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The public S805 datasheet only mentions that HHI_SYS_CPU_CLK_CNTL1[20:29] contains a divider called "cpu_scale_div". Unfortunately it does not mention how to use the register contents. The Amlogic 3.10 GPL kernel sources are using the following code to calculate the CPU clock based on that register (taken from arch/arm/mach-meson8/clock.c in the 3.10 Amlogic kernel, shortened to make it easier to read): N = (aml_read_reg32(P_HHI_SYS_CPU_CLK_CNTL1) >> 20) & 0x3FF; if (sel == 3) /* use cpu_scale_div */ div = 2 * N; else div = ... /* not relevant for this example */ cpu_clk = parent_clk / div; This suggests that the formula is: parent_rate / 2 * register_value However, running perf (which can measure the CPU clock rate thanks to the ARM PMU) shows that this formula is not correct. This can be reproduced with the following steps: 1. boot into u-boot 2. let the CPU clock run off the XTAL clock: mw.l 0xC110419C 0x30 1 3. set the cpu_scale_div register: to value 0x1: mw.l 0xC110415C 0x801016A2 1 to value 0x2: mw.l 0xC110415C 0x802016A2 1 to value 0x5: mw.l 0xC110415C 0x805016A2 1 4. let the CPU clock run off cpu_scale_div: mw.l 0xC110419C 0xbd 1 5. boot Linux 6. run: perf stat -aB stress --cpu 4 --timeout 10 7. check the "cycles" value I get the following results depending on the cpu_scale_div value: - (cpu_in_sel - this is the input clock for cpu_scale_div - runs at 1.2GHz) - 0x1 = 300MHz - 0x2 = 200MHz - 0x5 = 100MHz This means that the actual formula to calculate the output of the cpu_scale_div clock is: parent_rate / 2 * (register value + 1). The register value 0x0 is reserved. When letting the CPU clock run off the cpu_scale_div while the value is 0x0 the whole board hangs (even in u-boot). I also verified this with the TWD timer: when adding this to the .dts without specifying it's clock it will auto-detect the PERIPH (which is the input clock of the TWD) clock rate (and the result is shown in the kernel log). On Meson8, Meson8b and Meson8m2 the PERIPH clock is CPUCLK divided by 4. This also matched for all three test-cases from above (in all cases the TWD timer clock rate was approx. one fourth of the CPU clock rate). A small note regarding the "fixes" tag: the original issue seems to exist virtually since forever. Even commit |
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| .. | ||
| axg-aoclk.c | ||
| axg-aoclk.h | ||
| axg-audio.c | ||
| axg-audio.h | ||
| axg.c | ||
| axg.h | ||
| clk-mpll.c | ||
| clk-phase.c | ||
| clk-pll.c | ||
| clk-regmap.c | ||
| clk-regmap.h | ||
| clk-triphase.c | ||
| clkc-audio.h | ||
| clkc.h | ||
| gxbb-aoclk-32k.c | ||
| gxbb-aoclk.c | ||
| gxbb-aoclk.h | ||
| gxbb.c | ||
| gxbb.h | ||
| Kconfig | ||
| Makefile | ||
| meson8b.c | ||
| meson8b.h | ||
| meson-aoclk.c | ||
| meson-aoclk.h | ||
| sclk-div.c | ||
| vid-pll-div.c | ||