[PATCH v7 5/9] docs/clocks: add device's clock documentation

[Damien Hedde <damien.hedde@greensocs.com>]

Add the documentation about the clock inputs and outputs in devices.

This is based on the original work of Frederic Konrad.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
---

v7:
 + update ClockIn/Out types
---
 docs/devel/clocks.rst | 356 ++++++++++++++++++++++++++++++++++++++++++
 docs/devel/index.rst  |   1 +
 2 files changed, 357 insertions(+)
 create mode 100644 docs/devel/clocks.rst

diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst
new file mode 100644
index 0000000000..cc5563691d
--- /dev/null
+++ b/docs/devel/clocks.rst
@@ -0,0 +1,356 @@
+
+What are clocks
+===============
+
+Clocks are QOM objects developed for the purpose of modeling the
+distribution of clocks in QEMU.
+
+They allow us to model the clock distribution of a platform and detect
+configuration errors in the clock tree such as badly configured PLL, clock
+source selection or disabled clock.
+
+The object is *Clock* and its QOM name is ``CLOCK``.
+
+Clocks are typically used with devices where they are used to model inputs
+and outputs. They are created in a similar way as gpios. Inputs and outputs
+of different devices can be connect together.
+
+In these cases a Clock object is a child of a Device object but this is not
+a requirement. Clocks can be independent of devices. For example it is possible
+to create a clock outside of any device to model the main clock source of a
+machine.
+
+Here is an example of clocks::
+
+    +---------+      +----------------------+   +--------------+
+    | Clock 1 |      |       Device B       |   |   Device C   |
+    |         |      | +-------+  +-------+ |   | +-------+    |
+    |         |>>-+-->>|Clock 2|  |Clock 3|>>--->>|Clock 6|    |
+    +---------+   |  | | (in)  |  | (out) | |   | | (in)  |    |
+                  |  | +-------+  +-------+ |   | +-------+    |
+                  |  |            +-------+ |   +--------------+
+                  |  |            |Clock 4|>>
+                  |  |            | (out) | |   +--------------+
+                  |  |            +-------+ |   |   Device D   |
+                  |  |            +-------+ |   | +-------+    |
+                  |  |            |Clock 5|>>--->>|Clock 7|    |
+                  |  |            | (out) | |   | | (in)  |    |
+                  |  |            +-------+ |   | +-------+    |
+                  |  +----------------------+   |              |
+                  |                             | +-------+    |
+                  +----------------------------->>|Clock 8|    |
+                                                | | (in)  |    |
+                                                | +-------+    |
+                                                +--------------+
+
+Clocks are defined in include/hw/clock.h header and device related functions
+are defined in include/hw/qdev-clock.h header.
+
+The clock state
+===============
+
+The state of a clock is its period; it is stored as an integer representing
+it in 2^-32ns unit. The special value of 0 is used to represent the clock being
+inactive or gated. The clocks do not model the signal itself (pin toggling)
+or other properties such as the duty cycle.
+
+All clocks contain this state: outputs as well as inputs. It allows to fetch
+the current period of a clock at any time. When a clock is updated, the
+value is immediately propagated to all connected clocks in the tree.
+
+To ease interaction with clocks. Helpers with a unit suffix are defined for
+every clock state setter or getter. They are:
+  + ``_ns`` for handling periods in nanosecond, and
+  + ``_hz`` for handling frequencies in hertz.
+The 0 period value is converted to 0 in hertz and vice versa. 0 always means
+that the clock is disabled.
+
+Adding a new a clock
+====================
+
+Adding clocks to a device must be done during the init method of the Device
+instance.
+
+To add an input clock to a device, the function qdev_init_clock_in must be used.
+It takes the name, a callback and an opaque parameter for the callback (this will
+be explained in a following section below).
+Output is more simple, only the name is required. Typically::
+
+    qdev_init_clock_in(DEVICE(dev), "clk_in", clk_in_callback, dev);
+    qdev_init_clock_out(DEVICE(dev), "clk_out");
+
+Both functions return the created Clock pointer, which should be saved in the
+device's state structure for further use.
+
+These objects will be automatically deleted by the QOM reference mechanism.
+
+Note that it is possible to create a static array describing clock inputs and
+outputs. The function ``qdev_init_clocks()`` must be called with the array as
+parameter to initialize the clocks: it has the same behaviour as calling the
+``qdev_init_clock_in/out()`` for each clock in the array. To ease the array
+construction, some macros are defined in include/hw/qdev-clock.h.
+As an example, the following creates 2 clocks to a device: one input and one
+output.
+
+::
+
+    /* device structure containing pointer to the clock objects */
+    typedef struct MyDeviceState {
+        DeviceState parent_obj;
+        Clock *clk_in;
+        Clock *clk_out;
+    } MyDeviceState;
+
+    /*
+     * callback for the input clock (see "Callback on input clock
