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 238To enforce that the clock is wired up by the board code, you can 239call ``clock_has_source()`` in your device's realize method: 240 241.. code-block:: c 242 243 if (!clock_has_source(s->clk)) { 244 error_setg(errp, "MyDevice: clk input must be connected"); 245 return; 246 } 247 248Note that this only checks that the clock has been wired up; it is 249still possible that the output clock connected to it is disabled 250or has not yet been configured, in which case the period will be 251zero. You should use the clock callback to find out when the clock 252period changes. 253 254Fetching clock frequency/period 255------------------------------- 256 257To get the current state of a clock, use the functions ``clock_get()`` 258or ``clock_get_hz()``. 259 260``clock_get()`` returns the period of the clock in its fully precise 261internal representation, as an unsigned 64-bit integer in units of 2622^-32 nanoseconds. (For many purposes ``clock_ticks_to_ns()`` will 263be more convenient; see the section below on expiry deadlines.) 264 265``clock_get_hz()`` returns the frequency of the clock, rounded to the 266next lowest integer. This implies some inaccuracy due to the rounding, 267so be cautious about using it in calculations. 268 269It is also possible to register a callback on clock frequency changes. 270Here is an example: 271 272.. code-block:: c 273 274 void clock_callback(void *opaque) { 275 MyDeviceState *s = (MyDeviceState *) opaque; 276 /* 277 * 'opaque' is the argument passed to qdev_init_clock_in(); 278 * usually this will be the device state pointer. 279 */ 280 281 /* do something with the new period */ 282 fprintf(stdout, "device new period is %" PRIu64 "* 2^-32 ns\n", 283 clock_get(dev->my_clk_input)); 284 } 285 286If you are only interested in the frequency for displaying it to 287humans (for instance in debugging), use ``clock_display_freq()``, 288which returns a prettified string-representation, e.g. "33.3 MHz". 289The caller must free the string with g_free() after use. 290 291Calculating expiry deadlines 292---------------------------- 293 294A commonly required operation for a clock is to calculate how long 295it will take for the clock to tick N times; this can then be used 296to set a timer expiry deadline. Use the function ``clock_ticks_to_ns()``, 297which takes an unsigned 64-bit count of ticks and returns the length 298of time in nanoseconds required for the clock to tick that many times. 299 300It is important not to try to calculate expiry deadlines using a 301shortcut like multiplying a "period of clock in nanoseconds" value 302by the tick count, because clocks can have periods which are not a 303whole number of nanoseconds, and the accumulated error in the 304multiplication can be significant. 305 306For a clock with a very long period and a large number of ticks, 307the result of this function could in theory be too large to fit in 308a 64-bit value. To avoid overflow in this case, ``clock_ticks_to_ns()`` 309saturates the result to INT64_MAX (because this is the largest valid 310input to the QEMUTimer APIs). Since INT64_MAX nanoseconds is almost 311300 years, anything with an expiry later than that is in the "will 312never happen" category. Callers of ``clock_ticks_to_ns()`` should 313therefore generally not special-case the possibility of a saturated 314result but just allow the timer to be set to that far-future value. 315(If you are performing further calculations on the returned value 316rather than simply passing it to a QEMUTimer function like 317``timer_mod_ns()`` then you should be careful to avoid overflow 318in those calculations, of course.) 319 320Changing a clock period 321----------------------- 322 323A device can change its outputs using the ``clock_update()``, 324``clock_update_ns()`` or ``clock_update_hz()`` function. It will trigger 325updates on every connected input. 