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diff --git a/Documentation/remoteproc.txt b/Documentation/remoteproc.txt new file mode 100644 index 00000000000..23ff7349ffe --- /dev/null +++ b/Documentation/remoteproc.txt @@ -0,0 +1,324 @@ +Remote Processor Framework + +1. Introduction + +Modern SoCs typically have 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. +In a typical configuration, 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. + +The remoteproc framework allows different platforms/architectures to +control (power on, load firmware, power off) those remote processors while +abstracting the hardware differences, so the entire driver doesn't need to be +duplicated. In addition, this framework also adds rpmsg virtio devices +for remote processors that supports this kind of communication. This way, +platform-specific remoteproc drivers only need to provide a few low-level +handlers, and then all rpmsg drivers will then just work +(for more information about the virtio-based rpmsg bus and its drivers, +please read Documentation/rpmsg.txt). + +2. User API + + int rproc_boot(struct rproc *rproc) + - Boot a remote processor (i.e. load its firmware, power it on, ...). + If the remote processor is already powered on, this function immediately + returns (successfully). + Returns 0 on success, and an appropriate error value otherwise. + Note: to use this function you should already have a valid rproc + handle. There are several ways to achieve that cleanly (devres, pdata, + the way remoteproc_rpmsg.c does this, or, if this becomes prevalent, we + might also consider using dev_archdata for this). See also + rproc_get_by_name() below. + + void rproc_shutdown(struct rproc *rproc) + - Power off a remote processor (previously booted with rproc_boot()). + In case @rproc is still being used by an additional user(s), then + this function will just decrement the power refcount and exit, + without really powering off the device. + Every call to rproc_boot() must (eventually) be accompanied by a call + to rproc_shutdown(). Calling rproc_shutdown() redundantly is a bug. + Notes: + - we're not decrementing the rproc's refcount, only the power refcount. + which means that the @rproc handle stays valid even after + rproc_shutdown() returns, and users can still use it with a subsequent + rproc_boot(), if needed. + - don't call rproc_shutdown() to unroll rproc_get_by_name(), exactly + because rproc_shutdown() _does not_ decrement the refcount of @rproc. + To decrement the refcount of @rproc, use rproc_put() (but _only_ if + you acquired @rproc using rproc_get_by_name()). + + struct rproc *rproc_get_by_name(const char *name) + - Find an rproc handle using the remote processor's name, and then + boot it. If it's already powered on, then just immediately return + (successfully). Returns the rproc handle on success, and NULL on failure. + This function increments the remote processor's refcount, so always + use rproc_put() to decrement it back once rproc isn't needed anymore. + Note: currently rproc_get_by_name() and rproc_put() are not used anymore + by the rpmsg bus and its drivers. We need to scrutinize the use cases + that still need them, and see if we can migrate them to use the non + name-based boot/shutdown interface. + + void rproc_put(struct rproc *rproc) + - Decrement @rproc's power refcount and shut it down if it reaches zero + (essentially by just calling rproc_shutdown), and then decrement @rproc's + validity refcount too. + After this function returns, @rproc may _not_ be used anymore, and its + handle should be considered invalid. + This function should be called _iff_ the @rproc handle was grabbed by + calling rproc_get_by_name(). + +3. Typical usage + +#include <linux/remoteproc.h> + +/* in case we were given a valid 'rproc' handle */ +int dummy_rproc_example(struct rproc *my_rproc) +{ + int ret; + + /* let's power on and boot our remote processor */ + ret = rproc_boot(my_rproc); + if (ret) { + /* + * something went wrong. handle it and leave. + */ + } + + /* + * our remote processor is now powered on... give it some work + */ + + /* let's shut it down now */ + rproc_shutdown(my_rproc); +} + +4. API for implementors + + struct rproc *rproc_alloc(struct device *dev, const char *name, + const struct rproc_ops *ops, + const char *firmware, int len) + - Allocate a new remote processor handle, but don't register + it yet. Required parameters are the underlying device, the + name of this remote processor, platform-specific ops handlers, + the name of the firmware to boot this rproc with, and the + length of private data needed by the allocating rproc driver (in bytes). + + This function should be used by rproc implementations during + initialization