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 -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.

/* 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),

/* 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:

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


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:

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:

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

Fetching clock frequency/period

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

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' 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 "ns\n",

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 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:

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) |    | |
|         | +-------+    | |
|         +--------------+ |


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:

MyDeviceState {
    DeviceState parent_obj;
    [...] /* some fields */
    Clock *clk;

VMStateDescription my_device_vmstate = {
    .name = "my_device",
    .fields = (VMStateField[]) {
        [...], /* other migrated fields */
        VMSTATE_CLOCK(clk, MyDeviceState),

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.