen = iov->iov_len - offset;
+        }
+
+      memcpy((FAR uint8_t *)iov->iov_base + offset, buf, blen);
+
+      len -= blen;
+      buf = (const uint8_t *)buf + blen;
+      iov++;
+      offset = 0;
+    }
+}
+
+/****************************************************************************
+ * Name: uio_copyto
+ *
+ * Description:
+ *   Copy data to the linear buffer from uio.
+ *
+ ****************************************************************************/
+
+void uio_copyto(FAR struct uio *uio, size_t offset, FAR void *buf,
+                size_t len)
+{
+  FAR const struct iovec *iov = uio->uio_iov;
+
+  DEBUGASSERT(uio->uio_resid >= 0);
+  DEBUGASSERT(uio->uio_resid <= SSIZE_MAX);
+  DEBUGASSERT(len <= uio->uio_resid);
+  DEBUGASSERT(offset <= uio->uio_resid - len);
+  DEBUGASSERT(SSIZE_MAX - offset >= uio->uio_offset_in_iov);
+  DEBUGASSERT(uio->uio_offset_in_iov + uio->uio_resid ==
+              uio_calc_resid(uio));
+
+  offset += uio->uio_offset_in_iov;
+  while (offset > iov->iov_len)
+    {
+      offset -= iov->iov_len;
+      iov++;
+    }
+
+  while (len > 0)
+    {
+      size_t blen = len;
+      if (blen > iov->iov_len - offset)
+        {
+          blen = iov->iov_len - offset;
+        }
+
+      memcpy(buf, (FAR const uint8_t *)iov->iov_base + offset, blen);
+
+      len -= blen;
+      buf = (uint8_t *)buf + blen;
+      iov++;
+      offset = 0;
+    }
+}

fs/vfs/fs_write.c

@@ -51,7 +51,7 @@
  ****************************************************************************/
 
 static ssize_t file_writev_compat(FAR struct file *filep,
-                                  FAR const struct uio *uio)
+                                  FAR struct uio *uio)
 {
   FAR const struct iovec *iov = uio->uio_iov;
   int iovcnt = uio->uio_iovcnt;
@@ -102,6 +102,11 @@ static ssize_t file_writev_compat(FAR struct file *filep,
       remaining -= nwritten;
     }
 
+  if (ntotal >= 0)
+    {
+      uio_advance(uio, ntotal);
+    }
+
   return ntotal;
 }
 
@@ -133,7 +138,7 @@ static ssize_t file_writev_compat(FAR struct file *filep,
  *
  ****************************************************************************/
 
-ssize_t file_writev(FAR struct file *filep, FAR const struct uio *uio)
+ssize_t file_writev(FAR struct file *filep, FAR struct uio *uio)
 {
   FAR struct inode *inode;
   ssize_t ret = -EBADF;
@@ -202,11 +207,16 @@ ssize_t file_write(FAR struct file *filep, FAR const void *buf,
 {
   struct iovec iov;
   struct uio uio;
+  ssize_t ret;
 
   iov.iov_base = (FAR void *)buf;
   iov.iov_len = nbytes;
-  uio.uio_iov = &iov;
-  uio.uio_iovcnt = 1;
+  ret = uio_init(&uio, &iov, 1);
+  if (ret != 0)
+    {
+      return ret;
+    }
+
   return file_writev(filep, &uio);
 }
 
@@ -252,9 +262,12 @@ ssize_t nx_writev(int fd, FAR const struct iovec *iov, int iovcnt)
        * index.  Note that file_writev() will return the errno on failure.
        */
 
-      uio.uio_iov = iov;
-      uio.uio_iovcnt = iovcnt;
-      ret = file_writev(filep, &uio);
+      ret = uio_init(&uio, iov, iovcnt);
+      if (ret == 0)
+        {
+          ret = file_writev(filep, &uio);
+        }
+
       fs_putfilep(filep);
     }