Re: [PATCH 3/3] iio: adc: stm32: add optional min-sample-time

Jonathan Cameron
2017-07-24T15:06:44Z

On Mon, 24 Jul 2017 10:16:00 +0200
Fabrice Gasnier <fabrice.gasnier@st.com> wrote:

> On 07/23/2017 01:00 PM, Jonathan Cameron wrote:
> > On Tue, 18 Jul 2017 14:35:32 +0200
> > Fabrice Gasnier <fabrice.gasnier@st.com> wrote:
> >
> >> STM32 ADC allows each channel to be sampled with a different sampling time,
> >> by setting SMPR registers. Basically, value depends on local electrical
> >> properties. Selecting correct value for sampling time highly depends on
> >> analog source impedance. There is a manual that may help in this process:
> >> 'How to get the best ADC accuracy in STM32...'
> >>
> >> This patch allows to configure min-sample-time via device tree, either for:
> >> - all channels at once:
> >> min-sample-time = <10000>; /* nanosecs */
> >>
> >> - independently for each channel (must match "st,adc-channels" list):
> >> st,adc-channels = <0 1>;
> >> min-sample-time = <5000 10000>; /* nanosecs */
> >>
> >> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
> >
> > On question inline which may well just be down to me missing what
> > happens when you query index element 2 from a 1 element device tree
> > array.
>
> Hi Jonathan,
>
> I should probably comment on it, please see inline.
>
<snip>
> >> }
> >> @@ -1538,8 +1651,8 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
> >> struct property *prop;
> >> const __be32 *cur;
> >> struct iio_chan_spec *channels;
> >> - int scan_index = 0, num_channels;
> >> - u32 val;
> >> + int scan_index = 0, num_channels, ret;
> >> + u32 val, smp = 0;
> >>
> >> num_channels = of_property_count_u32_elems(node, "st,adc-channels");
> >> if (num_channels < 0 ||
> >> @@ -1548,6 +1661,13 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
> >> return num_channels < 0 ? num_channels : -EINVAL;
> >> }
> >>
> >> + /* Optional sample time is provided either for each, or all channels */
> >> + ret = of_property_count_u32_elems(node, "min-sample-time");
> >> + if (ret > 1 && ret != num_channels) {
> >> + dev_err(&indio_dev->dev, "Invalid min-sample-time\n");
> >> + return -EINVAL;
> >> + }
> >> +
> >> channels = devm_kcalloc(&indio_dev->dev, num_channels,
> >> sizeof(struct iio_chan_spec), GFP_KERNEL);
> >> if (!channels)
> >> @@ -1558,9 +1678,13 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
> >> dev_err(&indio_dev->dev, "Invalid channel %d\n", val);
> >> return -EINVAL;
> >> }
> >> +
> >> + of_property_read_u32_index(node, "min-sample-time", scan_index,
> >> + &smp);
> >> +
> > I might be missing something, but doesn't this fail for the single shared
> > value case?
>
> Yes, this fails in the single shared value case, with index >= 1.
> Rewinding... when index is 0, it picks up 1st (shared) value.
> Then fails for other index values.
>
> of_property_read_u32_index() documentation states out value remains
> untouched in case of error:
> /**
> * of_property_read_u32_index...
> [...]
> * The out_value is modified only if a valid u32 value can be decoded.
>
> This is what I use here (as far as I tested it, it worked like a charm).
> Do you wish I add a comment to describe this ?
Probably best or I'll forget sometime down the line and think it looks
crazy again ;)

Thanks for the explanation.

Jonathan
>
> Please let me know,
> Best Regards,
> Fabrice
>
> >> stm32_adc_chan_init_one(indio_dev, &channels[scan_index],
> >> &adc_info->channels[val],
> >> - scan_index);
> >> + scan_index, smp);
> >> scan_index++;
> >> }
> >>
> >> @@ -1755,6 +1879,7 @@ static int stm32_adc_remove(struct platform_device *pdev)
> >> .clk_required = true,
> >> .start_conv = stm32f4_adc_start_conv,
> >> .stop_conv = stm32f4_adc_stop_conv,
> >> + .smp_cycles = stm32f4_adc_smp_cycles,
> >> };
> >>
> >> static const struct stm32_adc_cfg stm32h7_adc_cfg = {
> >> @@ -1766,6 +1891,7 @@ static int stm32_adc_remove(struct platform_device *pdev)
> >> .stop_conv = stm32h7_adc_stop_conv,
> >> .prepare = stm32h7_adc_prepare,
> >> .unprepare = stm32h7_adc_unprepare,
> >> + .smp_cycles = stm32h7_adc_smp_cycles,
> >> };
> >>
> >> static const struct of_device_id stm32_adc_of_match[] = {
> >
> --
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