linux/linux-5.18.11/Documentation/driver-api/surface_aggregator/client.rst

.. 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_request_sync| replace:: :c:func:`ssam_request_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 = 0x01;
           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_sync(ctrl, &rqst, &resp);

           /*
            * Alternatively use
            *
            *   ssam_request_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_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       = 0x01,
           .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.

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.