+     * change" section below for more information).
+     */
+    static void clk_in_callback(void *opaque);
+
+    /*
+     * static array describing clocks:
+     * + a clock input named "clk_in", whose pointer is stored in
+     *   clk_in field of a MyDeviceState structure with callback
+     *   clk_in_callback.
+     * + a clock output named "clk_out" whose pointer is stored in
+     *   clk_out field of a MyDeviceState structure.
+     */
+    static const ClockPortInitArray mydev_clocks = {
+        QDEV_CLOCK_IN(MyDeviceState, clk_in, clk_in_callback),
+        QDEV_CLOCK_OUT(MyDeviceState, clk_out),
+        QDEV_CLOCK_END
+    };
+
+    /* device initialization function */
+    static void mydev_init(Object *obj)
+    {
+        /* cast to MyDeviceState */
+        MyDeviceState *mydev = MYDEVICE(obj);
+        /* create and fill the pointer fields in the MyDeviceState */
+        qdev_init_clocks(mydev, mydev_clocks);
+        [...]
+    }
+
+An alternative way to create a clock is to simply call
+``object_new(TYPE_CLOCK)``. In that case the clock will neither be an input nor
+an output of a device. After the whole QOM hieracrhy of the clock has been set
+``clock_setup_canonical_path()`` should be called.
+
+At creation, the period of the clock is 0: the clock is disabled. You can
+change it using ``clock_set[_ns|_hz]()``.
+
+Note that if you are creating a clock with a fixed period which will never
+change (for example the main clock source of a board), then you'll have
+nothing else to do. This value will be propagated to other clocks when
+connecting the clocks together and devices will fetch the right value during
+the first reset.
+
+Retrieving clocks from a device
+===============================
+
+``qdev_get_clock_in()`` and ``dev_get_clock_out()`` are available to get the clock inputs or outputs of a device. For example::
+
+    Clock *clk = qdev_get_clock_in(DEVICE(mydev), "clk_in");
+
+or::
+
+    Clock *clk = qdev_get_clock_out(DEVICE(mydev), "clk_out");
+
+Connecting two clocks together
+==============================
+
+To connect two clocks together, use the ``clock_set_source()`` function.
+Given two clocks ``clk1``, and ``clk2``, ``clock_set_source(clk2, clk1);``
+configure ``clk2`` to follow the ``clk1`` period changes. Every time ``clk1``
+is updated, ``clk2`` will be updated too.
+
+When connecting clock between devices, prefer using the
+``qdev_connect_clock_in()`` function set the source of an input device clock.
+For example, to connect the input clock ``clk2`` of ``devB`` to the output
+clock ``clk1`` of ``devA``, do::
+
+    qdev_connect_clock_in(devB, "clk2", qdev_get_clock_out(devA, "clk1"))
+
+We used ``qdev_get_clock_out()`` above, but any clock can drive an input clock,
+even another input clock. The following diagram shows some
+examples of connections. Note also that a clock can drive several other clocks.
+
+::
+
+  +------------+  +--------------------------------------------------+
+  |  Device A  |  |                   Device B                       |
+  |            |  |               +---------------------+            |
+  |            |  |               |       Device C      |            |
+  |  +-------+ |  | +-------+     | +-------+ +-------+ |  +-------+ |
+  |  |Clock 1|>>-->>|Clock 2|>>+-->>|Clock 3| |Clock 5|>>>>|Clock 6|>>
+  |  | (out) | |  | | (in)  |  |  | | (in)  | | (out) | |  | (out) | |
+  |  +-------+ |  | +-------+  |  | +-------+ +-------+ |  +-------+ |
+  +------------+  |            |  +---------------------+            |
+                  |            |                                     |
+                  |            |  +--------------+                   |
+                  |            |  |   Device D   |                   |
+                  |            |  | +-------+    |                   |
+                  |            +-->>|Clock 4|    |                   |
+                  |               | | (in)  |    |                   |
+                  |               | +-------+    |                   |
+                  |               +--------------+                   |
+                  +--------------------------------------------------+
+
+In the above example, when *Clock 1* is updated by *Device A*, three clocks gets the new clock period value: *Clock 2*, Clock 3* and *Clock 4*.
+
+It is not possible to disconnect a clock or to change the clock connection
+after it is done.
+
+Unconnected input clocks
+========================
+
+A newly created input clock is disabled (period of 0). It means the clock will
+be considered as disabled until the period is updated. If the clock remains
+unconnected it will always keep its initial value of 0. If this is not the
+wanted behaviour, ``clock_set()``, ``clock_set_ns()`` or ``clock_set_hz()``
+should be called on the Clock object during device instance init. For example::
+
+    clk = qdev_init_clock_in(DEVICE(dev), "clk-in", clk_in_callback,
+                             dev);
+    /* set initial value to 10ns / 100MHz */
+    clock_set_ns(clk, 10);
+
+Fetching clock frequency/period
+===============================
+
+To get the current state of a clock, the function ``clock_get()``,
+``clock_get_ns()`` or ``clock_get_hz()`` must be used.
+
+It is also possible to register a callback on clock frequency changes.
