Douglas Anderson, recently pointed out an interesting problem due to which udelay() was expiring earlier than it should.
While transitioning between frequencies few platforms may temporarily switch to a stable frequency, waiting for the main PLL to stabilize.
For example: When we transition between very low frequencies on exynos, like between 200MHz and 300MHz, we may temporarily switch to a PLL running at 800MHz. No CPUFREQ notification is sent for that. That means there's a period of time when we're running at 800MHz but loops_per_jiffy is calibrated at between 200MHz and 300MHz. And so udelay behaves badly.
To get this fixed in a generic way, lets introduce another set of callbacks get_intermediate() and target_intermediate(), only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.
get_intermediate should return a stable intermediate frequency platform wants to switch to, and target_intermediate() should set CPU to to that frequency, before jumping to the frequency corresponding to 'index'. Core will take care of sending notifications and driver doesn't have to handle them in target_intermediate() or target_index().
This patchset also update Tegra to use this new infrastructure.
NOTE: Once set to intermediate frequency, driver isn't expected to fail for the following ->target_index() call, if it fails core will issue a WARN().
@Stephen: Can you please test this? I haven't as I didn't had a board. :)
V1-V2: Almost changed completely, V1 was here: https://lkml.org/lkml/2014/5/15/40
Viresh Kumar (3): cpufreq: handle calls to ->target_index() in separate routine cpufreq: add support for intermediate (stable) frequencies cpufreq: Tegra: implement intermediate frequency callbacks
Documentation/cpu-freq/cpu-drivers.txt | 19 +++++++- drivers/cpufreq/cpufreq.c | 82 ++++++++++++++++++++++++---------- drivers/cpufreq/tegra-cpufreq.c | 81 ++++++++++++++++----------------- include/linux/cpufreq.h | 15 +++++++ 4 files changed, 128 insertions(+), 69 deletions(-)