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+
+What is a QEMU_CLOCK
+====================
+
+A QEMU_CLOCK is a QOM Object developed for the purpose of modeling a clock tree
+with QEMU.
+
+It only simulates the clock by keeping a copy of the current frequency and
+doesn't model the signal itself such as pin toggle or duty cycle.
+
+It allows to model the impact of badly configured PLL, clock source selection
+or disabled clock on the models.
+
+Binding the clock together to create a tree
+===========================================
+
+In order to create a clock tree with QEMU_CLOCK two or more clock must be bound
+together. Let's say there are two clocks clk_a and clk_b:
+Using qemu_clk_bind(clk_a, clk_b) will bind clk_a and clk_b.
+
+Binding two qemu-clk together creates a unidirectional link which means that
+changing the rate of clk_a will propagate to clk_b and not the opposite.
+The binding process automatically refreshes clk_b rate.
+
+Clock can be bound and unbound during execution for modeling eg: a clock
+selector.
+
+A clock can drive more than one other clock. eg with this code:
+qemu_clk_bind(clk_a, clk_b);
+qemu_clk_bind(clk_a, clk_c);
+
+A clock rate change one clk_a will propagate to clk_b and clk_c.
+
+Implementing a callback on a rate change
+========================================
+
+The function prototype is the following:
+typedef uint64_t QEMUClkRateUpdateCallback(void *opaque, uint64_t rate);
+
+It's main goal is to modify the rate before it's passed to the next clocks in
+the tree.
+
+eg: for a 4x PLL the function will be:
+uint64_t qemu_clk_rate_change_cb(void *opaque, uint64_t rate)
+{
+    return 4 * rate;
+}
+
+To set the callback for the clock:
+void qemu_clk_set_callback(qemu_clk clk, QEMUClkRateUpdateCallback *cb,
+                           void *opaque);
+can be called.
+
+The rate update process
+=======================
+
+The rate update happen in this way:
+When a model wants to update a clock frequency (eg: based on a register change
+or something similar) it will call qemu_clk_update_rate(..) on the clock:
+  * The callback associated to the clock is called with the new rate.
+  * qemu_clk_update_rate(..) is then called on all bound clocks with the value
+    returned by the callback.
+
+NOTE: When no callback is attached, the clock qemu_clk_update_rate(..) is called
+on the next clock in the tree with the rate unmodified.
+
+Adding a QEMU_CLOCK to a DeviceState
+====================================
+
+Adding a qemu-clk to a DeviceState is required to be able to get the clock
+outside the model through qemu_clk_device_get_clock(..).
+
+It is also required to be able to print the clock and its rate with info qtree.
+For example:
+
+  type System
+  dev: xlnx.zynqmp_crf, id ""
+    gpio-out "sysbus-irq" 1
+    gpio-out "RST_A9" 4
+    qemu-clk "dbg_trace" 0
+    qemu-clk "vpll_to_lpd" 625000000
+    qemu-clk "dp_stc_ref" 0
+    qemu-clk "dpll_to_lpd" 12500000
+    qemu-clk "acpu_clk" 0
+    qemu-clk "pcie_ref" 0
+    qemu-clk "topsw_main" 0
+    qemu-clk "topsw_lsbus" 0
+    qemu-clk "dp_audio_ref" 0
+    qemu-clk "sata_ref" 0
+    qemu-clk "dp_video_ref" 71428568
+    qemu-clk "vpll_clk" 2500000000
+    qemu-clk "apll_to_lpd" 12500000
+    qemu-clk "dpll_clk" 50000000
+    qemu-clk "gpu_ref" 0
+    qemu-clk "aux_refclk" 0
+    qemu-clk "video_clk" 27000000
+    qemu-clk "gdma_ref" 0
+    qemu-clk "gt_crx_ref_clk" 0
+    qemu-clk "dbg_fdp" 0
+    qemu-clk "apll_clk" 50000000
+    qemu-clk "pss_alt_ref_clk" 0
+    qemu-clk "ddr" 0
+    qemu-clk "pss_ref_clk" 50000000
+    qemu-clk "dpdma_ref" 0
+    qemu-clk "dbg_tstmp" 0
+    mmio 00000000fd1a0000/000000000000010c
+
+This way a DeviceState can have multiple clock input or output.
+
+Examples
+========
+
+Those are the different way of using the QEMUClock object.
+
+Modelling a fixed clock generator
+=================================
+
+Here is a brief example of a device acting as a clock source:
+
+typedef struct {
+    DeviceState parent_obj;
+
+    uint32_t rate;
+    QEMUClock out;
+} FixedClock;
+
+During the initialization the device must initialize its clock object:
+
+static void fixed_clock_instance_init(Object *obj)
+{
+    FixedClock *s = FIXED_CLOCK(obj);
+
+    object_initialize(&s->out, sizeof(s->out), TYPE_CLOCK);
+    qemu_clk_device_add_clock(DEVICE(obj), &s->out, "clk_out");
+}
+
+As the device acts as a clock source it must refresh the clock tree during the
+realize phase:
+
+static void fixed_clock_realizefn(DeviceState *dev, Error **errp)
+{
+    FixedClock *s = FIXED_CLOCK(dev);
+
+    qemu_clk_update_rate(&s->out, s->rate);
+}
+
+This means that the clock tree must be finished before realize is called on the
+fixed clock.
