[PATCH v1] meson saradc: fix clock divider mask length

[Martin Blumenstingl]

Hi George,

On Thu, Jun 1, 2023 at 10:56 PM George Stark <gnstark at sberdevices.ru> wrote:
[...]
> Here the test I promised:
> Question: what's the real size of clock divder field in SAR_ADC_REG3 register in saradc in meson8 socs?
> The current kernel code says 5 bits
> The datasheet says 6 bit
>
> The parent clock of adc clock is 24Mhz
> I can check it here by:
>
> # cat /sys/kernel/debug/clk/clk_summary
>   xtal                                 4        4        1    24000000          0     0  50000         Y
>      c1108680.adc#adc_div              1        1        0      1142858         0     0  50000         Y
>         c1108680.adc#adc_en            1        1        0      1142858         0     0  50000         Y
>
> for divider width 5bit min adc clock is 24Mhz / 32 =  750KHZ
> for divider width 6bit min adc clock is 24Mhz / 64 =  375KHz
>
> I suppose that the lower adc clock rate the higher measurement time
> so I need to get measurement time at both clk freqs and the times differ so
> 6bit divider is really applied
>
> I performed test at Odroid-C1, kernel 6.2-rc8
> Two kernel patches must be applied:
>
> the topic starter patch and the helper patch at the end of the letter
> In the helper patch I turn on CLOCK_ALLOW_WRITE_DEBUGFS to change clock rate from she shell
> and use ktime_get_raw_ts64 to measure measurement time
>
> So the the test itself:
> cat /sys/devices/platform/soc/c1100000.cbus/c1108680.adc/iio:device0/in_voltage3_raw
> [ 1781.226309] ==== freq: 1142858 time 42408000
>
> # echo 750000 > /sys/kernel/debug/clk/c1108680.adc#adc_en/clk_rate
> # cat /sys/devices/platform/soc/c1100000.cbus/c1108680.adc/iio:device0/in_voltage3_raw
> [ 1790.728656] ==== freq: 750000 time 49173000
>
> # echo 375000 > /sys/kernel/debug/clk/c1108680.adc#adc_en/clk_rate
> # cat /sys/devices/platform/soc/c1100000.cbus/c1108680.adc/iio:device0/in_voltage3_raw
> [ 1816.955477] ==== freq: 375000 time 68245000
>
> # cat /sys/kernel/debug/clk/clk_summary
>   xtal                                 4        4        1    24000000          0     0  50000         Y
>      c1108680.adc#adc_div              1        1        0      375000          0     0  50000         Y
>         c1108680.adc#adc_en            1        1        0      375000          0     0  50000         Y
These results looks excellent - thanks for sharing the test results!
Could you please check one last thing:
$ grep -i adc /sys/kernel/debug/meson-clk-msr/measure_summary
It should confirm that the clock rate is 375kHz (or close to it, SoC
internal clock measurement is not 100% precise)

Once we have that confirmation: can you please re-send the patch with
the description updated so it's clear which SoC generations are
affected and by stating that the fix was tested on a Meson8b Odroid-C1
board.


Thank you and best regards,
Martin