Re: [PATCH 6/7] iio: adc: stm32-adc: add vrefint calibration support

From: Olivier MOYSAN
Date: Wed Sep 22 2021 - 03:54:30 EST

Hi Jonathan,

On 9/18/21 8:42 PM, Jonathan Cameron wrote:
On Wed, 15 Sep 2021 12:02:45 +0200
Olivier MOYSAN <olivier.moysan@xxxxxxxxxxx> wrote:

Hi Jonathan,

On 9/11/21 6:28 PM, Jonathan Cameron wrote:
On Wed, 8 Sep 2021 17:54:51 +0200
Olivier Moysan <olivier.moysan@xxxxxxxxxxx> wrote:
Add support of vrefint calibration.
If a channel is labeled as vrefint, get vrefint calibration
from non volatile memory for this channel.
A conversion on vrefint channel allows to update scale
factor according to vrefint deviation, compared to vrefint
calibration value.

As I mention inline, whilst technically the ABI doesn't demand it
the expectation of much of userspace software is that _scale is
pseudo constant - that is it doesn't tend to change very often and when
it does it's normally because someone deliberately made it change.
As such most software reads it just once.

Normally we work around this by applying the maths in kernel and
not exposing the scale at all. Is this something that could be done here?

Jonathan

Signed-off-by: Olivier Moysan <olivier.moysan@xxxxxxxxxxx>
---
drivers/iio/adc/stm32-adc.c | 88 ++++++++++++++++++++++++++++++++++---
1 file changed, 82 insertions(+), 6 deletions(-)

diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
index ef3d2af98025..9e52a7de9b16 100644
--- a/drivers/iio/adc/stm32-adc.c
+++ b/drivers/iio/adc/stm32-adc.c
@@ -21,6 +21,7 @@
#include <linux/io.h>
#include <linux/iopoll.h>
#include <linux/module.h>
+#include <linux/nvmem-consumer.h>
#include <linux/platform_device.h>
#include <linux/pm_runtime.h>
#include <linux/of.h>
@@ -42,6 +43,7 @@
#define STM32_ADC_TIMEOUT (msecs_to_jiffies(STM32_ADC_TIMEOUT_US / 1000))
#define STM32_ADC_HW_STOP_DELAY_MS 100
#define STM32_ADC_CHAN_NONE -1
+#define STM32_ADC_VREFINT_VOLTAGE 3300
#define STM32_DMA_BUFFER_SIZE PAGE_SIZE
@@ -79,6 +81,7 @@ enum stm32_adc_extsel {
};
enum stm32_adc_int_ch {
+ STM32_ADC_INT_CH_NONE = -1,
STM32_ADC_INT_CH_VDDCORE,
STM32_ADC_INT_CH_VREFINT,
STM32_ADC_INT_CH_VBAT,
@@ -137,6 +140,16 @@ struct stm32_adc_regs {
int shift;
};
+/**
+ * struct stm32_adc_vrefint - stm32 ADC internal reference voltage data
+ * @vrefint_cal: vrefint calibration value from nvmem
+ * @vrefint_data: vrefint actual value
+ */
+struct stm32_adc_vrefint {
+ u32 vrefint_cal;
+ u32 vrefint_data;
+};
+
/**
* struct stm32_adc_regspec - stm32 registers definition
* @dr: data register offset
@@ -186,6 +199,7 @@ struct stm32_adc;
* @unprepare: optional unprepare routine (disable, power-down)
* @irq_clear: routine to clear irqs
* @smp_cycles: programmable sampling time (ADC clock cycles)
+ * @ts_vrefint_ns: vrefint minimum sampling time in ns
*/
struct stm32_adc_cfg {
const struct stm32_adc_regspec *regs;
@@ -199,6 +213,7 @@ struct stm32_adc_cfg {
void (*unprepare)(struct iio_dev *);
void (*irq_clear)(struct iio_dev *indio_dev, u32 msk);
const unsigned int *smp_cycles;
+ const unsigned int ts_vrefint_ns;
};
/**
@@ -223,6 +238,7 @@ struct stm32_adc_cfg {
* @pcsel: bitmask to preselect channels on some devices
* @smpr_val: sampling time settings (e.g. smpr1 / smpr2)
* @cal: optional calibration data on some devices
+ * @vrefint: internal reference voltage data
* @chan_name: channel name array
* @num_diff: number of differential channels
* @int_ch: internal channel indexes array
@@ -248,6 +264,7 @@ struct stm32_adc {
u32 pcsel;
u32 smpr_val[2];
struct stm32_adc_calib cal;
+ struct stm32_adc_vrefint vrefint;
char chan_name[STM32_ADC_CH_MAX][STM32_ADC_CH_SZ];
u32 num_diff;
int int_ch[STM32_ADC_INT_CH_NB];
@@ -1331,15 +1348,35 @@ static int stm32_adc_read_raw(struct iio_dev *indio_dev,
ret = stm32_adc_single_conv(indio_dev, chan, val);
else
ret = -EINVAL;
+
+ /* If channel mask corresponds to vrefint, store data */
+ if (adc->int_ch[STM32_ADC_INT_CH_VREFINT] == chan->channel)
+ adc->vrefint.vrefint_data = *val;
+
iio_device_release_direct_mode(indio_dev);
return ret;
case IIO_CHAN_INFO_SCALE:
if (chan->differential) {
- *val = adc->common->vref_mv * 2;
+ if (adc->vrefint.vrefint_data &&
+ adc->vrefint.vrefint_cal) {
+ *val = STM32_ADC_VREFINT_VOLTAGE * 2 *
+ adc->vrefint.vrefint_cal /
+ adc->vrefint.vrefint_data;

