1Modelling a clock tree in QEMU 2============================== 3 4What are clocks? 5---------------- 6 7Clocks are QOM objects developed for the purpose of modelling the 8distribution of clocks in QEMU. 9 10They allow us to model the clock distribution of a platform and detect 11configuration errors in the clock tree such as badly configured PLL, clock 12source selection or disabled clock. 13 14The object is *Clock* and its QOM name is ``clock`` (in C code, the macro 15``TYPE_CLOCK``). 16 17Clocks are typically used with devices where they are used to model inputs 18and outputs. They are created in a similar way to GPIOs. Inputs and outputs 19of different devices can be connected together. 20 21In these cases a Clock object is a child of a Device object, but this 22is not a requirement. Clocks can be independent of devices. For 23example it is possible to create a clock outside of any device to 24model the main clock source of a machine. 25 26Here is an example of clocks:: 27 28 +---------+ +----------------------+ +--------------+ 29 | Clock 1 | | Device B | | Device C | 30 | | | +-------+ +-------+ | | +-------+ | 31 | |>>-+-->>|Clock 2| |Clock 3|>>--->>|Clock 6| | 32 +---------+ | | | (in) | | (out) | | | | (in) | | 33 | | +-------+ +-------+ | | +-------+ | 34 | | +-------+ | +--------------+ 35 | | |Clock 4|>> 36 | | | (out) | | +--------------+ 37 | | +-------+ | | Device D | 38 | | +-------+ | | +-------+ | 39 | | |Clock 5|>>--->>|Clock 7| | 40 | | | (out) | | | | (in) | | 41 | | +-------+ | | +-------+ | 42 | +----------------------+ | | 43 | | +-------+ | 44 +----------------------------->>|Clock 8| | 45 | | (in) | | 46 | +-------+ | 47 +--------------+ 48 49Clocks are defined in the ``include/hw/clock.h`` header and device 50related functions are defined in the ``include/hw/qdev-clock.h`` 51header. 52 53The clock state 54--------------- 55 56The state of a clock is its period; it is stored as an integer 57representing it in units of 2 :sup:`-32` ns. The special value of 0 is used to 58represent the clock being inactive or gated. The clocks do not model 59the signal itself (pin toggling) or other properties such as the duty 60cycle. 61 62All clocks contain this state: outputs as well as inputs. This allows 63the current period of a clock to be fetched at any time. When a clock 64is updated, the value is immediately propagated to all connected 65clocks in the tree. 66 67To ease interaction with clocks, helpers with a unit suffix are defined for 68every clock state setter or getter. The suffixes are: 69 70- ``_ns`` for handling periods in nanoseconds 71- ``_hz`` for handling frequencies in hertz 72 73The 0 period value is converted to 0 in hertz and vice versa. 0 always means 74that the clock is disabled. 75 76Adding a new clock 77------------------ 78 79Adding clocks to a device must be done during the init method of the Device 80instance. 81 82To add an input clock to a device, the function ``qdev_init_clock_in()`` 83must be used. It takes the name, a callback and an opaque parameter 84for the callback (this will be explained in a following section). 85Output is simpler; only the name is required. Typically:: 86 87 qdev_init_clock_in(DEVICE(dev), "clk_in", clk_in_callback, dev); 88 qdev_init_clock_out(DEVICE(dev), "clk_out"); 89 90Both functions return the created Clock pointer, which should be saved in the 91device's state structure for further use. 92 93These objects will be automatically deleted by the QOM reference mechanism. 94 95Note that it is possible to create a static array describing clock inputs and 96outputs. The function ``qdev_init_clocks()`` must be called with the array as 97parameter to initialize the clocks: it has the same behaviour as calling the 98``qdev_init_clock_in/out()`` for each clock in the array. To ease the array 99construction, some macros are defined in ``include/hw/qdev-clock.h``. 100As an example, the following creates 2 clocks to a device: one input and one 101output. 102 103.. code-block:: c 104 105 /* device structure containing pointers to the clock objects */ 106 typedef struct MyDeviceState { 107 DeviceState parent_obj; 108 Clock *clk_in; 109 Clock *clk_out; 110 } MyDeviceState; 111 112 /* 113 * callback for the input clock (see "Callback on input clock 114 * change" section below for more information). 115 */ 116 static void clk_in_callback(void *opaque); 117 118 /* 119 * static array describing clocks: 120 * + a clock input named "clk_in", whose pointer is stored in 121 * the clk_in field of a MyDeviceState structure with callback 122 * clk_in_callback. 123 * + a clock output named "clk_out" whose pointer is stored in 124 * the clk_out field of a MyDeviceState structure. 