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