openamp: change rx/tx buffer hold flag to count

Before this commit, if rpmsg_hold_rx_buffer() is called in the endpoint
callback to hold current rx buffer and call rpmsg_release_rx_buffer()
to release this buffer immediately, this rx buffer will be returned to
the virtqueue twice:
1. the first time is in rpmsg_virtio_release_rx_buffer()
2. and the second time is in rpmsg_virtio_return_buffer() after ept->cb()
Follow shows this process:

rpmsg_virtio_rx_callback()
  - get rp_hdr
  - ept->cb(data = RPMSG_LOCATE_DATA(rp_hdr))
    - rpsmg_hold_rx_buffer(data)
    - rpmsg_release_rx_buffer(data) return buffer to virtqueue
  - rpmsg_virtio_return_buffer(data) return same buffer to virtqueue again

So this commit changes the HELD flag in rp_hdr to count to avoid this
use case and also supports hold/release rx buffer recursively.

Signed-off-by: Guiding Li <liguiding1@xiaomi.com>
Signed-off-by: Yin Tao <yintao@xiaomi.com>
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

lib/rpmsg/rpmsg_internal.h

@@ -28,7 +28,9 @@ extern "C" {
 #define RPMSG_ASSERT(_exp, _msg) metal_assert(_exp)
 #endif
 
-#define RPMSG_BUF_HELD (1U << 31) /* Flag to suggest to hold the buffer */
+/* Mask to get the rpmsg buffer held counter from rpmsg_hdr reserved field */
+#define RPMSG_BUF_HELD_SHIFT 16
+#define RPMSG_BUF_HELD_MASK  (0xFFFFU << RPMSG_BUF_HELD_SHIFT)
 
 #define RPMSG_LOCATE_HDR(p) \
 	((struct rpmsg_hdr *)((unsigned char *)(p) - sizeof(struct rpmsg_hdr)))

lib/rpmsg/rpmsg_virtio.c

@@ -24,6 +24,25 @@
 /* Time to wait - In multiple of 1 msecs. */
 #define RPMSG_TICKS_PER_INTERVAL                1000
 
+/*
+ * Get the buffer held counter value.
+ * If 0 the buffer can be released
+ */
+#define RPMSG_BUF_HELD_COUNTER(rp_hdr)          \
+	(((rp_hdr)->reserved & RPMSG_BUF_HELD_MASK) >> RPMSG_BUF_HELD_SHIFT)
+
+/* Increase buffer held counter */
+#define RPMSG_BUF_HELD_INC(rp_hdr)              \
+	((rp_hdr)->reserved += 1 << RPMSG_BUF_HELD_SHIFT)
+
+/* Decrease buffer held counter */
+#define RPMSG_BUF_HELD_DEC(rp_hdr)              \
+	((rp_hdr)->reserved -= 1 << RPMSG_BUF_HELD_SHIFT)
+
+/* Get the buffer index */
+#define RPMSG_BUF_INDEX(rphdr)                  \
+	((uint16_t)((rp_hdr)->reserved & ~RPMSG_BUF_HELD_MASK))
+
 /**
  * struct vbuff_reclaimer_t - vring buffer recycler
  *
@@ -272,37 +291,70 @@ static int rpmsg_virtio_wait_remote_ready(struct rpmsg_virtio_device *rvdev)
 }
 #endif /*!VIRTIO_DRIVER_ONLY*/
 
+/**
+ * @internal
+ *
+ * @brief Check whether rpmsg buffer needs to be released or not
+ *
+ * @param rp_hdr	Pointer to rpmsg buffer header
+ *
+ * @return true indicates this buffer needs to be released
+ */
+static bool rpmsg_virtio_buf_held_dec_test(struct rpmsg_hdr *rp_hdr)
+{
+	/* Check the held counter first */
+	if (RPMSG_BUF_HELD_COUNTER(rp_hdr) <= 0) {
+		metal_err("unexpected buffer held counter\r\n");
+		return false;
+	}
+
+	/* Decrease the held counter */
+	RPMSG_BUF_HELD_DEC(rp_hdr);
+
+	/* Check whether to release the buffer */
+	if (RPMSG_BUF_HELD_COUNTER(rp_hdr) > 0)
+		return false;
+
+	return true;
+}
+
 static void rpmsg_virtio_hold_rx_buffer(struct rpmsg_device *rdev, void *rxbuf)
 {
-	struct rpmsg_hdr *rp_hdr;
+	metal_mutex_acquire(&rdev->lock);
+	RPMSG_BUF_HELD_INC(RPMSG_LOCATE_HDR(rxbuf));
+	metal_mutex_release(&rdev->lock);
+}
 
-	(void)rdev;
+static bool rpmsg_virtio_release_rx_buffer_nolock(struct rpmsg_virtio_device *rvdev,
+						  struct rpmsg_hdr *rp_hdr)
+{
+	uint16_t idx;
+	uint32_t len;
 