of the remote processor. + After creating an rproc handle using this function, and when ready, + implementations should then call rproc_register() to complete + the registration of the remote processor. + On success, the new rproc is returned, and on failure, NULL. + + Note: _never_ directly deallocate @rproc, even if it was not registered + yet. Instead, if you just need to unroll rproc_alloc(), use rproc_free(). + + void rproc_free(struct rproc *rproc) + - Free an rproc handle that was allocated by rproc_alloc. + This function should _only_ be used if @rproc was only allocated, + but not registered yet. + If @rproc was already successfully registered (by calling + rproc_register()), then use rproc_unregister() instead. + + int rproc_register(struct rproc *rproc) + - Register @rproc with the remoteproc framework, after it has been + allocated with rproc_alloc(). + This is called by the platform-specific rproc implementation, whenever + a new remote processor device is probed. + Returns 0 on success and an appropriate error code otherwise. + Note: this function initiates an asynchronous firmware loading + context, which will look for virtio devices supported by the rproc's + firmware. + If found, those virtio devices will be created and added, so as a result + of registering this remote processor, additional virtio drivers might get + probed. + Currently, though, we only support a single RPMSG virtio vdev per remote + processor. + + int rproc_unregister(struct rproc *rproc) + - Unregister a remote processor, and decrement its refcount. + If its refcount drops to zero, then @rproc will be freed. If not, + it will be freed later once the last reference is dropped. + + This function should be called when the platform specific rproc + implementation decides to remove the rproc device. it should + _only_ be called if a previous invocation of rproc_register() + has completed successfully. + + After rproc_unregister() returns, @rproc is _not_ valid anymore and + it shouldn't be used. More specifically, don't call rproc_free() + or try to directly free @rproc after rproc_unregister() returns; + none of these are needed, and calling them is a bug. + + Returns 0 on success and -EINVAL if @rproc isn't valid. + +5. Implementation callbacks + +These callbacks should be provided by platform-specific remoteproc +drivers: + +/** + * struct rproc_ops - platform-specific device handlers + * @start: power on the device and boot it + * @stop: power off the device + * @kick: kick a virtqueue (virtqueue id given as a parameter) + */ +struct rproc_ops { + int (*start)(struct rproc *rproc); + int (*stop)(struct rproc *rproc); + void (*kick)(struct rproc *rproc, int vqid); +}; + +Every remoteproc implementation should at least provide the ->start and ->stop +handlers. If rpmsg functionality is also desired, then the ->kick handler +should be provided as well. + +The ->start() handler takes an rproc handle and should then power on the +device and boot it (use rproc->priv to access platform-specific private data). +The boot address, in case needed, can be found in rproc->bootaddr (remoteproc +core puts there the ELF entry point). +On success, 0 should be returned, and on failure, an appropriate error code. + +The ->stop() handler takes an rproc handle and powers the device down. +On success, 0 is returned, and on failure, an appropriate error code. + +The ->kick() handler takes an rproc handle, and an index of a virtqueue +where new message was placed in. Implementations should interrupt the remote +processor and let it know it has pending messages. Notifying remote processors +the exact virtqueue index to look in is optional: it is easy (and not +too expensive) to go through the existing virtqueues and look for new buffers +in the used rings. + +6. Binary Firmware Structure + +At this point remoteproc only supports ELF32 firmware binaries. However, +it is quite expected that other platforms/devices which we'd want to +support with this framework will be based on different binary formats. + +When those use cases show up, we will have to decouple the binary format +from the framework core, so we can support several binary formats without +duplicating common code. + +When the firmware is parsed, its various segments are loaded to memory +according to the specified device address (might be a physical address +if the remote processor is accessing memory directly). + +In addition to the standard ELF segments, most remote processors would +also include a special section which we call "the resource table". + +The resource table contains system resources that the remote processor +requires before it should be powered on, such as allocation of physically +contiguous memory, or iommu mapping