From: KONRAD Frederic <fred.kon...@greensocs.com> This adds the qemu-clock documentation.
Signed-off-by: KONRAD Frederic <fred.kon...@greensocs.com> V1 -> V2: * Fixed in accordance with the changes in the previous patches. --- docs/clock.txt | 278 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 278 insertions(+) create mode 100644 docs/clock.txt diff --git a/docs/clock.txt b/docs/clock.txt new file mode 100644 index 0000000..010ae50 --- /dev/null +++ b/docs/clock.txt @@ -0,0 +1,278 @@ + +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); +} -- 1.8.3.1