From 99abcbc7600c62c294e973db340adf6939932a93 Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Thu, 12 Aug 2021 10:33:40 +0100 Subject: clock: Provide builtin multiplier/divider It is quite common for a clock tree to involve possibly programmable clock multipliers or dividers, where the frequency of a clock is for instance divided by 8 to produce a slower clock to feed to a particular device. Currently we provide no convenient mechanism for modelling this. You can implement it by having an input Clock and an output Clock, and manually setting the period of the output clock in the period-changed callback of the input clock, but that's quite clunky. This patch adds support in the Clock objects themselves for setting a multiplier or divider. The effect of setting this on a clock is that when the clock's period is changed, all the children of the clock are set to period * multiplier / divider, rather than being set to the same period as the parent clock. Signed-off-by: Peter Maydell Reviewed-by: Alexandre Iooss Reviewed-by: Alistair Francis Reviewed-by: Philippe Mathieu-Daudé Reviewed-by: Luc Michel Reviewed-by: Damien Hedde Message-id: 20210812093356.1946-10-peter.maydell@linaro.org --- docs/devel/clocks.rst | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) (limited to 'docs/devel') diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst index 956bd147ea..675fbeb6ab 100644 --- a/docs/devel/clocks.rst +++ b/docs/devel/clocks.rst @@ -260,6 +260,29 @@ clocks get 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 connected. +Clock multiplier and divider settings +------------------------------------- + +By default, when clocks are connected together, the child +clocks run with the same period as their source (parent) clock. +The Clock API supports a built-in period multiplier/divider +mechanism so you can configure a clock to make its children +run at a different period from its own. If you call the +``clock_set_mul_div()`` function you can specify the clock's +multiplier and divider values. The children of that clock +will all run with a period of ``parent_period * multiplier / divider``. +For instance, if the clock has a frequency of 8MHz and you set its +multiplier to 2 and its divider to 3, the child clocks will run +at 12MHz. + +You can change the multiplier and divider of a clock at runtime, +so you can use this to model clock controller devices which +have guest-programmable frequency multipliers or dividers. + +Note that ``clock_set_mul_div()`` does not automatically call +``clock_propagate()``. If you make a runtime change to the +multiplier or divider you must call clock_propagate() yourself. + Unconnected input clocks ------------------------ -- cgit v1.2.3-55-g7522