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.