+Here is an example::
+
+    void clock_callback(void *opaque) {
+        MyDeviceState *s = (MyDeviceState *) opaque;
+        /*
+         * opaque may not be the device state pointer, but most
+         * probably it is. (It depends on what is given to the
+         * qdev_init_clock_in function)
+         */
+
+        /* do something with the new period */
+        fprintf(stdout, "device new period is %" PRIu64 "ns\n",
+                        clock_get_ns(dev->my_clk_input));
+    }
+
+Changing a clock period
+=======================
+
+A device can change its outputs using the ``clock_update()``,
+``clock_update_ns()`` or ``clock_update_hz()`` function. It will trigger
+updates on every connected input.
+
+For example, let's say that we have an output clock *clkout* and we have a
+pointer to it in the device state because we did the following in init phase::
+
+    dev->clkout = qdev_init_clock_out(DEVICE(dev), "clkout");
+
+Then at any time (apart from the cases listed below), it is possible to
+change the clock value by doing::
+
+    clock_update_hz(dev->clkout, 1000 * 1000 * 1000); /* 1Ghz */
+
+Because updating a clock may trigger any side effects through connected clocks
+and their callbacks, this operation must be done while holding the qemu io lock.
+
+For the same reason, one can updates clocks only when it is allowed to have
+side effects on other objects. In consequence, it is forbidden:
++ during migration,
++ and in the enter phase of reset.
+
+Note that calling ``clock_update[_ns|_hz]()`` is equivalent to call
+``clock_set[_ns|_hz]()`` (with the same arguments) then ``clock_propagate()`` on
+the clock. Thus, setting the clock value can separated from triggering the
+side-effects. This is often required to factorize code to handle reset and
+migration in devices.
+
+Aliasing clocks
+=================
+
+Sometimes, one needs to forward, or inherit, a clock from another device.
+Typically, when doing device composition, a device might expose a sub-device's
+clock without interfering with it.
+The function ``qdev_alias_clock()`` can be used to achieve this behaviour. Note
+that it is possible to expose the clock under a different name. This works for
+both inputs and outputs.
+
+For example, if device B is a child of device A, ``device_a_instance_init()``
+may do something like this::
+
+    void device_a_instance_init(Object *obj)
+    {
+        AState *A = DEVICE_A(obj);
+        BState *B;
+        /* create B object as child of A */
+        [...]
+        qdev_alias_clock(B, "clk", A, "b_clk");
+        /*
+         * Now A has a clock "b_clk" which is an alias to
+         * the clock "clk" of its child B.
+         */
+    }
+
+This function does not return any clock object. The new clock has the same
+direction (input or output) as the original one. This function only adds a link
+to the existing clock. In the above example, B object remains the only object
+allowed to use the clock and device A must not try to change the clock period
+or set a callback to the clock. Here follows a diagram describing the example
+with an input clock::
+
+    +--------------------------+
+    |        Device A          |
+    |         +--------------+ |
+    |         |   Device B   | |
+    |         | +-------+    | |
+    >>"b_clk">>>| "clk" |    | |
+    |  (in)   | |  (in) |    | |
+    |         | +-------+    | |
+    |         +--------------+ |
+    +--------------------------+
+
+Migration
+=========
+
+Clock state are not migrated automatically. Every device must handle its
+clock migration. Alias clocks must not be migrated.
+
+To ensure clock states are restored correctly during migration, there is two
+solutions.
+
+Clocks states can be migrated by adding an entry into the device
+vmstate description. To this purpose, the ``VMSTATE_CLOCK`` macro defines
+such an entry and should be used. This is typically used to migrate an input
+clock state. The following example describes it::
+
+    MyDeviceState {
+        DeviceState parent_obj;
+        [...] /* some fields */
+        Clock *clk;
+    };
+
+    VMStateDescription my_device_vmstate = {
+        .name = "my_device",
+        .fields = (VMStateField[]) {
+            [...], /* other migrated fields */
+            VMSTATE_CLOCK(clk, MyDeviceState),
+            VMSTATE_END_OF_LIST()
+        }
+    };
+
+The second solution is to restore the clock state using information already
+at our disposal. This can be used to restore output clocks states using the
+device state. The functions ``clock_set[_ns|_hz]()`` can be used during
+``post_load()`` migration callback.
+
+When adding a clock support to an existing device, if you care about migration
+compatibility. To this end, you can use ``clock_set()`` in a ``pre_load()``
+function to setup a default value in case the source virtual machine does not
+send the clock state. You may also need to use a vmstate subsection.
+
+Care should be taken not to use ``clock_update[_ns|_hz]()`` or
+``clock_propagate()`` during the whole migration procedure because it will
+trigger side effects to other devices in an unknown state.
diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index 4dc2ca8d71..f9c8c668ee 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -25,3 +25,4 @@ Contents:
    tcg-plugins
    bitops
    reset
+   clocks
-- 
2.24.1