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diff --git a/kernel/Documentation/rpmsg.txt b/kernel/Documentation/rpmsg.txt new file mode 100644 index 000000000..f7edc3aa1 --- /dev/null +++ b/kernel/Documentation/rpmsg.txt @@ -0,0 +1,293 @@ +Remote Processor Messaging (rpmsg) Framework + +Note: this document describes the rpmsg bus and how to write rpmsg drivers. +To learn how to add rpmsg support for new platforms, check out remoteproc.txt +(also a resident of Documentation/). + +1. Introduction + +Modern SoCs typically employ heterogeneous remote processor devices in +asymmetric multiprocessing (AMP) configurations, which may be running +different instances of operating system, whether it's Linux or any other +flavor of real-time OS. + +OMAP4, for example, has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP. +Typically, the dual cortex-A9 is running Linux in a SMP configuration, +and each of the other three cores (two M3 cores and a DSP) is running +its own instance of RTOS in an AMP configuration. + +Typically AMP remote processors employ dedicated DSP codecs and multimedia +hardware accelerators, and therefore are often used to offload CPU-intensive +multimedia tasks from the main application processor. + +These remote processors could also be used to control latency-sensitive +sensors, drive random hardware blocks, or just perform background tasks +while the main CPU is idling. + +Users of those remote processors can either be userland apps (e.g. multimedia +frameworks talking with remote OMX components) or kernel drivers (controlling +hardware accessible only by the remote processor, reserving kernel-controlled +resources on behalf of the remote processor, etc..). + +Rpmsg is a virtio-based messaging bus that allows kernel drivers to communicate +with remote processors available on the system. In turn, drivers could then +expose appropriate user space interfaces, if needed. + +When writing a driver that exposes rpmsg communication to userland, please +keep in mind that remote processors might have direct access to the +system's physical memory and other sensitive hardware resources (e.g. on +OMAP4, remote cores and hardware accelerators may have direct access to the +physical memory, gpio banks, dma controllers, i2c bus, gptimers, mailbox +devices, hwspinlocks, etc..). Moreover, those remote processors might be +running RTOS where every task can access the entire memory/devices exposed +to the processor. To minimize the risks of rogue (or buggy) userland code +exploiting remote bugs, and by that taking over the system, it is often +desired to limit userland to specific rpmsg channels (see definition below) +it can send messages on, and if possible, minimize how much control +it has over the content of the messages. + +Every rpmsg device is a communication channel with a remote processor (thus +rpmsg devices are called channels). Channels are identified by a textual name +and have a local ("source") rpmsg address, and remote ("destination") rpmsg +address. + +When a driver starts listening on a channel, its rx callback is bound with +a unique rpmsg local address (a 32-bit integer). This way when inbound messages +arrive, the rpmsg core dispatches them to the appropriate driver according +to their destination address (this is done by invoking the driver's rx handler +with the payload of the inbound message). + + +2. User API + + int rpmsg_send(struct rpmsg_channel *rpdev, void *data, int len); + - sends a message across to the remote processor on a given channel. + The caller should specify the channel, the data it wants to send, + and its length (in bytes). The message will be sent on the specified + channel, i.e. its source and destination address fields will be + set to the channel's src and dst addresses. + + In case there are no TX buffers available, the function will block until + one becomes available (i.e. until the remote processor consumes + a tx buffer and puts it back on virtio's used descriptor ring), + or a timeout of 15 seconds elapses. When the latter happens, + -ERESTARTSYS is returned. + The function can only be called from a process context (for now). + Returns 0 on success and an appropriate error value on failure. + + int rpmsg_sendto(struct rpmsg_channel *rpdev, void *data, int len, u32 dst); + - sends a message across to the remote processor on a given channel, + to a destination address provided by the caller. + The caller should specify the channel, the data it wants to send, + its length (in bytes), and an explicit destination address. + The message will then be sent to the remote processor to which the + channel belongs, using