125 */ 126 static const ClockPortInitArray mydev_clocks = { 127 QDEV_CLOCK_IN(MyDeviceState, clk_in, clk_in_callback), 128 QDEV_CLOCK_OUT(MyDeviceState, clk_out), 129 QDEV_CLOCK_END 130 }; 131 132 /* device initialization function */ 133 static void mydev_init(Object *obj) 134 { 135 /* cast to MyDeviceState */ 136 MyDeviceState *mydev = MYDEVICE(obj); 137 /* create and fill the pointer fields in the MyDeviceState */ 138 qdev_init_clocks(mydev, mydev_clocks); 139 [...] 140 } 141 142An alternative way to create a clock is to simply call 143``object_new(TYPE_CLOCK)``. In that case the clock will neither be an 144input nor an output of a device. After the whole QOM hierarchy of the 145clock has been set ``clock_setup_canonical_path()`` should be called. 146 147At creation, the period of the clock is 0: the clock is disabled. You can 148change it using ``clock_set_ns()`` or ``clock_set_hz()``. 149 150Note that if you are creating a clock with a fixed period which will never 151change (for example the main clock source of a board), then you'll have 152nothing else to do. This value will be propagated to other clocks when 153connecting the clocks together and devices will fetch the right value during 154the first reset. 155 156Retrieving clocks from a device 157------------------------------- 158 159``qdev_get_clock_in()`` and ``dev_get_clock_out()`` are available to 160get the clock inputs or outputs of a device. For example: 161 162.. code-block:: c 163 164 Clock *clk = qdev_get_clock_in(DEVICE(mydev), "clk_in"); 165 166or: 167 168.. code-block:: c 169 170 Clock *clk = qdev_get_clock_out(DEVICE(mydev), "clk_out"); 171 172Connecting two clocks together 173------------------------------ 174 175To connect two clocks together, use the ``clock_set_source()`` function. 176Given two clocks ``clk1``, and ``clk2``, ``clock_set_source(clk2, clk1);`` 177configures ``clk2`` to follow the ``clk1`` period changes. Every time ``clk1`` 178is updated, ``clk2`` will be updated too. 179 180When connecting clock between devices, prefer using the 181``qdev_connect_clock_in()`` function to set the source of an input 182device clock. For example, to connect the input clock ``clk2`` of 183``devB`` to the output clock ``clk1`` of ``devA``, do: 184 185.. code-block:: c 186 187 qdev_connect_clock_in(devB, "clk2", qdev_get_clock_out(devA, "clk1")) 188 189We used ``qdev_get_clock_out()`` above, but any clock can drive an 190input clock, even another input clock. The following diagram shows 191some examples of connections. Note also that a clock can drive several 192other clocks. 193 194:: 195 196 +------------+ +--------------------------------------------------+ 197 | Device A | | Device B | 198 | | | +---------------------+ | 199 | | | | Device C | | 200 | +-------+ | | +-------+ | +-------+ +-------+ | +-------+ | 201 | |Clock 1|>>-->>|Clock 2|>>+-->>|Clock 3| |Clock 5|>>>>|Clock 6|>> 202 | | (out) | | | | (in) | | | | (in) | | (out) | | | (out) | | 203 | +-------+ | | +-------+ | | +-------+ +-------+ | +-------+ | 204 +------------+ | | +---------------------+ | 205 | | | 206 | | +--------------+ | 207 | | | Device D | | 208 | | | +-------+ | | 209 | +-->>|Clock 4| | | 210 | | | (in) | | | 211 | | +-------+ | | 212 | +--------------+ | 213 +--------------------------------------------------+ 214 215In the above example, when *Clock 1* is updated by *Device A*, three 216clocks get the new clock period value: *Clock 2*, *Clock 3* and *Clock 4*. 217 218It is not possible to disconnect a clock or to change the clock connection 219after it is connected. 220 221Unconnected input clocks 222------------------------ 223 224A newly created input clock is disabled (period of 0). This means the 225clock will be considered as disabled until the period is updated. If 226the clock remains unconnected it will always keep its initial value 227of 0. If this is not the desired behaviour, ``clock_set()``, 228``clock_set_ns()`` or ``clock_set_hz()`` should be called on the Clock 229object during device instance init. For example: 230 231.. code-block:: c 232 233 clk = qdev_init_clock_in(DEVICE(dev), "clk-in", clk_in_callback, 234 dev); 235 /* set initial value to 10ns / 100MHz */ 236 clock_set_ns(clk, 10); 237 238Fetching clock frequency/period 239------------------------------- 240 241To get the current state of a clock, use the functions ``clock_get()``, 242``clock_get_ns()`` or ``clock_get_hz()``. 243 244It is also possible to register a callback on clock frequency changes. 245Here is an example: 246 247.. code-block:: c 248 249 void clock_callback(void *opaque) { 250 MyDeviceState *s = (MyDeviceState *) opaque; 251 /* 252 * 'opaque' is the argument passed to qdev_init_clock_in(); 253 * usually this will be the device state pointer. 