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This patch adds rate change notification support for the parent clock; should that clock change, then we try to adjust the our prescaler in order to compensate (i.e. we adjust to still get the same timer frequency). This is loosely based on what it's done in timer-cadence-ttc. timer-sun51, mips-gic-timer and smp_twd.c also seem to look at their parent clock rate and to perform some kind of adjustment whenever needed. In this particular case we have only one single counter and prescaler for all clocksource, clockevent and timer_delay, and we just update it for all (i.e. we don't let it go and call clockevents_update_freq() to notify to the kernel that our rate has changed). Note that, there is apparently no other way to fixup things, because once we call register_current_timer_delay(), specifying the timer rate, it seems that that rate is not supposed to change ever. In order for this mechanism to work, we have to make assumptions about how much the initial clock is supposed to eventually decrease from the initial one, and set our initial prescaler to a value that we can eventually decrease enough to compensate. We provide an option in KConfig for this. In case we end up in a situation in which we are not able to compensate the parent clock change, we fail returning NOTIFY_BAD. This fixes a real-world problem with Zynq arch not being able to use this driver and CPU_FREQ at the same time (because ARM global timer is fed by the CPU clock, which may keep changing when CPU_FREQ is enabled). Signed-off-by: Andrea Merello <andrea.merello@gmail.com> Cc: Patrice Chotard <patrice.chotard@st.com> Cc: linux-kernel@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org Cc: Michal Simek <michal.simek@xilinx.com> Cc: Sören Brinkmann <soren.brinkmann@xilinx.com> Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> Link: https://lore.kernel.org/r/20210406130045.15491-2-andrea.merello@gmail.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.