the channel's src address, and the user-provided + dst address (thus the channel's dst address will be ignored). + + In case there are no TX buffers available, the function will block until + one becomes available (i.e. until the remote processor consumes + a tx buffer and puts it back on virtio's used descriptor ring), + or a timeout of 15 seconds elapses. When the latter happens, + -ERESTARTSYS is returned. + The function can only be called from a process context (for now). + Returns 0 on success and an appropriate error value on failure. + + int rpmsg_send_offchannel(struct rpmsg_channel *rpdev, u32 src, u32 dst, + void *data, int len); + - sends a message across to the remote processor, using the src and dst + addresses provided by the user. + The caller should specify the channel, the data it wants to send, + its length (in bytes), and explicit source and destination addresses. + The message will then be sent to the remote processor to which the + channel belongs, but the channel's src and dst addresses will be + ignored (and the user-provided addresses will be used instead). + + In case there are no TX buffers available, the function will block until + one becomes available (i.e. until the remote processor consumes + a tx buffer and puts it back on virtio's used descriptor ring), + or a timeout of 15 seconds elapses. When the latter happens, + -ERESTARTSYS is returned. + The function can only be called from a process context (for now). + Returns 0 on success and an appropriate error value on failure. + + int rpmsg_trysend(struct rpmsg_channel *rpdev, void *data, int len); + - sends a message across to the remote processor on a given channel. + The caller should specify the channel, the data it wants to send, + and its length (in bytes). The message will be sent on the specified + channel, i.e. its source and destination address fields will be + set to the channel's src and dst addresses. + + In case there are no TX buffers available, the function will immediately + return -ENOMEM without waiting until one becomes available. + The function can only be called from a process context (for now). + Returns 0 on success and an appropriate error value on failure. + + int rpmsg_trysendto(struct rpmsg_channel *rpdev, void *data, int len, u32 dst) + - sends a message across to the remote processor on a given channel, + to a destination address provided by the user. + The user should specify the channel, the data it wants to send, + its length (in bytes), and an explicit destination address. + The message will then be sent to the remote processor to which the + channel belongs, using the channel's src address, and the user-provided + dst address (thus the channel's dst address will be ignored). + + In case there are no TX buffers available, the function will immediately + return -ENOMEM without waiting until one becomes available. + The function can only be called from a process context (for now). + Returns 0 on success and an appropriate error value on failure. + + int rpmsg_trysend_offchannel(struct rpmsg_channel *rpdev, u32 src, u32 dst, + void *data, int len); + - sends a message across to the remote processor, using source and + destination addresses provided by the user. + The user should specify the channel, the data it wants to send, + its length (in bytes), and explicit source and destination addresses. + The message will then be sent to the remote processor to which the + channel belongs, but the channel's src and dst addresses will be + ignored (and the user-provided addresses will be used instead). + + In case there are no TX buffers available, the function will immediately + return -ENOMEM without waiting until one becomes available. + The function can only be called from a process context (for now). + Returns 0 on success and an appropriate error value on failure. + + struct rpmsg_endpoint *rpmsg_create_ept(struct rpmsg_channel *rpdev, + void (*cb)(struct rpmsg_channel *, void *, int, void *, u32), + void *priv, u32 addr); + - every rpmsg address in the system is bound to an rx callback (so when + inbound messages arrive, they are dispatched by the rpmsg bus using the + appropriate callback handler) by means of an rpmsg_endpoint struct. + + This function allows drivers to create such an endpoint, and by that, + bind a callback, and possibly some private data too, to an rpmsg address + (either one that is known in advance, or one that will be dynamically + assigned for them). + + Simple rpmsg drivers need not call rpmsg_create_ept, because an endpoint + is already created for them when they are probed by the rpmsg bus + (using the rx callback they provide