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:
_nsfor handling periods in nanoseconds_hzfor 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, an opaque parameter
for the callback and a mask of events when the callback should be
called (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, ClockUpdate);
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, ClockEvent event);
/*
* 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, ClockUpdate),
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.
Clock callbacks¶
You can give a clock a callback function in several ways:
by passing it as an argument to
qdev_init_clock_in()as an argument to the
QDEV_CLOCK_IN()macro initializing an array to be passed toqdev_init_clocks()by directly calling the
clock_set_callback()function
The callback function must be of this type:
typedef void ClockCallback(void *opaque, ClockEvent event);
The opaque argument is the pointer passed to qdev_init_clock_in()
or clock_set_callback(); for qdev_init_clocks() it is the
dev device pointer.
The event argument specifies why the callback has been called.
When you register the callback you specify a mask of ClockEvent values
that you are interested in. The callback will only be called for those
events.
The events currently supported are:
ClockPreUpdate: called when the input clock’s period is about to update. This is useful if the device needs to do some action for which it needs to know the old value of the clock period. During this callback, Clock API functions likeclock_get()orclock_ticks_to_ns()will use the old period.
ClockUpdate: called after the input clock’s period has changed. During this callback, Clock API functions likeclock_ticks_to_ns()will use the new period.
Note that a clock only has one callback: it is not possible to register
different functions for different events. You must register a single
callback which listens for all of the events you are interested in,
and use the event argument to identify which event has happened.
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);
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,
dev, ClockUpdate);
/* 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:
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, which assumes that clock_callback has been
specified as the callback for the ClockUpdate event:
void clock_callback(void *opaque, ClockEvent event) {
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.)
Obtaining tick counts¶
For calculations where you need to know the number of ticks in
a given duration, use clock_ns_to_ticks(). This function handles
possible non-whole-number-of-nanoseconds periods and avoids
potential rounding errors. It will return ‘0’ if the clock is stopped
(i.e. it has period zero). If the inputs imply a tick count that
overflows a 64-bit value (a very long duration for a clock with a
very short period) the output value is truncated, so effectively
the 64-bit output wraps around.
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) | | |
| | +-------+ | |
| +--------------+ |
+--------------------------+
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:
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.