xref: /kernel/linux/linux-6.6/Documentation/driver-api/surface_aggregator/clients/cdev.rst

.. SPDX-License-Identifier: GPL-2.0+

.. |ssam_cdev_request| replace:: :c:type:`struct ssam_cdev_request <ssam_cdev_request>`
.. |ssam_cdev_request_flags| replace:: :c:type:`enum ssam_cdev_request_flags <ssam_cdev_request_flags>`
.. |ssam_cdev_event| replace:: :c:type:`struct ssam_cdev_event <ssam_cdev_event>`

==============================
User-Space EC Interface (cdev)
==============================

The ``surface_aggregator_cdev`` module provides a misc-device for the SSAM
controller to allow for a (more or less) direct connection from user-space to
the SAM EC. It is intended to be used for development and debugging, and
therefore should not be used or relied upon in any other way. Note that this
module is not loaded automatically, but instead must be loaded manually.

The provided interface is accessible through the ``/dev/surface/aggregator``
device-file. All functionality of this interface is provided via IOCTLs.
These IOCTLs and their respective input/output parameter structs are defined in
``include/uapi/linux/surface_aggregator/cdev.h``.

A small python library and scripts for accessing this interface can be found
at https://github.com/linux-surface/surface-aggregator-module/tree/master/scripts/ssam.

.. contents::


Receiving Events
================

Events can be received by reading from the device-file. The are represented by
the |ssam_cdev_event| datatype.

Before events are available to be read, however, the desired notifiers must be
registered via the ``SSAM_CDEV_NOTIF_REGISTER`` IOCTL. Notifiers are, in
essence, callbacks, called when the EC sends an event. They are, in this
interface, associated with a specific target category and device-file-instance.
They forward any event of this category to the buffer of the corresponding
instance, from which it can then be read.

Notifiers themselves do not enable events on the EC. Thus, it may additionally
be necessary to enable events via the ``SSAM_CDEV_EVENT_ENABLE`` IOCTL. While
notifiers work per-client (i.e. per-device-file-instance), events are enabled
globally, for the EC and all of its clients (regardless of userspace or
non-userspace). The ``SSAM_CDEV_EVENT_ENABLE`` and ``SSAM_CDEV_EVENT_DISABLE``
IOCTLs take care of reference counting the events, such that an event is
enabled as long as there is a client that has requested it.

Note that enabled events are not automatically disabled once the client
instance is closed. Therefore any client process (or group of processes) should
balance their event enable calls with the corresponding event disable calls. It
is, however, perfectly valid to enable and disable events on different client
instances. For example, it is valid to set up notifiers and read events on
client instance ``A``, enable those events on instance ``B`` (note that these
will also be received by A since events are enabled/disabled globally), and
after no more events are desired, disable the previously enabled events via
instance ``C``.


Controller IOCTLs
=================

The following IOCTLs are provided:

.. flat-table:: Controller IOCTLs
   :widths: 1 1 1 1 4
   :header-rows: 1

   * - Type
     - Number
     - Direction
     - Name
     - Description

   * - ``0xA5``
     - ``1``
     - ``WR``
     - ``REQUEST``
     - Perform synchronous SAM request.

   * - ``0xA5``
     - ``2``
     - ``W``
     - ``NOTIF_REGISTER``
     - Register event notifier.

   * - ``0xA5``
     - ``3``
     - ``W``
     - ``NOTIF_UNREGISTER``
     - Unregister event notifier.

   * - ``0xA5``
     - ``4``
     - ``W``
     - ``EVENT_ENABLE``
     - Enable event source.

   * - ``0xA5``
     - ``5``
     - ``W``
     - ``EVENT_DISABLE``
     - Disable event source.


``SSAM_CDEV_REQUEST``
---------------------

Defined as ``_IOWR(0xA5, 1, struct ssam_cdev_request)``.

Executes a synchronous SAM request. The request specification is passed in
as argument of type |ssam_cdev_request|, which is then written to/modified
by the IOCTL to return status and result of the request.

Request payload data must be allocated separately and is passed in via the
``payload.data`` and ``payload.length`` members. If a response is required,
the response buffer must be allocated by the caller and passed in via the
``response.data`` member. The ``response.length`` member must be set to the
capacity of this buffer, or if no response is required, zero. Upon
completion of the request, the call will write the response to the response
buffer (if its capacity allows it) and overwrite the length field with the
actual size of the response, in bytes.

Additionally, if the request has a response, this must be indicated via the
request flags, as is done with in-kernel requests. Request flags can be set
via the ``flags`` member and the values correspond to the values found in
|ssam_cdev_request_flags|.

Finally, the status of the request itself is returned in the ``status``
member (a negative errno value indicating failure). Note that failure
indication of the IOCTL is separated from failure indication of the request:
The IOCTL returns a negative status code if anything failed during setup of
the request (``-EFAULT``) or if the provided argument or any of its fields
are invalid (``-EINVAL``). In this case, the status value of the request
argument may be set, providing more detail on what went wrong (e.g.
``-ENOMEM`` for out-of-memory), but this value may also be zero. The IOCTL
will return with a zero status code in case the request has been set up,
submitted, and completed (i.e. handed back to user-space) successfully from
inside the IOCTL, but the request ``status`` member may still be negative in
case the actual execution of the request failed after it has been submitted.

A full definition of the argument struct is provided below.

``SSAM_CDEV_NOTIF_REGISTER``
----------------------------

Defined as ``_IOW(0xA5, 2, struct ssam_cdev_notifier_desc)``.

Register a notifier for the event target category specified in the given
notifier description with the specified priority. Notifiers registration is
required to receive events, but does not enable events themselves. After a
notifier for a specific target category has been registered, all events of that
category will be forwarded to the userspace client and can then be read from
the device file instance. Note that events may have to be enabled, e.g. via the
``SSAM_CDEV_EVENT_ENABLE`` IOCTL, before the EC will send them.

Only one notifier can be registered per target category and client instance. If
a notifier has already been registered, this IOCTL will fail with ``-EEXIST``.

Notifiers will automatically be removed when the device file instance is
closed.

``SSAM_CDEV_NOTIF_UNREGISTER``
------------------------------

Defined as ``_IOW(0xA5, 3, struct ssam_cdev_notifier_desc)``.

Unregisters the notifier associated with the specified target category. The
priority field will be ignored by this IOCTL. If no notifier has been
registered for this client instance and the given category, this IOCTL will
fail with ``-ENOENT``.

``SSAM_CDEV_EVENT_ENABLE``
--------------------------

Defined as ``_IOW(0xA5, 4, struct ssam_cdev_event_desc)``.

Enable the event associated with the given event descriptor.

Note that this call will not register a notifier itself, it will only enable
events on the controller. If you want to receive events by reading from the
device file, you will need to register the corresponding notifier(s) on that
instance.

Events are not automatically disabled when the device file is closed. This must
be done manually, via a call to the ``SSAM_CDEV_EVENT_DISABLE`` IOCTL.

``SSAM_CDEV_EVENT_DISABLE``
---------------------------

Defined as ``_IOW(0xA5, 5, struct ssam_cdev_event_desc)``.

Disable the event associated with the given event descriptor.

Note that this will not unregister any notifiers. Events may still be received
and forwarded to user-space after this call. The only safe way of stopping
events from being received is unregistering all previously registered
notifiers.


Structures and Enums
====================

.. kernel-doc:: include/uapi/linux/surface_aggregator/cdev.h