staging: axis-fifo: flush RX FIFO on read errors

Flush stale data from the RX FIFO in case of errors, to avoid reading
old data when new packets arrive.

Commit c6e8d85 ("staging: axis-fifo: Remove hardware resets for
user errors") removed full FIFO resets from the read error paths, which
fixed potential TX data losses, but introduced this RX issue.

Fixes: c6e8d85 ("staging: axis-fifo: Remove hardware resets for user errors")
Cc: stable@vger.kernel.org
Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Link: https://lore.kernel.org/r/20250912101322.1282507-2-ovidiu.panait.oss@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ovpanait

drivers/staging/axis-fifo/axis-fifo.c

@@ -227,6 +227,7 @@ static ssize_t axis_fifo_read(struct file *f, char __user *buf,
 	}
 
 	bytes_available = ioread32(fifo->base_addr + XLLF_RLR_OFFSET);
+	words_available = bytes_available / sizeof(u32);
 	if (!bytes_available) {
 		dev_err(fifo->dt_device, "received a packet of length 0\n");
 		ret = -EIO;
@@ -237,7 +238,7 @@ static ssize_t axis_fifo_read(struct file *f, char __user *buf,
 		dev_err(fifo->dt_device, "user read buffer too small (available bytes=%zu user buffer bytes=%zu)\n",
 			bytes_available, len);
 		ret = -EINVAL;
-		goto end_unlock;
+		goto err_flush_rx;
 	}
 
 	if (bytes_available % sizeof(u32)) {
@@ -246,11 +247,9 @@ static ssize_t axis_fifo_read(struct file *f, char __user *buf,
 		 */
 		dev_err(fifo->dt_device, "received a packet that isn't word-aligned\n");
 		ret = -EIO;
-		goto end_unlock;
+		goto err_flush_rx;
 	}
 
-	words_available = bytes_available / sizeof(u32);
-
 	/* read data into an intermediate buffer, copying the contents
 	 * to userspace when the buffer is full
 	 */
@@ -262,18 +261,23 @@ static ssize_t axis_fifo_read(struct file *f, char __user *buf,
 			tmp_buf[i] = ioread32(fifo->base_addr +
 					      XLLF_RDFD_OFFSET);
 		}
+		words_available -= copy;
 
 		if (copy_to_user(buf + copied * sizeof(u32), tmp_buf,
 				 copy * sizeof(u32))) {
 			ret = -EFAULT;
-			goto end_unlock;
+			goto err_flush_rx;
 		}
 
 		copied += copy;
-		words_available -= copy;
 	}
+	mutex_unlock(&fifo->read_lock);
+
+	return bytes_available;
 
-	ret = bytes_available;
+err_flush_rx:
+	while (words_available--)
+		ioread32(fifo->base_addr + XLLF_RDFD_OFFSET);
 
 end_unlock:
 	mutex_unlock(&fifo->read_lock);