docs/devel/clocks.rst - qemu - Git at Google

 Modelling a clock tree in QEMU
 ==============================

 What are clocks?
 ----------------

 Clocks are QOM objects developed for the purpose of modelling 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`` (in C code, the macro
 ``TYPE_CLOCK``).

 Clocks are typically used with devices where they are used to model inputs
 and outputs. They are created in a similar way to GPIOs. Inputs and outputs
 of different devices can be connected 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 the ``include/hw/clock.h`` header and device
 related functions are defined in the ``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 units of 2 :sup:`-32` ns. 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. This allows
 the current period of a clock to be fetched 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. The suffixes are:

 - ``_ns`` for handling periods in nanoseconds
 - ``_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 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).
 Output is simpler; 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.

 .. code-block:: c

     /* device structure containing pointers 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
      *   the clk_in field of a MyDeviceState structure with callback
      *   clk_in_callback.
      * + a clock output named "clk_out" whose pointer is stored in
      *   the 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 hierarchy 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()`` or ``clock_set_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:

 .. code-block:: c

    Clock *clk = qdev_get_clock_in(DEVICE(mydev), "clk_in");

 or:

 .. code-block:: c

    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);``
 configures ``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 to 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:

 .. code-block:: c

     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 get 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 connected.

 Unconnected input clocks
 ------------------------

 A newly created input clock is disabled (period of 0). This 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 desired behaviour, ``clock_set()``,
 ``clock_set_ns()`` or ``clock_set_hz()`` should be called on the Clock
 object during device instance init. For example:

 .. code-block:: c

     clk = qdev_init_clock_in(DEVICE(dev), "clk-in", clk_in_callback,
                              dev);
     /* set initial value to 10ns / 100MHz */
     clock_set_ns(clk, 10);

 To enforce that the clock is wired up by the board code, you can
 call ``clock_has_source()`` in your device's realize method:

 .. code-block:: c

    if (!clock_has_source(s->clk)) {
        error_setg(errp, "MyDevice: clk input must be connected");
        return;
    }

 Note that this only checks that the clock has been wired up; it is
 still possible that the output clock connected to it is disabled
 or has not yet been configured, in which case the period will be
 zero. You should use the clock callback to find out when the clock
 period changes.

 Fetching clock frequency/period
 -------------------------------

 To get the current state of a clock, use the functions ``clock_get()``
 or ``clock_get_hz()``.

 ``clock_get()`` returns the period of the clock in its fully precise
 internal representation, as an unsigned 64-bit integer in units of
 2^-32 nanoseconds. (For many purposes ``clock_ticks_to_ns()`` will
 be more convenient; see the section below on expiry deadlines.)

 ``clock_get_hz()`` returns the frequency of the clock, rounded to the
 next lowest integer. This implies some inaccuracy due to the rounding,
 so be cautious about using it in calculations.

 It is also possible to register a callback on clock frequency changes.
 Here is an example:

 .. code-block:: c

     void clock_callback(void *opaque) {
         MyDeviceState *s = (MyDeviceState *) opaque;
         /*
          * 'opaque' is the argument passed to qdev_init_clock_in();
          * usually this will be the device state pointer.
          */

         /* do something with the new period */
         fprintf(stdout, "device new period is %" PRIu64 "* 2^-32 ns\n",
                         clock_get(dev->my_clk_input));
     }

 If you are only interested in the frequency for displaying it to
 humans (for instance in debugging), use ``clock_display_freq()``,
 which returns a prettified string-representation, e.g. "33.3 MHz".
 The caller must free the string with g_free() after use.

 Calculating expiry deadlines
 ----------------------------

 A commonly required operation for a clock is to calculate how long
 it will take for the clock to tick N times; this can then be used
 to set a timer expiry deadline. Use the function ``clock_ticks_to_ns()``,
 which takes an unsigned 64-bit count of ticks and returns the length
 of time in nanoseconds required for the clock to tick that many times.

 It is important not to try to calculate expiry deadlines using a
 shortcut like multiplying a "period of clock in nanoseconds" value
 by the tick count, because clocks can have periods which are not a
 whole number of nanoseconds, and the accumulated error in the
 multiplication can be significant.

