1 Remote Processor Messaging (rpmsg) Framework
3 Note: this document describes the rpmsg bus and how to write rpmsg drivers.
4 To learn how to add rpmsg support for new platforms, check out remoteproc.txt
5 (also a resident of Documentation/).
9 Modern SoCs typically employ heterogeneous remote processor devices in
10 asymmetric multiprocessing (AMP) configurations, which may be running
11 different instances of operating system, whether it's Linux or any other
12 flavor of real-time OS.
14 OMAP4, for example, has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP.
15 Typically, the dual cortex-A9 is running Linux in a SMP configuration,
16 and each of the other three cores (two M3 cores and a DSP) is running
17 its own instance of RTOS in an AMP configuration.
19 Typically AMP remote processors employ dedicated DSP codecs and multimedia
20 hardware accelerators, and therefore are often used to offload CPU-intensive
21 multimedia tasks from the main application processor.
23 These remote processors could also be used to control latency-sensitive
24 sensors, drive random hardware blocks, or just perform background tasks
25 while the main CPU is idling.
27 Users of those remote processors can either be userland apps (e.g. multimedia
28 frameworks talking with remote OMX components) or kernel drivers (controlling
29 hardware accessible only by the remote processor, reserving kernel-controlled
30 resources on behalf of the remote processor, etc..).
32 Rpmsg is a virtio-based messaging bus that allows kernel drivers to communicate
33 with remote processors available on the system. In turn, drivers could then
34 expose appropriate user space interfaces, if needed.
36 When writing a driver that exposes rpmsg communication to userland, please
37 keep in mind that remote processors might have direct access to the
38 system's physical memory and other sensitive hardware resources (e.g. on
39 OMAP4, remote cores and hardware accelerators may have direct access to the
40 physical memory, gpio banks, dma controllers, i2c bus, gptimers, mailbox
41 devices, hwspinlocks, etc..). Moreover, those remote processors might be
42 running RTOS where every task can access the entire memory/devices exposed
43 to the processor. To minimize the risks of rogue (or buggy) userland code
44 exploiting remote bugs, and by that taking over the system, it is often
45 desired to limit userland to specific rpmsg channels (see definition below)
46 it can send messages on, and if possible, minimize how much control
47 it has over the content of the messages.
49 Every rpmsg device is a communication channel with a remote processor (thus
50 rpmsg devices are called channels). Channels are identified by a textual name
51 and have a local ("source") rpmsg address, and remote ("destination") rpmsg
54 When a driver starts listening on a channel, its rx callback is bound with
55 a unique rpmsg local address (a 32-bit integer). This way when inbound messages
56 arrive, the rpmsg core dispatches them to the appropriate driver according
57 to their destination address (this is done by invoking the driver's rx handler
58 with the payload of the inbound message).
63 int rpmsg_send(struct rpmsg_channel *rpdev, void *data, int len);
64 - sends a message across to the remote processor on a given channel.
65 The caller should specify the channel, the data it wants to send,
66 and its length (in bytes). The message will be sent on the specified
67 channel, i.e. its source and destination address fields will be
68 set to the channel's src and dst addresses.
70 In case there are no TX buffers available, the function will block until
71 one becomes available (i.e. until the remote processor consumes
72 a tx buffer and puts it back on virtio's used descriptor ring),
73 or a timeout of 15 seconds elapses. When the latter happens,
74 -ERESTARTSYS is returned.
75 The function can only be called from a process context (for now).
76 Returns 0 on success and an appropriate error value on failure.
78 int rpmsg_sendto(struct rpmsg_channel *rpdev, void *data, int len, u32 dst);
79 - sends a message across to the remote processor on a given channel,
80 to a destination address provided by the caller.
81 The caller should specify the channel, the data it wants to send,
82 its length (in bytes), and an explicit destination address.
83 The message will then be sent to the remote processor to which the
84 channel belongs, using the channel's src address, and the user-provided
85 dst address (thus the channel's dst address will be ignored).
87 In case there are no TX buffers available, the function will block until
88 one becomes available (i.e. until the remote processor consumes
89 a tx buffer and puts it back on virtio's used descriptor ring),
90 or a timeout of 15 seconds elapses. When the latter happens,
91 -ERESTARTSYS is returned.