254 */ 255 256 /* do something with the new period */ 257 fprintf(stdout, "device new period is %" PRIu64 "ns\n", 258 clock_get_ns(dev->my_clk_input)); 259 } 260 261Changing a clock period 262----------------------- 263 264A device can change its outputs using the ``clock_update()``, 265``clock_update_ns()`` or ``clock_update_hz()`` function. It will trigger 266updates on every connected input. 267 268For example, let's say that we have an output clock *clkout* and we 269have a pointer to it in the device state because we did the following 270in init phase: 271 272.. code-block:: c 273 274 dev->clkout = qdev_init_clock_out(DEVICE(dev), "clkout"); 275 276Then at any time (apart from the cases listed below), it is possible to 277change the clock value by doing: 278 279.. code-block:: c 280 281 clock_update_hz(dev->clkout, 1000 * 1000 * 1000); /* 1GHz */ 282 283Because updating a clock may trigger any side effects through 284connected clocks and their callbacks, this operation must be done 285while holding the qemu io lock. 286 287For the same reason, one can update clocks only when it is allowed to have 288side effects on other objects. In consequence, it is forbidden: 289 290* during migration, 291* and in the enter phase of reset. 292 293Note that calling ``clock_update[_ns|_hz]()`` is equivalent to calling 294``clock_set[_ns|_hz]()`` (with the same arguments) then 295``clock_propagate()`` on the clock. Thus, setting the clock value can 296be separated from triggering the side-effects. This is often required 297to factorize code to handle reset and migration in devices. 298 299Aliasing clocks 300--------------- 301 302Sometimes, one needs to forward, or inherit, a clock from another 303device. Typically, when doing device composition, a device might 304expose a sub-device's clock without interfering with it. The function 305``qdev_alias_clock()`` can be used to achieve this behaviour. Note 306that it is possible to expose the clock under a different name. 307``qdev_alias_clock()`` works for both input and output clocks. 308 309For example, if device B is a child of device A, 310``device_a_instance_init()`` may do something like this: 311 312.. code-block:: c 313 314 void device_a_instance_init(Object *obj) 315 { 316 AState *A = DEVICE_A(obj); 317 BState *B; 318 /* create object B as child of A */ 319 [...] 320 qdev_alias_clock(B, "clk", A, "b_clk"); 321 /* 322 * Now A has a clock "b_clk" which is an alias to 323 * the clock "clk" of its child B. 324 */ 325 } 326 327This function does not return any clock object. The new clock has the 328same direction (input or output) as the original one. This function 329only adds a link to the existing clock. In the above example, object B 330remains the only object allowed to use the clock and device A must not 331try to change the clock period or set a callback to the clock. This 332diagram describes the example with an input clock:: 333 334 +--------------------------+ 335 | Device A | 336 | +--------------+ | 337 | | Device B | | 338 | | +-------+ | | 339 >>"b_clk">>>| "clk" | | | 340 | (in) | | (in) | | | 341 | | +-------+ | | 342 | +--------------+ | 343 +--------------------------+ 344 345Migration 346--------- 347 348Clock state is not migrated automatically. Every device must handle its 349clock migration. Alias clocks must not be migrated. 350 351To ensure clock states are restored correctly during migration, there 352are two solutions. 353 354Clock states can be migrated by adding an entry into the device 355vmstate description. You should use the ``VMSTATE_CLOCK`` macro for this. 356This is typically used to migrate an input clock state. For example: 357 358.. code-block:: c 359 360 MyDeviceState { 361 DeviceState parent_obj; 362 [...] /* some fields */ 363 Clock *clk; 364 }; 365 366 VMStateDescription my_device_vmstate = { 367 .name = "my_device", 368 .fields = (VMStateField[]) { 369 [...], /* other migrated fields */ 370 VMSTATE_CLOCK(clk, MyDeviceState), 371 VMSTATE_END_OF_LIST() 372 } 373 }; 374 375The second solution is to restore the clock state using information already 376at our disposal. This can be used to restore output clock states using the 377device state. The functions ``clock_set[_ns|_hz]()`` can be used during the 378``post_load()`` migration callback. 379 380When adding clock support to an existing device, if you care about 381migration compatibility you will need to be careful, as simply adding 382a ``VMSTATE_CLOCK()`` line will break compatibility. Instead, you can 383put the ``VMSTATE_CLOCK()`` line into a vmstate subsection with a 384suitable ``needed`` function, and use ``clock_set()`` in a 385``pre_load()`` function to set the default value that will be used if 386the source virtual machine in the migration does not send the clock 387state. 388 389Care should be taken not to use ``clock_update[_ns|_hz]()`` or 390``clock_propagate()`` during the whole migration procedure because it 391will trigger side effects to other devices in an unknown state. 392