 For a clock with a very long period and a large number of ticks,
 the result of this function could in theory be too large to fit in
 a 64-bit value. To avoid overflow in this case, ``clock_ticks_to_ns()``
 saturates the result to INT64_MAX (because this is the largest valid
 input to the QEMUTimer APIs). Since INT64_MAX nanoseconds is almost
 300 years, anything with an expiry later than that is in the "will
 never happen" category. Callers of ``clock_ticks_to_ns()`` should
 therefore generally not special-case the possibility of a saturated
 result but just allow the timer to be set to that far-future value.
 (If you are performing further calculations on the returned value
 rather than simply passing it to a QEMUTimer function like
 ``timer_mod_ns()`` then you should be careful to avoid overflow
 in those calculations, of course.)

 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:

 .. code-block:: c

    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:

 .. code-block:: c

    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 update 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 calling
 ``clock_set[_ns|_hz]()`` (with the same arguments) then
 ``clock_propagate()`` on the clock. Thus, setting the clock value can
 be 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.
 ``qdev_alias_clock()`` works for both input and output clocks.

 For example, if device B is a child of device A,
 ``device_a_instance_init()`` may do something like this:

 .. code-block:: c

     void device_a_instance_init(Object *obj)
     {
         AState *A = DEVICE_A(obj);
         BState *B;
         /* create object B 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, object B
 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. This
 diagram describes the example with an input clock::

     +--------------------------+
     |        Device A          |
     |         +--------------+ |
     |         |   Device B   | |
     |         | +-------+    | |
     >>"b_clk">>>| "clk" |    | |
     |  (in)   | |  (in) |    | |
     |         | +-------+    | |
     |         +--------------+ |
     +--------------------------+

 Migration
 ---------

 Clock state is 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
 are two solutions.

 Clock states can be migrated by adding an entry into the device
 vmstate description. You should use the ``VMSTATE_CLOCK`` macro for this.
 This is typically used to migrate an input clock state. For example:

 .. code-block:: c

     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 clock states using the
 device state. The functions ``clock_set[_ns|_hz]()`` can be used during the
 ``post_load()`` migration callback.

 When adding clock support to an existing device, if you care about
 migration compatibility you will need to be careful, as simply adding
 a ``VMSTATE_CLOCK()`` line will break compatibility. Instead, you can
 put the ``VMSTATE_CLOCK()`` line into a vmstate subsection with a
 suitable ``needed`` function, and use ``clock_set()`` in a
 ``pre_load()`` function to set the default value that will be used if
 the source virtual machine in the migration does not send the clock
 state.

 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.
	Modelling a clock tree in QEMU
	==============================

	What are clocks?
	----------------

	Clocks are QOM objects developed for the purpose of modelling 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`` (in C code, the macro
	``TYPE_CLOCK``).

	Clocks are typically used with devices where they are used to model inputs
	and outputs. They are created in a similar way to GPIOs. Inputs and outputs
	of different devices can be connected 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 the ``include/hw/clock.h`` header and device
	related functions are defined in the ``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 units of 2 :sup:`-32` ns. 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. This allows
	the current period of a clock to be fetched 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. The suffixes are:

	- ``_ns`` for handling periods in nanoseconds
	- ``_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 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).
	Output is simpler; 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.

	.. code-block:: c

	/* device structure containing pointers 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
	* the clk_in field of a MyDeviceState structure with callback
	* clk_in_callback.
	* + a clock output named "clk_out" whose pointer is stored in
	* the 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 hierarchy 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()`` or ``clock_set_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:

	.. code-block:: c

	Clock *clk = qdev_get_clock_in(DEVICE(mydev), "clk_in");

	or:

	.. code-block:: c

	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);``
	configures ``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 to 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:

	.. code-block:: c

	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 get 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 connected.

	Unconnected input clocks
	------------------------

	A newly created input clock is disabled (period of 0). This 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 desired behaviour, ``clock_set()``,
	``clock_set_ns()`` or ``clock_set_hz()`` should be called on the Clock
	object during device instance init. For example:

	.. code-block:: c

	clk = qdev_init_clock_in(DEVICE(dev), "clk-in", clk_in_callback,
	dev);
	/* set initial value to 10ns / 100MHz */
	clock_set_ns(clk, 10);

	To enforce that the clock is wired up by the board code, you can
	call ``clock_has_source()`` in your device's realize method:

	.. code-block:: c

	if (!clock_has_source(s->clk)) {
	error_setg(errp, "MyDevice: clk input must be connected");
	return;
	}

	Note that this only checks that the clock has been wired up; it is
	still possible that the output clock connected to it is disabled
	or has not yet been configured, in which case the period will be
	zero. You should use the clock callback to find out when the clock
	period changes.