326 327For example, let's say that we have an output clock *clkout* and we 328have a pointer to it in the device state because we did the following 329in init phase: 330 331.. code-block:: c 332 333 dev->clkout = qdev_init_clock_out(DEVICE(dev), "clkout"); 334 335Then at any time (apart from the cases listed below), it is possible to 336change the clock value by doing: 337 338.. code-block:: c 339 340 clock_update_hz(dev->clkout, 1000 * 1000 * 1000); /* 1GHz */ 341 342Because updating a clock may trigger any side effects through 343connected clocks and their callbacks, this operation must be done 344while holding the qemu io lock. 345 346For the same reason, one can update clocks only when it is allowed to have 347side effects on other objects. In consequence, it is forbidden: 348 349* during migration, 350* and in the enter phase of reset. 351 352Note that calling ``clock_update[_ns|_hz]()`` is equivalent to calling 353``clock_set[_ns|_hz]()`` (with the same arguments) then 354``clock_propagate()`` on the clock. Thus, setting the clock value can 355be separated from triggering the side-effects. This is often required 356to factorize code to handle reset and migration in devices. 357 358Aliasing clocks 359--------------- 360 361Sometimes, one needs to forward, or inherit, a clock from another 362device. Typically, when doing device composition, a device might 363expose a sub-device's clock without interfering with it. The function 364``qdev_alias_clock()`` can be used to achieve this behaviour. Note 365that it is possible to expose the clock under a different name. 366``qdev_alias_clock()`` works for both input and output clocks. 367 368For example, if device B is a child of device A, 369``device_a_instance_init()`` may do something like this: 370 371.. code-block:: c 372 373 void device_a_instance_init(Object *obj) 374 { 375 AState *A = DEVICE_A(obj); 376 BState *B; 377 /* create object B as child of A */ 378 [...] 379 qdev_alias_clock(B, "clk", A, "b_clk"); 380 /* 381 * Now A has a clock "b_clk" which is an alias to 382 * the clock "clk" of its child B. 383 */ 384 } 385 386This function does not return any clock object. The new clock has the 387same direction (input or output) as the original one. This function 388only adds a link to the existing clock. In the above example, object B 389remains the only object allowed to use the clock and device A must not 390try to change the clock period or set a callback to the clock. This 391diagram describes the example with an input clock:: 392 393 +--------------------------+ 394 | Device A | 395 | +--------------+ | 396 | | Device B | | 397 | | +-------+ | | 398 >>"b_clk">>>| "clk" | | | 399 | (in) | | (in) | | | 400 | | +-------+ | | 401 | +--------------+ | 402 +--------------------------+ 403 404Migration 405--------- 406 407Clock state is not migrated automatically. Every device must handle its 408clock migration. Alias clocks must not be migrated. 409 410To ensure clock states are restored correctly during migration, there 411are two solutions. 412 413Clock states can be migrated by adding an entry into the device 414vmstate description. You should use the ``VMSTATE_CLOCK`` macro for this. 415This is typically used to migrate an input clock state. For example: 416 417.. code-block:: c 418 419 MyDeviceState { 420 DeviceState parent_obj; 421 [...] /* some fields */ 422 Clock *clk; 423 }; 424 425 VMStateDescription my_device_vmstate = { 426 .name = "my_device", 427 .fields = (VMStateField[]) { 428 [...], /* other migrated fields */ 429 VMSTATE_CLOCK(clk, MyDeviceState), 430 VMSTATE_END_OF_LIST() 431 } 432 }; 433 434The second solution is to restore the clock state using information already 435at our disposal. This can be used to restore output clock states using the 436device state. The functions ``clock_set[_ns|_hz]()`` can be used during the 437``post_load()`` migration callback. 438 439When adding clock support to an existing device, if you care about 440migration compatibility you will need to be careful, as simply adding 441a ``VMSTATE_CLOCK()`` line will break compatibility. Instead, you can 442put the ``VMSTATE_CLOCK()`` line into a vmstate subsection with a 443suitable ``needed`` function, and use ``clock_set()`` in a 444``pre_load()`` function to set the default value that will be used if 445the source virtual machine in the migration does not send the clock 446state. 447 448Care should be taken not to use ``clock_update[_ns|_hz]()`` or 449``clock_propagate()`` during the whole migration procedure because it 450will trigger side effects to other devices in an unknown state. 451