Re: [PATCH] soc: amlogic: clk-measure: Optimize measurement accuracy

[Chuan Liu <chuan.liu@amlogic.com>]

hi Marti:


On 7/17/2025 11:43 PM, Martin Blumenstingl wrote:
> [ EXTERNAL EMAIL ]
>
> Hello,
>
> thank you for this patch!
>
> On Thu, Jul 17, 2025 at 5:08 AM Chuan Liu via B4 Relay
> <devnull+chuan.liu.amlogic.com@kernel.org> wrote:
>> From: Chuan Liu <chuan.liu@amlogic.com>
>>
>> The cycle count register has a 20-bit effective width, but the driver
>> only utilizes 16 bits. This reduces the sampling window when measuring
>> high-frequency clocks, resulting in (slightly) degraded measurement
>> accuracy.
> I checked the Meson8 downstream code [0] and it uses 0x000FFFFF to
> mask the register value -> this means that old SoCs also have a 20-bit
> wide width.
>
> [...]
>> Here are the test comparisons based on C3:
> [...]
>> Here are the test comparisons based on C3:
> I have tested this patch with Meson8b based Odroid-C1:
> pre-optimization:
> # time cat /sys/kernel/debug/meson-clk-msr/measure_summary | grep -v "  0  "
>    clock                     rate    precision
> ---------------------------------------------
>   clk81                 159372396    +/-5208Hz
>   a9_clk_div16           24000000    +/-3125Hz
>   rtc_osc_clk_out           31250    +/-3125Hz
>   hdmi_ch0_tmds         146399038    +/-4807Hz
>   sar_adc                 1140625    +/-3125Hz
>   sdhc_rx                94443750    +/-3125Hz
>   sdhc_sd                94443750    +/-3125Hz
>   pwm_d                 849921875    +/-31250Hz
>   pwm_c                 849921875    +/-31250Hz
>
> real    0m0.102s
> user    0m0.005s
> sys     0m0.069s
>
>
> post-optimization:
> # time cat /sys/kernel/debug/meson-clk-msr/measure_summary | grep -v "  0  "
>    clock                     rate    precision
> ---------------------------------------------
>   clk81                 159373438    +/-1562Hz
>   a9_clk_div16           12000000    +/-1562Hz
>   rtc_osc_clk_out           32813    +/-1562Hz
>   hdmi_ch0_tmds         146398438    +/-1562Hz
>   sar_adc                 1143750    +/-1562Hz
>   sdhc_rx                94443750    +/-1562Hz
>   sdhc_sd                94443750    +/-1562Hz
>   pwm_d                 849992188    +/-1562Hz
>   pwm_c                 849992188    +/-1562Hz
>
> real    0m0.173s
> user    0m0.008s
> sys     0m0.109s
>
> So there's also an improvement in accuracy. The only downside I'm
> seeing is that it takes 75% extra time for the measurement. For me
> this is irrelevant since we use this for debugging.
>
> [...]
>> +       /*
>> +        * HACK: The input clock signal path from gate (Controlled by MSR_RUN)
>> +        * to internal sampling circuit in clk-measure has a propagation delay
>> +        * requirement: 24 clock cycles must elapse after mux selection before
>> +        * sampling.
>> +        *
>> +        * For a 30kHz measurement clock, this translates to an 800μs delay:
>> +        * 800us = 24 / 30000Hz.
>> +        */
>> +       fsleep(800);
> What is needed to make this not a HACK anymore? Is there a register
> that we can poll for the number of clock cycles that have passed?


The required delay duration is frequency-dependent on the measurement
clock source. The current 800μs delay is calculated based on a
minimum input clock frequency of 30kHz. At higher input frequencies,
this delay could be proportionally reduced. Applying a uniform 800μs
delay therefore appears overly conservative for general use cases.


The IP currently lacks a status register to detect whether the input
clock signal has successfully propagated to the internal measurement
circuitry.


The design of this IP has been maintained for many years. From a
hardware design perspective, there is room for optimization in this
signal propagation delay. Future IP updates may not require such a
long delay.


>
> Best regards,
> Martin

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