	Fetching clock frequency/period
	-------------------------------

	To get the current state of a clock, use the functions ``clock_get()``
	or ``clock_get_hz()``.

	``clock_get()`` returns the period of the clock in its fully precise
	internal representation, as an unsigned 64-bit integer in units of
	2^-32 nanoseconds. (For many purposes ``clock_ticks_to_ns()`` will
	be more convenient; see the section below on expiry deadlines.)

	``clock_get_hz()`` returns the frequency of the clock, rounded to the
	next lowest integer. This implies some inaccuracy due to the rounding,
	so be cautious about using it in calculations.

	It is also possible to register a callback on clock frequency changes.
	Here is an example:

	.. code-block:: c

	void clock_callback(void *opaque) {
	MyDeviceState s = (MyDeviceState ) opaque;
	/*
	* 'opaque' is the argument passed to qdev_init_clock_in();
	* usually this will be the device state pointer.
	*/

	/* do something with the new period */
	fprintf(stdout, "device new period is %" PRIu64 "* 2^-32 ns\n",
	clock_get(dev->my_clk_input));
	}

	If you are only interested in the frequency for displaying it to
	humans (for instance in debugging), use ``clock_display_freq()``,
	which returns a prettified string-representation, e.g. "33.3 MHz".
	The caller must free the string with g_free() after use.

	Calculating expiry deadlines
	----------------------------

	A commonly required operation for a clock is to calculate how long
	it will take for the clock to tick N times; this can then be used
	to set a timer expiry deadline. Use the function ``clock_ticks_to_ns()``,
	which takes an unsigned 64-bit count of ticks and returns the length
	of time in nanoseconds required for the clock to tick that many times.

	It is important not to try to calculate expiry deadlines using a
	shortcut like multiplying a "period of clock in nanoseconds" value
	by the tick count, because clocks can have periods which are not a
	whole number of nanoseconds, and the accumulated error in the
	multiplication can be significant.

	For a clock with a very long period and a large number of ticks,
	the result of this function could in theory be too large to fit in
	a 64-bit value. To avoid overflow in this case, ``clock_ticks_to_ns()``
	saturates the result to INT64_MAX (because this is the largest valid
	input to the QEMUTimer APIs). Since INT64_MAX nanoseconds is almost
	300 years, anything with an expiry later than that is in the "will
	never happen" category. Callers of ``clock_ticks_to_ns()`` should
	therefore generally not special-case the possibility of a saturated
	result but just allow the timer to be set to that far-future value.
	(If you are performing further calculations on the returned value
	rather than simply passing it to a QEMUTimer function like
	``timer_mod_ns()`` then you should be careful to avoid overflow
	in those calculations, of course.)

	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:

	.. code-block:: c

	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:

	.. code-block:: c

	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 update 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 calling
	``clock_set[_ns\|_hz]()`` (with the same arguments) then
	``clock_propagate()`` on the clock. Thus, setting the clock value can
	be 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.
	``qdev_alias_clock()`` works for both input and output clocks.

	For example, if device B is a child of device A,
	``device_a_instance_init()`` may do something like this:

	.. code-block:: c

	void device_a_instance_init(Object *obj)
	{
	AState *A = DEVICE_A(obj);
	BState *B;
	/* create object B 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, object B
	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. This
	diagram describes the example with an input clock::

	+--------------------------+
	\| Device A \|
	\| +--------------+ \|
	\| \| Device B \| \|
	\| \| +-------+ \| \|
	>>"b_clk">>>\| "clk" \| \| \|
	\| (in) \| \| (in) \| \| \|
	\| \| +-------+ \| \|
	\| +--------------+ \|
	+--------------------------+

	Migration
	---------

	Clock state is 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
	are two solutions.

	Clock states can be migrated by adding an entry into the device
	vmstate description. You should use the ``VMSTATE_CLOCK`` macro for this.
	This is typically used to migrate an input clock state. For example:

	.. code-block:: c

	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 clock states using the
	device state. The functions ``clock_set[_ns\|_hz]()`` can be used during the
	``post_load()`` migration callback.

	When adding clock support to an existing device, if you care about
	migration compatibility you will need to be careful, as simply adding
	a ``VMSTATE_CLOCK()`` line will break compatibility. Instead, you can
	put the ``VMSTATE_CLOCK()`` line into a vmstate subsection with a
	suitable ``needed`` function, and use ``clock_set()`` in a
	``pre_load()`` function to set the default value that will be used if
	the source virtual machine in the migration does not send the clock
	state.

	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.