Ah.. Dynamic scale. This is always awkward when it occurs.
Given most / possibly all userspace software assumes a pseudo static scale
(not data dependent) we normally hide this by doing the maths internal to the
driver - sometimes meaning we need to present the particular channel as processed
not raw.

Is the expectation here that vrefint_data is actually very nearly constant? If
so then what you have here may be fine as anyone not aware the scale might change
will get very nearly the right value anyway.

The need here is to compare the measured value of vrefint with the
calibrated value saved in non volatile memory. The ratio between these
two values can be used as a correction factor for the acquisitions on
all other channels.

The vrefint data is expected to be close to the saved vrefint
calibration value, and it should not vary strongly over time.
So, yes, we can indeed consider the scale as a pseudo constant. If the
scale is not updated, the deviation with actual value should remain
limited, as well.

Ok, so in that case we could probably get away with having it as you have
here, though for maximum precision we'd need userspace to occasionally check
the scale.


You suggest above to hide scale tuning through processed channels.
If I follow this logic, when vrefint channel is available, all channels
should be defined as processed channels (excepted vrefint channel)
In this case no scale is exposed for these channels, and the vrefint
calibration ratio can be used to provide converted data directly.
Do you prefer this implementation ?


In this case I wonder how buffered data have to be managed. These data
are still provided as raw data, but the scale factor is not more
available to convert them. I guess that these data have to be converted
internally also, either in dma callback or irq handler.
Is this correct ?

This is one of the holes in what IIO does today. Without meta data in the
buffer (which is hard to define in a clean fashion) it is hard to have
a compact representation of the data in the presence of dynamic scaling.
The vast majority of devices don't inherently support such scaling so
this is only occasionally a problem.

To support this at the moment you would indeed need to scale the data
before pushing it to the buffer which is obviously really ugly.

My gut feeling here is there are three possible approaches.

1) Ignore the dynamic nature of the calibration and pretend it's static.
2) Add an explicit 'calibration' sysfs attribute.
This is a fairly common model for other sensor types which don't do
dynamic calibration but instead require to you to start some special
calibration sequence.
As the calibration is not updated, except on explicit userspace action
we can assume that the scale is static unless userspace is aware of
the dynamic aspect.
3) Add a userspace control to turn on dynamic calibration. That makes it
opt in. Everything will work reasonably well without it turned on
as we'll hopefully have a static estimate of scale which is good enough.
If aware software is using the device, it can enable this mode and
sample the scale as often as it wants to.

I slightly favour option 3. What do you think? If we ever figure out
the meta data question for buffered case then we can make that work on top
of this.

Jonathan

This discussion made me revisit the calibration aspects in the ADC driver.

We have three types of calibration in ADC:

- Linear calibration: this calibration is not voltage or temperature dependent. So, it can be done once at boot time, as this is done currently.

- offset calibration: this calibration has a voltage and temperature dependency. This calibration is currently done once at boot. But it would be relevant to allow application to request a new offset calibration, when supply or temperature change over time.
Here the 'calibration' sysfs attribute you suggested in option 2, would be convenient I think. I plan to submit this improvement in a separate patch.