92 The function can only be called from a process context (for now).
93 Returns 0 on success and an appropriate error value on failure.
95 int rpmsg_send_offchannel(struct rpmsg_channel *rpdev, u32 src, u32 dst,
97 - sends a message across to the remote processor, using the src and dst
98 addresses provided by the user.
99 The caller should specify the channel, the data it wants to send,
100 its length (in bytes), and explicit source and destination addresses.
101 The message will then be sent to the remote processor to which the
102 channel belongs, but the channel's src and dst addresses will be
103 ignored (and the user-provided addresses will be used instead).
105 In case there are no TX buffers available, the function will block until
106 one becomes available (i.e. until the remote processor consumes
107 a tx buffer and puts it back on virtio's used descriptor ring),
108 or a timeout of 15 seconds elapses. When the latter happens,
109 -ERESTARTSYS is returned.
110 The function can only be called from a process context (for now).
111 Returns 0 on success and an appropriate error value on failure.
113 int rpmsg_trysend(struct rpmsg_channel *rpdev, void *data, int len);
114 - sends a message across to the remote processor on a given channel.
115 The caller should specify the channel, the data it wants to send,
116 and its length (in bytes). The message will be sent on the specified
117 channel, i.e. its source and destination address fields will be
118 set to the channel's src and dst addresses.
120 In case there are no TX buffers available, the function will immediately
121 return -ENOMEM without waiting until one becomes available.
122 The function can only be called from a process context (for now).
123 Returns 0 on success and an appropriate error value on failure.
125 int rpmsg_trysendto(struct rpmsg_channel *rpdev, void *data, int len, u32 dst)
126 - sends a message across to the remote processor on a given channel,
127 to a destination address provided by the user.
128 The user should specify the channel, the data it wants to send,
129 its length (in bytes), and an explicit destination address.
130 The message will then be sent to the remote processor to which the
131 channel belongs, using the channel's src address, and the user-provided
132 dst address (thus the channel's dst address will be ignored).
134 In case there are no TX buffers available, the function will immediately
135 return -ENOMEM without waiting until one becomes available.
136 The function can only be called from a process context (for now).
137 Returns 0 on success and an appropriate error value on failure.
139 int rpmsg_trysend_offchannel(struct rpmsg_channel *rpdev, u32 src, u32 dst,
140 void *data, int len);
141 - sends a message across to the remote processor, using source and
142 destination addresses provided by the user.
143 The user should specify the channel, the data it wants to send,
144 its length (in bytes), and explicit source and destination addresses.
145 The message will then be sent to the remote processor to which the
146 channel belongs, but the channel's src and dst addresses will be
147 ignored (and the user-provided addresses will be used instead).
149 In case there are no TX buffers available, the function will immediately
150 return -ENOMEM without waiting until one becomes available.
151 The function can only be called from a process context (for now).
152 Returns 0 on success and an appropriate error value on failure.
154 struct rpmsg_endpoint *rpmsg_create_ept(struct rpmsg_channel *rpdev,
155 void (*cb)(struct rpmsg_channel *, void *, int, void *, u32),
156 void *priv, u32 addr);
157 - every rpmsg address in the system is bound to an rx callback (so when
158 inbound messages arrive, they are dispatched by the rpmsg bus using the
159 appropriate callback handler) by means of an rpmsg_endpoint struct.
161 This function allows drivers to create such an endpoint, and by that,
162 bind a callback, and possibly some private data too, to an rpmsg address
163 (either one that is known in advance, or one that will be dynamically
166 Simple rpmsg drivers need not call rpmsg_create_ept, because an endpoint
167 is already created for them when they are probed by the rpmsg bus
168 (using the rx callback they provide when they registered to the rpmsg bus).
170 So things should just work for simple drivers: they already have an
171 endpoint, their rx callback is bound to their rpmsg address, and when
172 relevant inbound messages arrive (i.e. messages which their dst address
173 equals to the src address of their rpmsg channel), the driver's handler
174 is invoked to process it.
176 That said, more complicated drivers might do need to allocate
177 additional rpmsg addresses, and bind them to different rx callbacks.