of certain on-chip peripherals. +Remotecore will only power up the device after all the resource table's +requirement are met. + +In addition to system resources, the resource table may also contain +resource entries that publish the existence of supported features +or configurations by the remote processor, such as trace buffers and +supported virtio devices (and their configurations). + +Currently the resource table is just an array of: + +/** + * struct fw_resource - describes an entry from the resource section + * @type: resource type + * @id: index number of the resource + * @da: device address of the resource + * @pa: physical address of the resource + * @len: size, in bytes, of the resource + * @flags: properties of the resource, e.g. iommu protection required + * @reserved: must be 0 atm + * @name: name of resource + */ +struct fw_resource { + u32 type; + u32 id; + u64 da; + u64 pa; + u32 len; + u32 flags; + u8 reserved[16]; + u8 name[48]; +} __packed; + +Some resources entries are mere announcements, where the host is informed +of specific remoteproc configuration. Other entries require the host to +do something (e.g. reserve a requested resource) and possibly also reply +by overwriting a member inside 'struct fw_resource' with info about the +allocated resource. + +Different resource entries use different members of this struct, +with different meanings. This is pretty limiting and error-prone, +so the plan is to move to variable-length TLV-based resource entries, +where each resource will begin with a type and length fields, followed by +its own specific structure. + +Here are the resource types that are currently being used: + +/** + * enum fw_resource_type - types of resource entries + * + * @RSC_CARVEOUT: request for allocation of a physically contiguous + * memory region. + * @RSC_DEVMEM: request to iommu_map a memory-based peripheral. + * @RSC_TRACE: announces the availability of a trace buffer into which + * the remote processor will be writing logs. In this case, + * 'da' indicates the device address where logs are written to, + * and 'len' is the size of the trace buffer. + * @RSC_VRING: request for allocation of a virtio vring (address should + * be indicated in 'da', and 'len' should contain the number + * of buffers supported by the vring). + * @RSC_VIRTIO_DEV: announces support for a virtio device, and serves as + * the virtio header. 'da' contains the virtio device + * features, 'pa' holds the virtio guest features (host + * will write them here after they're negotiated), 'len' + * holds the virtio status, and 'flags' holds the virtio + * device id (currently only VIRTIO_ID_RPMSG is supported). + */ +enum fw_resource_type { + RSC_CARVEOUT = 0, + RSC_DEVMEM = 1, + RSC_TRACE = 2, + RSC_VRING = 3, + RSC_VIRTIO_DEV = 4, + RSC_VIRTIO_CFG = 5, +}; + +Most of the resource entries share the basic idea of address/length +negotiation with the host: the firmware usually asks for memory +of size 'len' bytes, and the host needs to allocate it and provide +the device/physical address (when relevant) in 'da'/'pa' respectively. + +If the firmware is compiled with hard coded device addresses, and +can't handle dynamically allocated 'da' values, then the 'da' field +will contain the expected device addresses (today we actually only support +this scheme, as there aren't yet any use cases for dynamically allocated +device addresses). + +We also expect that platform-specific resource entries will show up +at some point. When that happens, we could easily add a new RSC_PLAFORM +type, and hand those resources to the platform-specific rproc driver to handle. + +7. Virtio and remoteproc + +The firmware should provide remoteproc information about virtio devices +that it supports, and their configurations: a RSC_VIRTIO_DEV resource entry +should specify the virtio device id, and subsequent RSC_VRING resource entries +should indicate the vring size (i.e. how many buffers do they support) and +where should they be mapped (i.e. which device address). Note: the alignment +between the consumer and producer parts of the vring is assumed to be 4096. + +At this point we only support a single virtio rpmsg device per remote +processor, but the plan is to remove this limitation. In addition, once we +move to TLV-based resource table, the plan is to have a single RSC_VIRTIO +entry per supported virtio device, which will include the virtio header, +the vrings information and the virtio config space. + +Of course, RSC_VIRTIO resource entries are only good enough for static +allocation of virtio devices. Dynamic allocations will also be made possible +using the rpmsg bus (similar to how we already do dynamic allocations of +rpmsg channels; read more about it in rpmsg.txt). |