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Re: [PATCH v8 5/9] docs/clocks: add device's clock documentation
From: |
Alistair Francis |
Subject: |
Re: [PATCH v8 5/9] docs/clocks: add device's clock documentation |
Date: |
Wed, 26 Feb 2020 13:56:34 -0800 |
On Tue, Feb 25, 2020 at 6:01 AM Damien Hedde <address@hidden> wrote:
>
> Add the documentation about the clock inputs and outputs in devices.
>
> This is based on the original work of Frederic Konrad.
>
> Signed-off-by: Damien Hedde <address@hidden>
> ---
>
> v8:
> + fix list indentation
> + reduce title size
>
> v7:
> + update ClockIn/Out types
> + switch to rst format
> ---
> docs/devel/clocks.rst | 360 ++++++++++++++++++++++++++++++++++++++++++
> docs/devel/index.rst | 1 +
> 2 files changed, 361 insertions(+)
> create mode 100644 docs/devel/clocks.rst
>
> diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst
> new file mode 100644
> index 0000000000..d3c0146bd2
> --- /dev/null
> +++ b/docs/devel/clocks.rst
> @@ -0,0 +1,360 @@
> +Modeling a clock tree in QEMU
> +=============================
> +
> +What are clocks
> +---------------
> +
> +Clocks are QOM objects developed for the purpose of modeling 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``.
> +
> +Clocks are typically used with devices where they are used to model inputs
> +and outputs. They are created in a similar way as gpios. Inputs and outputs
> +of different devices can be connect 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 include/hw/clock.h header and device related functions
> +are defined in 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 2^-32ns unit. 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. It allows to fetch
> +the current period of a clock 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. They are:
> +
> +- ``_ns`` for handling periods in nanosecond,
> +- ``_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 a 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 below).
> +Output is more simple, 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 pointer 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
> + * clk_in field of a MyDeviceState structure with callback
> + * clk_in_callback.
> + * + a clock output named "clk_out" whose pointer is stored in
> + * 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 hieracrhy 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|_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");
> +
> +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);``
> +configure ``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 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
> gets 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 done.
> +
> +Unconnected input clocks
> +------------------------
> +
> +A newly created input clock is disabled (period of 0). It 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
> +wanted 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);
> + /* set initial value to 10ns / 100MHz */
> + clock_set_ns(clk, 10);
> +
> +Fetching clock frequency/period
> +-------------------------------
> +
> +To get the current state of a clock, the function ``clock_get()``,
> +``clock_get_ns()`` or ``clock_get_hz()`` must be used.
> +
> +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 may not be the device state pointer, but most
> + * probably it is. (It depends on what is given to the
> + * qdev_init_clock_in function)
> + */
> +
> + /* do something with the new period */
> + fprintf(stdout, "device new period is %" PRIu64 "ns\n",
> + clock_get_ns(dev->my_clk_input));
> + }
> +
> +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 updates clocks only when it is allowed to have
s/updates/update/g
> +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 call
s/call/calling/g
> +``clock_set[_ns|_hz]()`` (with the same arguments) then
> ``clock_propagate()`` on
> +the clock. Thus, setting the clock value can separated from triggering the
can be separated
> +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. This works
> for
> +both inputs and outputs.
> +
> +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 B object 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, B object 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. Here follows a diagram describing the example
> +with an input clock::
> +
> + +--------------------------+
> + | Device A |
> + | +--------------+ |
> + | | Device B | |
> + | | +-------+ | |
> + >>"b_clk">>>| "clk" | | |
> + | (in) | | (in) | | |
> + | | +-------+ | |
> + | +--------------+ |
> + +--------------------------+
> +
> +Migration
> +---------
> +
> +Clock state are not migrated automatically. Every device must handle its
s/are/is/g
> +clock migration. Alias clocks must not be migrated.
> +
> +To ensure clock states are restored correctly during migration, there is two
> +solutions.
> +
> +Clocks states can be migrated by adding an entry into the device
> +vmstate description. To this purpose, the ``VMSTATE_CLOCK`` macro defines
> +such an entry and should be used. This is typically used to migrate an input
> +clock state. The following example describes it::
> +
> + 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 clocks states using the
> +device state. The functions ``clock_set[_ns|_hz]()`` can be used during
> +``post_load()`` migration callback.
during the ``post_load()`` migration callback.
Besides the small typos:
Reviewed-by: Alistair Francis <address@hidden>
Alistair
> +
> +When adding a clock support to an existing device, if you care about
> migration
> +compatibility. To this end, you can use ``clock_set()`` in a ``pre_load()``
> +function to setup a default value in case the source virtual machine does not
> +send the clock state. You may also need to use a vmstate subsection.
> +
> +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.
> diff --git a/docs/devel/index.rst b/docs/devel/index.rst
> index 4dc2ca8d71..f9c8c668ee 100644
> --- a/docs/devel/index.rst
> +++ b/docs/devel/index.rst
> @@ -25,3 +25,4 @@ Contents:
> tcg-plugins
> bitops
> reset
> + clocks
> --
> 2.25.1
>
>
- [PATCH v8 2/9] hw/core/clock-vmstate: define a vmstate entry for clock state, (continued)
- [PATCH v8 2/9] hw/core/clock-vmstate: define a vmstate entry for clock state, Damien Hedde, 2020/02/25
- [PATCH v8 4/9] qdev-clock: introduce an init array to ease the device construction, Damien Hedde, 2020/02/25
- [PATCH v8 3/9] qdev: add clock input&output support to devices., Damien Hedde, 2020/02/25
- [PATCH v8 9/9] qdev-monitor: print the device's clock with info qtree, Damien Hedde, 2020/02/25
- [PATCH v8 6/9] hw/misc/zynq_slcr: add clock generation for uarts, Damien Hedde, 2020/02/25
- [PATCH v8 7/9] hw/char/cadence_uart: add clock support, Damien Hedde, 2020/02/25
- [PATCH v8 8/9] hw/arm/xilinx_zynq: connect uart clocks to slcr, Damien Hedde, 2020/02/25
- [PATCH v8 5/9] docs/clocks: add device's clock documentation, Damien Hedde, 2020/02/25
- Re: [PATCH v8 5/9] docs/clocks: add device's clock documentation,
Alistair Francis <=