+
+Modelling a clock user device
+=============================
+
+Here is a brief example of a clock user:
+
+typedef struct {
+    DeviceState parent_obj;
+
+    QEMUClock in;
+} ClockUser;
+
+As before the clock must be initialized through the device initialize function:
+
+static void clock_user_instance_init(Object *obj)
+{
+    ClockUser *s = CLOCK_USER(obj);
+
+    object_initialize(&s->in, sizeof(s->in), TYPE_CLOCK);
+    qemu_clk_device_add_clock(DEVICE(obj), &s->in, "clk_in");
+    /*
+     * Call on_rate_change_cb when something change on clk_in.
+     */
+    qemu_clk_set_callback(s->in, on_rate_change_cb, obj);
+}
+
+The callback is in this case used as a notifier when the clock tree which
+sources the device change:
+
+static uint64_t on_rate_change_cb(void *opaque, uint64_t input_rate)
+{
+    printf("the new rate is %ld\n", input_rate);
+
+    /* The return is ignored if nothing is bound to clk_in. */
+    return input_rate;
+}
+
+Modelling a clock multiplier
+============================
+
+Here is a brief example of a device acting as a clock modifier:
+
+typedef struct {
+    DeviceState parent_obj;
+
+    uint32_t rate;
+    QEMUClock out;
+    QEMUClock in;
+} ClockMultiplier;
+
+As before the clocks must be initialized through the device initialize function
+but they must be bound together so a change on the input will propagate on the
+output:
+
+static void clock_multiplier_instance_init(Object *obj)
+{
+    ClockMultiplier *s = CLOCK_MULTIPLIER(obj);
+
+    object_initialize(&s->out, sizeof(s->out), TYPE_CLOCK);
+    object_initialize(&s->in, sizeof(s->in), TYPE_CLOCK);
+
+    qemu_clk_device_add_clock(DEVICE(obj), &s->in, "clk_in");
+    qemu_clk_device_add_clock(DEVICE(obj), &s->out, "clk_out");
+    /*
+     * Propagate the change from in to out, this can be done dynamically during
+     * the simulation but we need to do the initial binding here to get the
+     * initial refresh happening when the realize function is called on the
+     * fixed clock.
+     */
+    qemu_clk_bind(s->in, s->out);
+    /*
+     * But before propagating the rate modify it with multiplier_cb.
+     */
+    qemu_clk_set_callback(s->out, multiplier_cb, obj);
+}
+
+In this example when clk_in changes it will trigger a change on clk_out and this
+callback can modify the rate of clk_out (the return value of this callback)
+accordingly to clk_in rate (input_rate).
+
+static uint64_t multiplier_cb(void *opaque, uint64_t input_rate)
+{
+    return input_rate * 4;
+}
+
+This device doesn't refresh the clock tree as it will be done by the clock tree
+source.
+
+Modelling a clock selector
+==========================
+
+Here is a brief example of a device acting as a device which alter the clock
+topology such as a clock selector:
+
+typedef struct {
+    DeviceState parent_obj;
+
+    QEMUClock out;
+    QEMUClock in_a;
+    QEMUClock in_b;
+} ClockSelector;
+
+The clocks must be initialized through the device initialize function but they
+must be bound together like they will be when the device is reset so a change on
+the input during the realize of the fixed clock will propagate to the output:
+
+static void clock_selector_instance_init(Object *obj)
+{
+    ClockSelector *s = CLOCK_SELECTOR(obj);
+
+    object_initialize(&s->out, sizeof(s->out), TYPE_CLOCK);
+    object_initialize(&s->in_a, sizeof(s->in_a), TYPE_CLOCK);
+    object_initialize(&s->in_b, sizeof(s->in_b), TYPE_CLOCK);
+
+    qemu_clk_device_add_clock(DEVICE(obj), &s->in_a, "clk_in_a");
+    qemu_clk_device_add_clock(DEVICE(obj), &s->in_b, "clk_in_b");
+    qemu_clk_device_add_clock(DEVICE(obj), &s->out, "clk_out");
+
+    /* Assuming at the reset that the input_a is connected to output. */
+    qemu_clk_bind(s->in_a, s->out);
+}
+
+/* This is called for example by a register change or something like that */
+void update_topology(ClockSelector *s)
+{
+    /* Unbind the old clock */
+    qemu_clk_unbind(s->in_a, s->out);
+    /* Bind the new one, the rate is automatically refreshed. */
+    qemu_clk_bind(s->in_b, s->out);
+}