when they registered to the rpmsg bus). + + So things should just work for simple drivers: they already have an + endpoint, their rx callback is bound to their rpmsg address, and when + relevant inbound messages arrive (i.e. messages which their dst address + equals to the src address of their rpmsg channel), the driver's handler + is invoked to process it. + + That said, more complicated drivers might do need to allocate + additional rpmsg addresses, and bind them to different rx callbacks. + To accomplish that, those drivers need to call this function. + Drivers should provide their channel (so the new endpoint would bind + to the same remote processor their channel belongs to), an rx callback + function, an optional private data (which is provided back when the + rx callback is invoked), and an address they want to bind with the + callback. If addr is RPMSG_ADDR_ANY, then rpmsg_create_ept will + dynamically assign them an available rpmsg address (drivers should have + a very good reason why not to always use RPMSG_ADDR_ANY here). + + Returns a pointer to the endpoint on success, or NULL on error. + + void rpmsg_destroy_ept(struct rpmsg_endpoint *ept); + - destroys an existing rpmsg endpoint. user should provide a pointer + to an rpmsg endpoint that was previously created with rpmsg_create_ept(). + + int register_rpmsg_driver(struct rpmsg_driver *rpdrv); + - registers an rpmsg driver with the rpmsg bus. user should provide + a pointer to an rpmsg_driver struct, which contains the driver's + ->probe() and ->remove() functions, an rx callback, and an id_table + specifying the names of the channels this driver is interested to + be probed with. + + void unregister_rpmsg_driver(struct rpmsg_driver *rpdrv); + - unregisters an rpmsg driver from the rpmsg bus. user should provide + a pointer to a previously-registered rpmsg_driver struct. + Returns 0 on success, and an appropriate error value on failure. + + +3. Typical usage + +The following is a simple rpmsg driver, that sends an "hello!" message +on probe(), and whenever it receives an incoming message, it dumps its +content to the console. + +#include <linux/kernel.h> +#include <linux/module.h> +#include <linux/rpmsg.h> + +static void rpmsg_sample_cb(struct rpmsg_channel *rpdev, void *data, int len, + void *priv, u32 src) +{ + print_hex_dump(KERN_INFO, "incoming message:", DUMP_PREFIX_NONE, + 16, 1, data, len, true); +} + +static int rpmsg_sample_probe(struct rpmsg_channel *rpdev) +{ + int err; + + dev_info(&rpdev->dev, "chnl: 0x%x -> 0x%x\n", rpdev->src, rpdev->dst); + + /* send a message on our channel */ + err = rpmsg_send(rpdev, "hello!", 6); + if (err) { + pr_err("rpmsg_send failed: %d\n", err); + return err; + } + + return 0; +} + +static void rpmsg_sample_remove(struct rpmsg_channel *rpdev) +{ + dev_info(&rpdev->dev, "rpmsg sample client driver is removed\n"); +} + +static struct rpmsg_device_id rpmsg_driver_sample_id_table[] = { + { .name = "rpmsg-client-sample" }, + { }, +}; +MODULE_DEVICE_TABLE(rpmsg, rpmsg_driver_sample_id_table); + +static struct rpmsg_driver rpmsg_sample_client = { + .drv.name = KBUILD_MODNAME, + .drv.owner = THIS_MODULE, + .id_table = rpmsg_driver_sample_id_table, + .probe = rpmsg_sample_probe, + .callback = rpmsg_sample_cb, + .remove = rpmsg_sample_remove, +}; + +static int __init init(void) +{ + return register_rpmsg_driver(&rpmsg_sample_client); +} +module_init(init); + +static void __exit fini(void) +{ + unregister_rpmsg_driver(&rpmsg_sample_client); +} +module_exit(fini); + +Note: a similar sample which can be built and loaded can be found +in samples/rpmsg/. + +4. Allocations of rpmsg channels: + +At this point we only support dynamic allocations of rpmsg channels. + +This is possible only with remote processors that have the VIRTIO_RPMSG_F_NS +virtio device feature set. This feature bit means that the remote +processor supports dynamic name service announcement messages. + +When this feature is enabled, creation of rpmsg devices (i.e. channels) +is completely dynamic: the remote processor announces the existence of a +remote rpmsg service by sending a name service message (which contains +the name and rpmsg addr of the remote service, see struct rpmsg_ns_msg). + +This message is then handled by the rpmsg bus, which in turn dynamically +creates and registers an rpmsg channel (which represents the remote service). +If/when a relevant rpmsg driver is registered, it will be immediately probed +by the bus, and can then start sending messages to the remote service. + +The plan is also to add static creation of rpmsg channels via the virtio +config space, but it's not implemented yet. |