i2c: add extra check to safe DMA buffer helper

Make sure we report 'no buffer' for 0-length messages. This can only
happen if threshold is set to 0 which is kind of bogus but we should
still handle this situation. Update the docs and add a debug message
to educate callers of this function.
Reported-by: Hsin-Yi Wang <hsinyi@chromium.org>
Fixes: e94bc5d1 ("i2c: add helpers to ease DMA handling")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/i2c-core-base.c

@@ -2258,7 +2258,8 @@ EXPORT_SYMBOL(i2c_put_adapter);
 /**
  * i2c_get_dma_safe_msg_buf() - get a DMA safe buffer for the given i2c_msg
  * @msg: the message to be checked
- * @threshold: the minimum number of bytes for which using DMA makes sense
+ * @threshold: the minimum number of bytes for which using DMA makes sense.
+ *	       Should at least be 1.
  *
  * Return: NULL if a DMA safe buffer was not obtained. Use msg->buf with PIO.
  *	   Or a valid pointer to be used with DMA. After use, release it by
@@ -2268,7 +2269,11 @@ EXPORT_SYMBOL(i2c_put_adapter);
  */
 u8 *i2c_get_dma_safe_msg_buf(struct i2c_msg *msg, unsigned int threshold)
 {
-	if (msg->len < threshold)
+	/* also skip 0-length msgs for bogus thresholds of 0 */
+	if (!threshold)
+		pr_debug("DMA buffer for addr=0x%02x with length 0 is bogus\n",
+			 msg->addr);
+	if (msg->len < threshold || msg->len == 0)
 		return NULL;
 
 	if (msg->flags & I2C_M_DMA_SAFE)