Hi,
here is an initial implementation of EMC scaling for Tegra124, I'm most interested in feedback on the approach.
The present incarnation is very much specific to Tegra, but I'm sure that we could find an API that can be shared across SoC families.
I have looked at using existing frameworks (common clock, pm_qos and devfreq) to cover this use case, but it always felt like trying to fit a square peg in a round hole. These are our requirements:
1. Ceiling frequencies (for thermal and other power management components)
2. Floor frequencies (determined from load statistics that are often maintained by some firmware, to avoid frequent interrupts)
3. Let misc. device drivers such as display controllers or USB set their bandwidth requirements, which would be aggregated to calculate the final effective frequency.
4. The EMC on the Tegra124 has per-consumer latency allowance registers that influence how memory access requests are arbitrated, and these depend on the clock frequency.
1 and 2 could be implemented as additions to the common clock framework, but I feel that 3 should live at a higher level (as not all clocks are used to drive memory buses), and 4 doesn't seem related to clocks at all.
For past discussions on this see: https://lkml.org/lkml/2014/5/13/469
Tomeu Vizoso (4): memory: tegra124-emc: Add EMC driver ARM: tegra: Add Tegra124 EMC support drm/tegra: Request memory bandwidth for the display controller cpufreq: tegra: Register a minimum EMC frequency based on the CPU clock
.../bindings/arm/tegra/nvidia,tegra124-emc.txt | 26 ++++ arch/arm/boot/dts/tegra124.dtsi | 9 ++ drivers/cpufreq/tegra-cpufreq.c | 20 +-- drivers/gpu/drm/tegra/dc.c | 9 ++ drivers/memory/Kconfig | 8 + drivers/memory/Makefile | 1 + drivers/memory/tegra124-emc.c | 173 +++++++++++++++++++++ include/linux/platform_data/tegra_emc.h | 23 +++ 8 files changed, 254 insertions(+), 15 deletions(-) create mode 100644 Documentation/devicetree/bindings/arm/tegra/nvidia,tegra124-emc.txt create mode 100644 drivers/memory/tegra124-emc.c