- vref compensation: the vrefint channel offers a way to evaluate vref deviation. Here I need to change a bit the logic. I think that putting intelligence in the driver is not the best way at the end. This hides voltage deviation information, where it could be useful to check if an offset calibration is needed. Moreover we get a lack of consistency between raw and buffered data.
It looks that processed type is the good way to expose vrefint channel. This allows to provide the actual vref voltage. And we can let the application decide how to manage this information. (trigger offset calibration / compensate raw data)
I'm going to send a v2 serie with this change for vrefint support
(plus the corrections for other comments)

Regards
Olivier


Regards
Olivier

+ } else {
+ *val = adc->common->vref_mv * 2;
+ }
*val2 = chan->scan_type.realbits;
} else {
- *val = adc->common->vref_mv;
+ /* Use vrefint data if available */
+ if (adc->vrefint.vrefint_data &&
+ adc->vrefint.vrefint_cal) {
+ *val = STM32_ADC_VREFINT_VOLTAGE *
+ adc->vrefint.vrefint_cal /
+ adc->vrefint.vrefint_data;
+ } else {
+ *val = adc->common->vref_mv;
+ }
*val2 = chan->scan_type.realbits;
}
return IIO_VAL_FRACTIONAL_LOG2;
@@ -1907,6 +1944,35 @@ static int stm32_adc_legacy_chan_init(struct iio_dev *indio_dev,
return scan_index;
}
+static int stm32_adc_get_int_ch(struct iio_dev *indio_dev, const char *ch_name,
+ int chan)

Naming would suggest to me that it would return a channel rather than setting it
inside adc->int_ch[i] Perhaps something like st32_adc_populate_int_ch() ?

+{
+ struct stm32_adc *adc = iio_priv(indio_dev);
+ u16 vrefint;
+ int i, ret;
+
+ for (i = 0; i < STM32_ADC_INT_CH_NB; i++) {
+ if (!strncmp(stm32_adc_ic[i].name, ch_name, STM32_ADC_CH_SZ)) {
+ adc->int_ch[i] = chan;
+ /* If channel is vrefint get calibration data. */
+ if (stm32_adc_ic[i].idx == STM32_ADC_INT_CH_VREFINT) {

I would reduce indentation by reversing the logic.

if (stm32_adc_ic[i].idx != STM32_ADC_INT_CH_VREFINT)
continue;

ret =
+ ret = nvmem_cell_read_u16(&indio_dev->dev, "vrefint", &vrefint);
+ if (ret && ret != -ENOENT && ret != -EOPNOTSUPP) {
+ dev_err(&indio_dev->dev, "nvmem access error %d\n", ret);
+ return ret;
+ }
+ if (ret == -ENOENT)
+ dev_dbg(&indio_dev->dev,
+ "vrefint calibration not found\n");
+ else
+ adc->vrefint.vrefint_cal = vrefint;
+ }
+ }
+ }
+
+ return 0;
+}
+
static int stm32_adc_generic_chan_init(struct iio_dev *indio_dev,
struct stm32_adc *adc,
struct iio_chan_spec *channels)
@@ -1938,10 +2004,9 @@ static int stm32_adc_generic_chan_init(struct iio_dev *indio_dev,
return -EINVAL;
}
strncpy(adc->chan_name[val], name, STM32_ADC_CH_SZ);
- for (i = 0; i < STM32_ADC_INT_CH_NB; i++) {
- if (!strncmp(stm32_adc_ic[i].name, name, STM32_ADC_CH_SZ))
- adc->int_ch[i] = val;
- }
+ ret = stm32_adc_get_int_ch(indio_dev, name, val);
+ if (ret)
+ goto err;
} else if (ret != -EINVAL) {
dev_err(&indio_dev->dev, "Invalid label %d\n", ret);
goto err;
@@ -2044,6 +2109,16 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev, bool timestamping)
*/
of_property_read_u32_index(node, "st,min-sample-time-nsecs",
i, &smp);
+
+ /*
+ * For vrefint channel, ensure that the sampling time cannot
+ * be lower than the one specified in the datasheet
+ */
+ if (channels[i].channel == adc->int_ch[STM32_ADC_INT_CH_VREFINT] &&
+ smp < adc->cfg->ts_vrefint_ns) {
+ smp = adc->cfg->ts_vrefint_ns;
+ }

if (channels[i].channel == adc->int_ch[STM32_ADC_INT_CH_VREFINT])
smp = max(smp, adc->cfg->ts_vrefint_ns);
+
/* Prepare sampling time settings */
stm32_adc_smpr_init(adc, channels[i].channel, smp);
}
@@ -2350,6 +2425,7 @@ static const struct stm32_adc_cfg stm32mp1_adc_cfg = {
.unprepare = stm32h7_adc_unprepare,
.smp_cycles = stm32h7_adc_smp_cycles,
.irq_clear = stm32h7_adc_irq_clear,
+ .ts_vrefint_ns = 4300,
};
static const struct of_device_id stm32_adc_of_match[] = {