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Linux/Documentation/rpmsg.txt

  1 Remote Processor Messaging (rpmsg) Framework
  2 
  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/).
  6 
  7 1. Introduction
  8 
  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.
 13 
 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.
 18 
 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.
 22 
 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.
 26 
 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..).
 31 
 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.
 35 
 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.
 48 
 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
 52 address.
 53 
 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).
 59 
 60 
 61 2. User API
 62 
 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.
 69 
 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.
 77 
 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).
 86 
 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.
 94 
 95   int rpmsg_send_offchannel(struct rpmsg_channel *rpdev, u32 src, u32 dst,
 96                                                         void *data, int len);
 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).
104 
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.
112 
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.
119 
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.
124 
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).
133 
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.
138 
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).
148 
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.
153 
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.
160 
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
164      assigned for them).
165 
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).
169 
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.
175 
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).
186 
187      Returns a pointer to the endpoint on success, or NULL on error.
188 
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().
192 
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
198      be probed with.
199 
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.
204 
205 
206 3. Typical usage
207 
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.
211 
212 #include <linux/kernel.h>
213 #include <linux/module.h>
214 #include <linux/rpmsg.h>
215 
216 static void rpmsg_sample_cb(struct rpmsg_channel *rpdev, void *data, int len,
217                                                 void *priv, u32 src)
218 {
219         print_hex_dump(KERN_INFO, "incoming message:", DUMP_PREFIX_NONE,
220                                                 16, 1, data, len, true);
221 }
222 
223 static int rpmsg_sample_probe(struct rpmsg_channel *rpdev)
224 {
225         int err;
226 
227         dev_info(&rpdev->dev, "chnl: 0x%x -> 0x%x\n", rpdev->src, rpdev->dst);
228 
229         /* send a message on our channel */
230         err = rpmsg_send(rpdev, "hello!", 6);
231         if (err) {
232                 pr_err("rpmsg_send failed: %d\n", err);
233                 return err;
234         }
235 
236         return 0;
237 }
238 
239 static void rpmsg_sample_remove(struct rpmsg_channel *rpdev)
240 {
241         dev_info(&rpdev->dev, "rpmsg sample client driver is removed\n");
242 }
243 
244 static struct rpmsg_device_id rpmsg_driver_sample_id_table[] = {
245         { .name = "rpmsg-client-sample" },
246         { },
247 };
248 MODULE_DEVICE_TABLE(rpmsg, rpmsg_driver_sample_id_table);
249 
250 static struct rpmsg_driver rpmsg_sample_client = {
251         .drv.name       = KBUILD_MODNAME,
252         .id_table       = rpmsg_driver_sample_id_table,
253         .probe          = rpmsg_sample_probe,
254         .callback       = rpmsg_sample_cb,
255         .remove         = rpmsg_sample_remove,
256 };
257 module_rpmsg_driver(rpmsg_sample_client);
258 
259 Note: a similar sample which can be built and loaded can be found
260 in samples/rpmsg/.
261 
262 4. Allocations of rpmsg channels:
263 
264 At this point we only support dynamic allocations of rpmsg channels.
265 
266 This is possible only with remote processors that have the VIRTIO_RPMSG_F_NS
267 virtio device feature set. This feature bit means that the remote
268 processor supports dynamic name service announcement messages.
269 
270 When this feature is enabled, creation of rpmsg devices (i.e. channels)
271 is completely dynamic: the remote processor announces the existence of a
272 remote rpmsg service by sending a name service message (which contains
273 the name and rpmsg addr of the remote service, see struct rpmsg_ns_msg).
274 
275 This message is then handled by the rpmsg bus, which in turn dynamically
276 creates and registers an rpmsg channel (which represents the remote service).
277 If/when a relevant rpmsg driver is registered, it will be immediately probed
278 by the bus, and can then start sending messages to the remote service.
279 
280 The plan is also to add static creation of rpmsg channels via the virtio
281 config space, but it's not implemented yet.

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