Hi all, as some of you may recall I've been spending some time looking into providing 'cpufreq' support for the Raspberry Pi platform[1]. I think I'm close to something workable, so I'd love for you to comment on it. There has been some design changes since the last version. Namely the fact that I now make sure *only* the CPU frequency is updated. The firmware API we use has two modes, with or without turbo. Enabling turbo implies not only scaling the CPU clock but also the VPU and other peripheral related clocks. This is problematic as some of them are not prepared for this kind frequency changes. I spent some time adapting the peripheral drivers, but the result was disappointing as they poorly support live frequency changes (which most other chips accept, think for instance I2C and clock stretching) but also turned out hard to integrate into the kernel. As we were planning to use 'clk_notifiers' which turns out not to be such a good idea as it's prone to deadlocks and not recommended by the clock maintainers[2]. It's also worth mentioning that the foundation kernel doesn't support VPU frequency scaling either. With this in mind, and as suggested by clock maintainers[2], I've decided to integrate the firmware clock interface into the bcm2835 clock driver. This, in my opinion, provides the least friction with the firmware and lets us write very simple and portable higher level drivers. As I did with the 'cpufreq' driver which simply queries the max and min frequencies available, which are configurable in the firmware, to then trigger the generic 'cpufreq-dt'. In the future we could further integrate other firmware dependent clocks into the main driver. For instance to be able to scale the VPU clock, which should be operated through a 'devfreq' driver. This was tested on a RPi3b+ and if the series is well received I'll test it further on all platforms I own. That's all, kind regards, Nicolas [1] https://lists.infradead.org/pipermail/linux-rpi-kernel/2019-April/008634.html [2] https://www.spinics.net/lists/linux-clk/msg36937.html --- Nicolas Saenz Julienne (5): clk: bcm2835: set CLK_GET_RATE_NOCACHE on CPU clocks clk: bcm2835: set pllb_arm divisor as readonly clk: bcm2835: use firmware interface to update pllb dts: bcm2837: add per-cpu clock devices cpufreq: add driver for Raspbery Pi arch/arm/boot/dts/bcm2837.dtsi | 8 + drivers/clk/bcm/clk-bcm2835.c | 284 ++++++++++++++++++++++++-- drivers/cpufreq/Kconfig.arm | 8 + drivers/cpufreq/Makefile | 1 + drivers/cpufreq/raspberrypi-cpufreq.c | 79 +++++++ 5 files changed, 362 insertions(+), 18 deletions(-) create mode 100644 drivers/cpufreq/raspberrypi-cpufreq.c -- 2.21.0
