rpmsg: Add rpmsg_release_tx_buffer API

Add an API to be able to release unused TX buffer that will not be
sent.
For instance this API can be called in case of error between the
buffer reservation and the send to the remote side.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>

docs/rpmsg-design.md

@@ -87,6 +87,12 @@ running on two processors.
 			       uint32_t src, uint32_t dst,
 			       const void *data, int len)`
   ```
+
+* Releases unused Tx buffer reserved by rpmsg_get_tx_payload_buffer() function:
+  ```
+  int rpmsg_release_tx_buffer(struct rpmsg_endpoint *ept, void *txbuf)
+  ```
+
 ## RPMsg User Defined Callbacks
 * RPMsg endpoint message received callback:
   ```

lib/include/openamp/rpmsg.h

@@ -42,6 +42,7 @@ extern "C" {
 #define RPMSG_ERR_BUFF_SIZE		(RPMSG_ERROR_BASE - 5)
 #define RPMSG_ERR_INIT			(RPMSG_ERROR_BASE - 6)
 #define RPMSG_ERR_ADDR			(RPMSG_ERROR_BASE - 7)
+#define RPMSG_ERR_PERM			(RPMSG_ERROR_BASE - 8)
 
 struct rpmsg_endpoint;
 struct rpmsg_device;
@@ -87,6 +88,7 @@ struct rpmsg_endpoint {
  * @release_rx_buffer: release RPMsg RX buffer
  * @get_tx_payload_buffer: get RPMsg TX buffer
  * @send_offchannel_nocopy: send RPMsg data without copy
+ * @release_tx_buffer: release RPMsg TX buffer
  */
 struct rpmsg_device_ops {
 	int (*send_offchannel_raw)(struct rpmsg_device *rdev,
@@ -99,6 +101,7 @@ struct rpmsg_device_ops {
 	int (*send_offchannel_nocopy)(struct rpmsg_device *rdev,
 				      uint32_t src, uint32_t dst,
 				       const void *data, int len);
+	int (*release_tx_buffer)(struct rpmsg_device *rdev, void *txbuf);
 };
 
 /**
@@ -334,6 +337,29 @@ void rpmsg_release_rx_buffer(struct rpmsg_endpoint *ept, void *rxbuf);
 void *rpmsg_get_tx_payload_buffer(struct rpmsg_endpoint *ept,
 				  uint32_t *len, int wait);
 
+/**
+ * @brief Releases unused buffer.
+ *
+ * This API can be called at process context when the Tx buffer reserved by
+ * rpmsg_get_tx_payload_buffer needs to be released without been sent to the
+ * remote side.
+ *
+ * Note that the rpmsg virtio is not able to detect if a buffer has already been released.
+ * The user must prevent a double release (e.g. by resetting its buffer pointer to zero after
+ * the release).
+ *
+ * @ept: the rpmsg endpoint
+ * @txbuf: tx buffer with message payload
+ *
+ * @return:
+ *   - RPMSG_SUCCESS on success
+ *   - RPMSG_ERR_PARAM on invalid parameter
+ *   - RPMSG_ERR_PERM if service not implemented
+ *
+ * @see rpmsg_get_tx_payload_buffer
+ */
+int rpmsg_release_tx_buffer(struct rpmsg_endpoint *ept, void *txbuf);
+
 /**
  * rpmsg_send_offchannel_nocopy() - send a message in tx buffer reserved by
  * rpmsg_get_tx_payload_buffer() across to the remote processor.

lib/rpmsg/rpmsg.c

@@ -170,6 +170,21 @@ void rpmsg_release_rx_buffer(struct rpmsg_endpoint *ept, void *rxbuf)
 		rdev->ops.release_rx_buffer(rdev, rxbuf);
 }
 
+int rpmsg_release_tx_buffer(struct rpmsg_endpoint *ept, void *buf)
+{
+	struct rpmsg_device *rdev;
+
+	if (!ept || !ept->rdev || !buf)
+		return RPMSG_ERR_PARAM;
+
+	rdev = ept->rdev;
+
+	if (rdev->ops.release_tx_buffer)
+		return rdev->ops.release_tx_buffer(rdev, buf);
+
+	return RPMSG_ERR_PERM;
+}
+
 void *rpmsg_get_tx_payload_buffer(struct rpmsg_endpoint *ept,
 				  uint32_t *len, int wait)
 {