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diff --git a/Documentation/driver-api/surface_aggregator/client.rst b/Documentation/driver-api/surface_aggregator/client.rst new file mode 100644 index 000000000000..e100ab0a24cc --- /dev/null +++ b/Documentation/driver-api/surface_aggregator/client.rst @@ -0,0 +1,397 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +.. |ssam_controller| replace:: :c:type:`struct ssam_controller <ssam_controller>` +.. |ssam_device| replace:: :c:type:`struct ssam_device <ssam_device>` +.. |ssam_device_driver| replace:: :c:type:`struct ssam_device_driver <ssam_device_driver>` +.. |ssam_client_bind| replace:: :c:func:`ssam_client_bind` +.. |ssam_client_link| replace:: :c:func:`ssam_client_link` +.. |ssam_get_controller| replace:: :c:func:`ssam_get_controller` +.. |ssam_controller_get| replace:: :c:func:`ssam_controller_get` +.. |ssam_controller_put| replace:: :c:func:`ssam_controller_put` +.. |ssam_device_alloc| replace:: :c:func:`ssam_device_alloc` +.. |ssam_device_add| replace:: :c:func:`ssam_device_add` +.. |ssam_device_remove| replace:: :c:func:`ssam_device_remove` +.. |ssam_device_driver_register| replace:: :c:func:`ssam_device_driver_register` +.. |ssam_device_driver_unregister| replace:: :c:func:`ssam_device_driver_unregister` +.. |module_ssam_device_driver| replace:: :c:func:`module_ssam_device_driver` +.. |SSAM_DEVICE| replace:: :c:func:`SSAM_DEVICE` +.. |ssam_notifier_register| replace:: :c:func:`ssam_notifier_register` +.. |ssam_notifier_unregister| replace:: :c:func:`ssam_notifier_unregister` +.. |ssam_device_notifier_register| replace:: :c:func:`ssam_device_notifier_register` +.. |ssam_device_notifier_unregister| replace:: :c:func:`ssam_device_notifier_unregister` +.. |ssam_request_do_sync| replace:: :c:func:`ssam_request_do_sync` +.. |ssam_event_mask| replace:: :c:type:`enum ssam_event_mask <ssam_event_mask>` + + +====================== +Writing Client Drivers +====================== + +For the API documentation, refer to: + +.. toctree:: + :maxdepth: 2 + + client-api + + +Overview +======== + +Client drivers can be set up in two main ways, depending on how the +corresponding device is made available to the system. We specifically +differentiate between devices that are presented to the system via one of +the conventional ways, e.g. as platform devices via ACPI, and devices that +are non-discoverable and instead need to be explicitly provided by some +other mechanism, as discussed further below. + + +Non-SSAM Client Drivers +======================= + +All communication with the SAM EC is handled via the |ssam_controller| +representing that EC to the kernel. Drivers targeting a non-SSAM device (and +thus not being a |ssam_device_driver|) need to explicitly establish a +connection/relation to that controller. This can be done via the +|ssam_client_bind| function. Said function returns a reference to the SSAM +controller, but, more importantly, also establishes a device link between +client device and controller (this can also be done separate via +|ssam_client_link|). It is important to do this, as it, first, guarantees +that the returned controller is valid for use in the client driver for as +long as this driver is bound to its device, i.e. that the driver gets +unbound before the controller ever becomes invalid, and, second, as it +ensures correct suspend/resume ordering. This setup should be done in the +driver's probe function, and may be used to defer probing in case the SSAM +subsystem is not ready yet, for example: + +.. code-block:: c + + static int client_driver_probe(struct platform_device *pdev) + { + struct ssam_controller *ctrl; + + ctrl = ssam_client_bind(&pdev->dev); + if (IS_ERR(ctrl)) + return PTR_ERR(ctrl) == -ENODEV ? -EPROBE_DEFER : PTR_ERR(ctrl); + + // ... + + return 0; + } + +The controller may be separately obtained via |ssam_get_controller| and its +lifetime be guaranteed via |ssam_controller_get| and |ssam_controller_put|. +Note that none of these functions, however, guarantee that the controller +will not be shut down or suspended. These functions essentially only operate +on the reference, i.e. only guarantee a bare minimum of accessibility +without any guarantees at all on practical operability. + + +Adding SSAM Devices +=================== + +If a device does not already exist/is not already provided via conventional +means, it