new file mode 100644
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+
+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.
@@ -25,3 +25,4 @@ Contents:
tcg-plugins
bitops
reset
+ clocks
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