-	rp_hdr = RPMSG_LOCATE_HDR(rxbuf);
+	/* The reserved field contains buffer index */
+	idx = RPMSG_BUF_INDEX(rp_hdr);
+	/* Return buffer on virtqueue. */
+	len = virtqueue_get_buffer_length(rvdev->rvq, idx);
+	rpmsg_virtio_return_buffer(rvdev, rp_hdr, len, idx);
 
-	/* Set held status to keep buffer */
-	rp_hdr->reserved |= RPMSG_BUF_HELD;
+	return true;
 }
 
 static void rpmsg_virtio_release_rx_buffer(struct rpmsg_device *rdev,
 					   void *rxbuf)
 {
 	struct rpmsg_virtio_device *rvdev;
 	struct rpmsg_hdr *rp_hdr;
-	uint16_t idx;
-	uint32_t len;
 
 	rvdev = metal_container_of(rdev, struct rpmsg_virtio_device, rdev);
 	rp_hdr = RPMSG_LOCATE_HDR(rxbuf);
-	/* The reserved field contains buffer index */
-	idx = (uint16_t)(rp_hdr->reserved & ~RPMSG_BUF_HELD);
 
 	metal_mutex_acquire(&rdev->lock);
-	/* Return buffer on virtqueue. */
-	len = virtqueue_get_buffer_length(rvdev->rvq, idx);
-	rpmsg_virtio_return_buffer(rvdev, rp_hdr, len, idx);
-	/* Tell peer we return some rx buffers */
-	virtqueue_kick(rvdev->rvq);
+	if (rpmsg_virtio_buf_held_dec_test(rp_hdr)) {
+		rpmsg_virtio_release_rx_buffer_nolock(rvdev, rp_hdr);
+		/* Tell peer we return some rx buffers */
+		virtqueue_kick(rvdev->rvq);
+	}
 	metal_mutex_release(&rdev->lock);
 }
 
@@ -361,6 +413,9 @@ static void *rpmsg_virtio_get_tx_payload_buffer(struct rpmsg_device *rdev,
 	/* Store the index into the reserved field to be used when sending */
 	rp_hdr->reserved = idx;
 
+	/* Increase the held counter to hold this Tx buffer */
+	RPMSG_BUF_HELD_INC(rp_hdr);
+
 	/* Actual data buffer size is vring buffer size minus header length */
 	*len -= sizeof(struct rpmsg_hdr);
 	return RPMSG_LOCATE_DATA(rp_hdr);
@@ -424,20 +479,20 @@ static int rpmsg_virtio_release_tx_buffer(struct rpmsg_device *rdev, void *txbuf
 	struct rpmsg_hdr *rp_hdr = RPMSG_LOCATE_HDR(txbuf);
 	void *vbuff = rp_hdr;  /* only used to avoid warning on the cast of a packed structure */
 	struct vbuff_reclaimer_t *r_desc = (struct vbuff_reclaimer_t *)vbuff;
-	uint16_t idx;
-
-	/*
-	 * Reuse the RPMsg buffer to temporary store the vbuff_reclaimer_t structure.
-	 * Stores the index locally before overwriting the RPMsg header.
-	 */
-	idx = rp_hdr->reserved;
 
 	rvdev = metal_container_of(rdev, struct rpmsg_virtio_device, rdev);
 
 	metal_mutex_acquire(&rdev->lock);
 
-	r_desc->idx = idx;
-	metal_list_add_tail(&rvdev->reclaimer, &r_desc->node);
+	/* Check whether to release the Tx buffer */
+	if (rpmsg_virtio_buf_held_dec_test(rp_hdr)) {
+		/*
+		 * Reuse the RPMsg buffer to temporary store the vbuff_reclaimer_t structure.
+		 * Store the index locally before overwriting the RPMsg header.
+		 */
+		r_desc->idx = RPMSG_BUF_INDEX(rp_hdr);
+		metal_list_add_tail(&rvdev->reclaimer, &r_desc->node);
+	}
 
 	metal_mutex_release(&rdev->lock);
 
@@ -534,6 +589,7 @@ static void rpmsg_virtio_rx_callback(struct virtqueue *vq)
 		metal_mutex_acquire(&rdev->lock);
 		ept = rpmsg_get_ept_from_addr(rdev, rp_hdr->dst);
 		rpmsg_ept_incref(ept);
+		RPMSG_BUF_HELD_INC(rp_hdr);
 		metal_mutex_release(&rdev->lock);
 
 		if (ept) {
@@ -553,12 +609,8 @@ static void rpmsg_virtio_rx_callback(struct virtqueue *vq)
 
 		metal_mutex_acquire(&rdev->lock);
 		rpmsg_ept_decref(ept);
-
-		/* Check whether callback wants to hold buffer */
-		if (!(rp_hdr->reserved & RPMSG_BUF_HELD)) {
-			/* No, return used buffers. */
-			rpmsg_virtio_return_buffer(rvdev, rp_hdr, len, idx);
-		}
+		if (rpmsg_virtio_buf_held_dec_test(rp_hdr))
+			rpmsg_virtio_release_rx_buffer_nolock(rvdev, rp_hdr);
 
 		rp_hdr = rpmsg_virtio_get_rx_buffer(rvdev, &len, &idx);
 		if (!rp_hdr) {