should be provided as |ssam_device| via the SSAM client device +hub. New devices can be added to this hub by entering their UID into the +corresponding registry. SSAM devices can also be manually allocated via +|ssam_device_alloc|, subsequently to which they have to be added via +|ssam_device_add| and eventually removed via |ssam_device_remove|. By +default, the parent of the device is set to the controller device provided +for allocation, however this may be changed before the device is added. Note +that, when changing the parent device, care must be taken to ensure that the +controller lifetime and suspend/resume ordering guarantees, in the default +setup provided through the parent-child relation, are preserved. If +necessary, by use of |ssam_client_link| as is done for non-SSAM client +drivers and described in more detail above. + +A client device must always be removed by the party which added the +respective device before the controller shuts down. Such removal can be +guaranteed by linking the driver providing the SSAM device to the controller +via |ssam_client_link|, causing it to unbind before the controller driver +unbinds. Client devices registered with the controller as parent are +automatically removed when the controller shuts down, but this should not be +relied upon, especially as this does not extend to client devices with a +different parent. + + +SSAM Client Drivers +=================== + +SSAM client device drivers are, in essence, no different than other device +driver types. They are represented via |ssam_device_driver| and bind to a +|ssam_device| via its UID (:c:type:`struct ssam_device.uid <ssam_device>`) +member and the match table +(:c:type:`struct ssam_device_driver.match_table <ssam_device_driver>`), +which should be set when declaring the driver struct instance. Refer to the +|SSAM_DEVICE| macro documentation for more details on how to define members +of the driver's match table. + +The UID for SSAM client devices consists of a ``domain``, a ``category``, +a ``target``, an ``instance``, and a ``function``. The ``domain`` is used +differentiate between physical SAM devices +(:c:type:`SSAM_DOMAIN_SERIALHUB <ssam_device_domain>`), i.e. devices that can +be accessed via the Surface Serial Hub, and virtual ones +(:c:type:`SSAM_DOMAIN_VIRTUAL <ssam_device_domain>`), such as client-device +hubs, that have no real representation on the SAM EC and are solely used on +the kernel/driver-side. For physical devices, ``category`` represents the +target category, ``target`` the target ID, and ``instance`` the instance ID +used to access the physical SAM device. In addition, ``function`` references +a specific device functionality, but has no meaning to the SAM EC. The +(default) name of a client device is generated based on its UID. + +A driver instance can be registered via |ssam_device_driver_register| and +unregistered via |ssam_device_driver_unregister|. For convenience, the +|module_ssam_device_driver| macro may be used to define module init- and +exit-functions registering the driver. + +The controller associated with a SSAM client device can be found in its +:c:type:`struct ssam_device.ctrl <ssam_device>` member. This reference is +guaranteed to be valid for at least as long as the client driver is bound, +but should also be valid for as long as the client device exists. Note, +however, that access outside of the bound client driver must ensure that the +controller device is not suspended while making any requests or +(un-)registering event notifiers (and thus should generally be avoided). This +is guaranteed when the controller is accessed from inside the bound client +driver. + + +Making Synchronous Requests +=========================== + +Synchronous requests are (currently) the main form of host-initiated +communication with the EC. There are a couple of ways to define and execute +such requests, however, most of them boil down to something similar as shown +in the example below. This example defines a write-read request, meaning +that the caller provides an argument to the SAM EC and receives a response. +The caller needs to know the (maximum) length of the response payload and +provide a buffer for it. + +Care must be taken to ensure that any command payload data passed to the SAM +EC is provided in little-endian format and, similarly, any response payload +data received from it