On Fri, May 16, 2014 at 2:33 PM, Viresh Kumar <viresh.ku...@linaro.org> wrote: > 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.
Hi Viresh, In the given example above, the reworked implementation of cpufreq for exynos maintains the transition frequency at 800MHz / 4 = 200MHz by using a clock divider. So the transition frequency is ensured to be less than or equal to the pre-transition cpu clock frequency. Thanks, Thomas. > > 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(-) > > -- > 2.0.0.rc2 > > > _______________________________________________ > linaro-kernel mailing list > linaro-ker...@lists.linaro.org > http://lists.linaro.org/mailman/listinfo/linaro-kernel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