178 To accomplish that, those drivers need to call this function.
179 Drivers should provide their channel (so the new endpoint would bind
180 to the same remote processor their channel belongs to), an rx callback
181 function, an optional private data (which is provided back when the
182 rx callback is invoked), and an address they want to bind with the
183 callback. If addr is RPMSG_ADDR_ANY, then rpmsg_create_ept will
184 dynamically assign them an available rpmsg address (drivers should have
185 a very good reason why not to always use RPMSG_ADDR_ANY here).
187 Returns a pointer to the endpoint on success, or NULL on error.
189 void rpmsg_destroy_ept(struct rpmsg_endpoint *ept);
190 - destroys an existing rpmsg endpoint. user should provide a pointer
191 to an rpmsg endpoint that was previously created with rpmsg_create_ept().
193 int register_rpmsg_driver(struct rpmsg_driver *rpdrv);
194 - registers an rpmsg driver with the rpmsg bus. user should provide
195 a pointer to an rpmsg_driver struct, which contains the driver's
196 ->probe() and ->remove() functions, an rx callback, and an id_table
197 specifying the names of the channels this driver is interested to
200 void unregister_rpmsg_driver(struct rpmsg_driver *rpdrv);
201 - unregisters an rpmsg driver from the rpmsg bus. user should provide
202 a pointer to a previously-registered rpmsg_driver struct.
203 Returns 0 on success, and an appropriate error value on failure.
208 The following is a simple rpmsg driver, that sends an "hello!" message
209 on probe(), and whenever it receives an incoming message, it dumps its
210 content to the console.
212 #include <linux/kernel.h>
213 #include <linux/module.h>
214 #include <linux/rpmsg.h>
216 static void rpmsg_sample_cb(struct rpmsg_channel *rpdev, void *data, int len,
219 print_hex_dump(KERN_INFO, "incoming message:", DUMP_PREFIX_NONE,
220 16, 1, data, len, true);
223 static int rpmsg_sample_probe(struct rpmsg_channel *rpdev)
227 dev_info(&rpdev->dev, "chnl: 0x%x -> 0x%x\n", rpdev->src, rpdev->dst);
229 /* send a message on our channel */
230 err = rpmsg_send(rpdev, "hello!", 6);
232 pr_err("rpmsg_send failed: %d\n", err);
239 static void __devexit rpmsg_sample_remove(struct rpmsg_channel *rpdev)
241 dev_info(&rpdev->dev, "rpmsg sample client driver is removed\n");
244 static struct rpmsg_device_id rpmsg_driver_sample_id_table[] = {
245 { .name = "rpmsg-client-sample" },
248 MODULE_DEVICE_TABLE(rpmsg, rpmsg_driver_sample_id_table);
250 static struct rpmsg_driver rpmsg_sample_client = {
251 .drv.name = KBUILD_MODNAME,
252 .drv.owner = THIS_MODULE,
253 .id_table = rpmsg_driver_sample_id_table,
254 .probe = rpmsg_sample_probe,
255 .callback = rpmsg_sample_cb,
256 .remove = __devexit_p(rpmsg_sample_remove),
259 static int __init init(void)
261 return register_rpmsg_driver(&rpmsg_sample_client);
265 static void __exit fini(void)
267 unregister_rpmsg_driver(&rpmsg_sample_client);
271 Note: a similar sample which can be built and loaded can be found
274 4. Allocations of rpmsg channels:
276 At this point we only support dynamic allocations of rpmsg channels.
278 This is possible only with remote processors that have the VIRTIO_RPMSG_F_NS
279 virtio device feature set. This feature bit means that the remote
280 processor supports dynamic name service announcement messages.
282 When this feature is enabled, creation of rpmsg devices (i.e. channels)
283 is completely dynamic: the remote processor announces the existence of a
284 remote rpmsg service by sending a name service message (which contains
285 the name and rpmsg addr of the remote service, see struct rpmsg_ns_msg).
287 This message is then handled by the rpmsg bus, which in turn dynamically
288 creates and registers an rpmsg channel (which represents the remote service).
289 If/when a relevant rpmsg driver is registered, it will be immediately probed
290 by the bus, and can then start sending messages to the remote service.
292 The plan is also to add static creation of rpmsg channels via the virtio
293 config space, but it's not implemented yet.