is converted from little-endian to host endianness. + +.. code-block:: c + + int perform_request(struct ssam_controller *ctrl, u32 arg, u32 *ret) + { + struct ssam_request rqst; + struct ssam_response resp; + int status; + + /* Convert request argument to little-endian. */ + __le32 arg_le = cpu_to_le32(arg); + __le32 ret_le = cpu_to_le32(0); + + /* + * Initialize request specification. Replace this with your values. + * The rqst.payload field may be NULL if rqst.length is zero, + * indicating that the request does not have any argument. + * + * Note: The request parameters used here are not valid, i.e. + * they do not correspond to an actual SAM/EC request. + */ + rqst.target_category = SSAM_SSH_TC_SAM; + rqst.target_id = SSAM_SSH_TID_SAM; + rqst.command_id = 0x02; + rqst.instance_id = 0x03; + rqst.flags = SSAM_REQUEST_HAS_RESPONSE; + rqst.length = sizeof(arg_le); + rqst.payload = (u8 *)&arg_le; + + /* Initialize request response. */ + resp.capacity = sizeof(ret_le); + resp.length = 0; + resp.pointer = (u8 *)&ret_le; + + /* + * Perform actual request. The response pointer may be null in case + * the request does not have any response. This must be consistent + * with the SSAM_REQUEST_HAS_RESPONSE flag set in the specification + * above. + */ + status = ssam_request_do_sync(ctrl, &rqst, &resp); + + /* + * Alternatively use + * + * ssam_request_do_sync_onstack(ctrl, &rqst, &resp, sizeof(arg_le)); + * + * to perform the request, allocating the message buffer directly + * on the stack as opposed to allocation via kzalloc(). + */ + + /* + * Convert request response back to native format. Note that in the + * error case, this value is not touched by the SSAM core, i.e. + * 'ret_le' will be zero as specified in its initialization. + */ + *ret = le32_to_cpu(ret_le); + + return status; + } + +Note that |ssam_request_do_sync| in its essence is a wrapper over lower-level +request primitives, which may also be used to perform requests. Refer to its +implementation and documentation for more details. + +An arguably more user-friendly way of defining such functions is by using +one of the generator macros, for example via: + +.. code-block:: c + + SSAM_DEFINE_SYNC_REQUEST_W(__ssam_tmp_perf_mode_set, __le32, { + .target_category = SSAM_SSH_TC_TMP, + .target_id = SSAM_SSH_TID_SAM, + .command_id = 0x03, + .instance_id = 0x00, + }); + +This example defines a function + +.. code-block:: c + + static int __ssam_tmp_perf_mode_set(struct ssam_controller *ctrl, const __le32 *arg); + +executing the specified request, with the controller passed in when calling +said function. In this example, the argument is provided via the ``arg`` +pointer. Note that the generated function allocates the message buffer on +the stack. Thus, if the argument provided via the request is large, these +kinds of macros should be avoided. Also note that, in contrast to the +previous non-macro example, this function does not do any endianness +conversion, which has to be handled by the caller. Apart from those +differences the function generated by the macro is similar to the one +provided in the non-macro example above. + +The full list of such function-generating macros is + +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_N` for requests without return value and + without argument. +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_R` for requests with return value but no + argument. +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_W` for requests without return value but + with argument. + +Refer to their respective documentation for more details. For each one of +these macros, a special variant is provided, which targets request types +applicable to multiple instances of the same device type: + +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_MD_N` +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_MD_R` +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_MD_W` + +The difference of those macros to the previously mentioned versions is, that +the device target and instance IDs are not fixed for the generated function, +but instead have to be provided by the caller of said function. + +Additionally, variants for direct use with client devices, i.e. +|ssam_device|, are also provided. These can, for example, be used as +follows: + +.. code-block:: c + + SSAM_DEFINE_SYNC_REQUEST_CL_R(ssam_bat_get_sta, __le32, { + .target_category = SSAM_SSH_TC_BAT, + .command_id = 0x01, + }); + +This invocation of the macro defines a function + +.. code-block:: c + + static int ssam_bat_get_sta(struct ssam_device *sdev, __le32 *ret); + +executing the specified request, using the device IDs and controller given +in the client device. The full list of such macros for client devices is: + +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_CL_N` +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_CL_R` +- :c:func:`SSAM_DEFINE_SYNC_REQUEST_CL_W` + + +Handling Events +=============== + +To receive events from the SAM EC, an event notifier must be registered for +the desired event via |ssam_notifier_register|. The notifier must be +unregistered via |ssam_notifier_unregister| once it is not required any +more. For |ssam_device| type clients, the |ssam_device_notifier_register| and +|ssam_device_notifier_unregister| wrappers should be preferred as they properly +handle hot-removal of client devices. + +Event notifiers are registered by providing (at minimum) a callback to call +in case an event has been received, the registry specifying how the event +should be enabled, an event ID specifying for which target category and, +optionally and depending on the registry used, for which instance ID events +should be enabled, and finally, flags describing how the EC will send these +events. If the specific registry does not enable events by instance ID, the +instance ID must be set to zero. Additionally, a priority for the respective +notifier may be specified, which determines its order in relation to any +other notifier registered for the same target category. + +By default, event notifiers will receive all events for the specific target +category, regardless of the instance ID specified when registering the +notifier. The core may be instructed to only call a notifier if the target +ID or instance ID (or both) of the event match the ones implied by the +notifier IDs (in case of target ID, the target ID of the registry), by +providing an event mask (see |ssam_event_mask|). + +In general, the target ID of the registry is also the target ID of the +enabled event (with the notable exception being keyboard input events on the +Surface Laptop 1 and 2, which are enabled via a registry with target ID 1, +but provide events with target ID 2). + +A full example for registering an event notifier and handling received +events is provided below: + +.. code-block:: c + + u32 notifier_callback(struct ssam_event_notifier *nf, + const struct ssam_event *event) + { + int status = ... + + /* Handle the event here ... */ + + /* Convert return value and indicate that we handled the event. */ + return ssam_notifier_from_errno(status) | SSAM_NOTIF_HANDLED; + } + + int setup_notifier(struct ssam_device *sdev, + struct ssam_event_notifier *nf) + { + /* Set priority wrt. other handlers of same target category. */ + nf->base.priority = 1; + + /* Set event/notifier callback. */ + nf->base.fn = notifier_callback; + + /* Specify event registry, i.e. how events get enabled/disabled. */ + nf->event.reg = SSAM_EVENT_REGISTRY_KIP; + + /* Specify which event to enable/disable */ + nf->event.id.target_category = sdev->uid.category; + nf->event.id.instance = sdev->uid.instance; + + /* + * Specify for which events the notifier callback gets executed. + * This essentially tells the core if it can skip notifiers that + * don't have target or instance IDs matching those of the event. + */ + nf->event.mask = SSAM_EVENT_MASK_STRICT; + + /* Specify event flags. */ + nf->event.flags = SSAM_EVENT_SEQUENCED; + + return ssam_notifier_register(sdev->ctrl, nf); + } + +Multiple event notifiers can be registered for the same event. The event +handler core takes care of enabling and disabling events when notifiers are +registered and unregistered, by keeping track of how many notifiers for a +specific event (combination of registry, event target category, and event +instance ID) are currently registered. This means that a specific event will +be enabled when the first notifier for it is being registered and disabled +when the last notifier for it is being unregistered. Note that the event +flags are therefore only used on the first registered notifier, however, one +should take care that notifiers for a specific event are always registered +with the same flag and it is considered a bug to do otherwise. |