BACKPORT: rpmsg: core: add API to get MTU

Return the rpmsg buffer MTU for sending message, so rpmsg users
can split a long message in several sub rpmsg buffers.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20211015094701.5732-2-arnaud.pouliquen@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Change-Id: I978024488709916034606d78d00ce935f26e3fbc
(cherry picked from commit e279317e9a)
[Jiahang: Resolving merge conflicts ]
Signed-off-by: Jiahang Zheng <jiahang.zheng@rock-chips.com>

drivers/rpmsg/rpmsg_core.c

@@ -319,6 +319,29 @@ int rpmsg_set_signals(struct rpmsg_endpoint *ept, u32 set, u32 clear)
 }
 EXPORT_SYMBOL(rpmsg_set_signals);
 
+#ifdef CONFIG_NO_GKI
+/**
+ * rpmsg_get_mtu() - get maximum transmission buffer size for sending message.
+ * @ept: the rpmsg endpoint
+ *
+ * This function returns maximum buffer size available for a single outgoing message.
+ *
+ * Return: the maximum transmission size on success and an appropriate error
+ * value on failure.
+ */
+
+ssize_t rpmsg_get_mtu(struct rpmsg_endpoint *ept)
+{
+	if (WARN_ON(!ept))
+		return -EINVAL;
+	if (!ept->ops->get_mtu)
+		return -ENOTSUPP;
+
+	return ept->ops->get_mtu(ept);
+}
+EXPORT_SYMBOL(rpmsg_get_mtu);
+#endif
+
 /*
  * match a rpmsg channel with a channel info struct.
  * this is used to make sure we're not creating rpmsg devices for channels

drivers/rpmsg/rpmsg_internal.h

@@ -50,6 +50,7 @@ struct rpmsg_device_ops {
  * @poll:		see @rpmsg_poll(), optional
  * @get_signals:	see @rpmsg_get_signals(), optional
  * @set_signals:	see @rpmsg_set_signals(), optional
+ * @get_mtu:		see @rpmsg_get_mtu(), optional
  *
  * Indirection table for the operations that a rpmsg backend should implement.
  * In addition to @destroy_ept, the backend must at least implement @send and
@@ -71,6 +72,9 @@ struct rpmsg_endpoint_ops {
 			     poll_table *wait);
 	int (*get_signals)(struct rpmsg_endpoint *ept);
 	int (*set_signals)(struct rpmsg_endpoint *ept, u32 set, u32 clear);
+#ifdef CONFIG_NO_GKI
+	ssize_t (*get_mtu)(struct rpmsg_endpoint *ept);
+#endif
 };
 
 int rpmsg_register_device(struct rpmsg_device *rpdev);

drivers/rpmsg/virtio_rpmsg_bus.c

@@ -181,6 +181,7 @@ static int virtio_rpmsg_trysendto(struct rpmsg_endpoint *ept, void *data,
 				  int len, u32 dst);
 static int virtio_rpmsg_trysend_offchannel(struct rpmsg_endpoint *ept, u32 src,
 					   u32 dst, void *data, int len);
+static ssize_t virtio_rpmsg_get_mtu(struct rpmsg_endpoint *ept);
 
 static const struct rpmsg_endpoint_ops virtio_endpoint_ops = {
 	.destroy_ept = virtio_rpmsg_destroy_ept,
@@ -190,6 +191,7 @@ static const struct rpmsg_endpoint_ops virtio_endpoint_ops = {
 	.trysend = virtio_rpmsg_trysend,
 	.trysendto = virtio_rpmsg_trysendto,
 	.trysend_offchannel = virtio_rpmsg_trysend_offchannel,
+	.get_mtu = virtio_rpmsg_get_mtu,
 };
 
 /**
@@ -705,6 +707,14 @@ static int virtio_rpmsg_trysend_offchannel(struct rpmsg_endpoint *ept, u32 src,
 	return rpmsg_send_offchannel_raw(rpdev, src, dst, data, len, false);
 }
 
+static ssize_t virtio_rpmsg_get_mtu(struct rpmsg_endpoint *ept)
+{
+	struct rpmsg_device *rpdev = ept->rpdev;
+	struct virtio_rpmsg_channel *vch = to_virtio_rpmsg_channel(rpdev);
+
+	return vch->vrp->buf_size - sizeof(struct rpmsg_hdr);
+}
+
 static int rpmsg_recv_single(struct virtproc_info *vrp, struct device *dev,
 			     struct rpmsg_hdr *msg, unsigned int len)
 {

include/linux/rpmsg.h

@@ -145,6 +145,8 @@ __poll_t rpmsg_poll(struct rpmsg_endpoint *ept, struct file *filp,
 int rpmsg_get_signals(struct rpmsg_endpoint *ept);
 int rpmsg_set_signals(struct rpmsg_endpoint *ept, u32 set, u32 clear);
 
+ssize_t rpmsg_get_mtu(struct rpmsg_endpoint *ept);
+
 #else
 
 static inline int register_rpmsg_device(struct rpmsg_device *dev)
@@ -269,6 +271,14 @@ static inline int rpmsg_set_signals(struct rpmsg_endpoint *ept,
 	return -ENXIO;
 }
 
+static inline ssize_t rpmsg_get_mtu(struct rpmsg_endpoint *ept)
+{
+	/* This shouldn't be possible */
+	WARN_ON(1);
+
+	return -ENXIO;
+}
+
 #endif /* IS_ENABLED(CONFIG_RPMSG) */
 
 /* use a macro to avoid include chaining to get THIS_MODULE */