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

Fabrice Gasnier
2017-07-18T12:38:07Z

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>
---
drivers/iio/adc/stm32-adc.c | 134 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 130 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
index 5bfcc1f..e6f6b5e 100644
--- a/drivers/iio/adc/stm32-adc.c
+++ b/drivers/iio/adc/stm32-adc.c
@@ -83,6 +83,8 @@
#define STM32H7_ADC_IER 0x04
#define STM32H7_ADC_CR 0x08
#define STM32H7_ADC_CFGR 0x0C
+#define STM32H7_ADC_SMPR1 0x14
+#define STM32H7_ADC_SMPR2 0x18
#define STM32H7_ADC_PCSEL 0x1C
#define STM32H7_ADC_SQR1 0x30
#define STM32H7_ADC_SQR2 0x34
@@ -151,6 +153,7 @@ enum stm32h7_adc_dmngt {
#define STM32H7_BOOST_CLKRATE 20000000UL

#define STM32_ADC_MAX_SQ 16 /* SQ1..SQ16 */
+#define STM32_ADC_MAX_SMP 7 /* SMPx range is [0..7] */
#define STM32_ADC_TIMEOUT_US 100000
#define STM32_ADC_TIMEOUT (msecs_to_jiffies(STM32_ADC_TIMEOUT_US / 1000))

@@ -227,6 +230,8 @@ struct stm32_adc_regs {
* @exten: trigger control register & bitfield
* @extsel: trigger selection register & bitfield
* @res: resolution selection register & bitfield
+ * @smpr: smpr1 & smpr2 registers offset array
+ * @smp_bits: smpr1 & smpr2 index and bitfields
*/
struct stm32_adc_regspec {
const u32 dr;
@@ -236,6 +241,8 @@ struct stm32_adc_regspec {
const struct stm32_adc_regs exten;
const struct stm32_adc_regs extsel;
const struct stm32_adc_regs res;
+ const u32 smpr[2];
+ const struct stm32_adc_regs *smp_bits;
};

struct stm32_adc;
@@ -251,6 +258,7 @@ struct stm32_adc_regspec {
* @start_conv: routine to start conversions
* @stop_conv: routine to stop conversions
* @unprepare: optional unprepare routine (disable, power-down)
+ * @smp_cycles: programmable sampling time (ADC clock cycles)
*/
struct stm32_adc_cfg {
const struct stm32_adc_regspec *regs;
@@ -262,6 +270,7 @@ struct stm32_adc_cfg {
void (*start_conv)(struct stm32_adc *, bool dma);
void (*stop_conv)(struct stm32_adc *);
void (*unprepare)(struct stm32_adc *);
+ const unsigned int *smp_cycles;
};

/**
@@ -283,6 +292,7 @@ struct stm32_adc_cfg {
* @rx_dma_buf: dma rx buffer bus address
* @rx_buf_sz: dma rx buffer size
* @pcsel bitmask to preselect channels on some devices
+ * @smpr_val: sampling time settings (e.g. smpr1 / smpr2)
* @cal: optional calibration data on some devices
*/
struct stm32_adc {
@@ -303,6 +313,7 @@ struct stm32_adc {
dma_addr_t rx_dma_buf;
unsigned int rx_buf_sz;
u32 pcsel;
+ u32 smpr_val[2];
struct stm32_adc_calib cal;
};

@@ -431,6 +442,39 @@ struct stm32_adc_info {
{}, /* sentinel */
};

+/**
+ * stm32f4_smp_bits[] - describe sampling time register index & bit fields
+ * Sorted so it can be indexed by channel number.
+ */
+static const struct stm32_adc_regs stm32f4_smp_bits[] = {
+ /* STM32F4_ADC_SMPR2: smpr[] index, mask, shift for SMP0 to SMP9 */
+ { 1, GENMASK(2, 0), 0 },
+ { 1, GENMASK(5, 3), 3 },
+ { 1, GENMASK(8, 6), 6 },
+ { 1, GENMASK(11, 9), 9 },
+ { 1, GENMASK(14, 12), 12 },
+ { 1, GENMASK(17, 15), 15 },
+ { 1, GENMASK(20, 18), 18 },
+ { 1, GENMASK(23, 21), 21 },
+ { 1, GENMASK(26, 24), 24 },
+ { 1, GENMASK(29, 27), 27 },
+ /* STM32F4_ADC_SMPR1, smpr[] index, mask, shift for SMP10 to SMP18 */
+ { 0, GENMASK(2, 0), 0 },
+ { 0, GENMASK(5, 3), 3 },
+ { 0, GENMASK(8, 6), 6 },
+ { 0, GENMASK(11, 9), 9 },
+ { 0, GENMASK(14, 12), 12 },
+ { 0, GENMASK(17, 15), 15 },
+ { 0, GENMASK(20, 18), 18 },
+ { 0, GENMASK(23, 21), 21 },
+ { 0, GENMASK(26, 24), 24 },
+};
+
+/* STM32F4 programmable sampling time (ADC clock cycles) */
+static const unsigned int stm32f4_adc_smp_cycles[STM32_ADC_MAX_SMP + 1] = {
+ 3, 15, 28, 56, 84, 112, 144, 480,
+};
+
static const struct stm32_adc_regspec stm32f4_adc_regspec = {
.dr = STM32F4_ADC_DR,
.ier_eoc = { STM32F4_ADC_CR1, STM32F4_EOCIE },
@@ -440,6 +484,8 @@ struct stm32_adc_info {
.extsel = { STM32F4_ADC_CR2, STM32F4_EXTSEL_MASK,
STM32F4_EXTSEL_SHIFT },
.res = { STM32F4_ADC_CR1, STM32F4_RES_MASK, STM32F4_RES_SHIFT },
+ .smpr = { STM32F4_ADC_SMPR1, STM32F4_ADC_SMPR2 },
+ .smp_bits = stm32f4_smp_bits,
};

static const struct stm32_adc_regs stm32h7_sq[STM32_ADC_MAX_SQ + 1] = {
@@ -483,6 +529,40 @@ struct stm32_adc_info {
{},
};

+/**
+ * stm32h7_smp_bits - describe sampling time register index & bit fields
+ * Sorted so it can be indexed by channel number.
+ */
+static const struct stm32_adc_regs stm32h7_smp_bits[] = {
+ /* STM32H7_ADC_SMPR1, smpr[] index, mask, shift for SMP0 to SMP9 */
+ { 0, GENMASK(2, 0), 0 },
+ { 0, GENMASK(5, 3), 3 },
+ { 0, GENMASK(8, 6), 6 },
+ { 0, GENMASK(11, 9), 9 },
+ { 0, GENMASK(14, 12), 12 },
+ { 0, GENMASK(17, 15), 15 },
+ { 0, GENMASK(20, 18), 18 },
+ { 0, GENMASK(23, 21), 21 },
+ { 0, GENMASK(26, 24), 24 },
+ { 0, GENMASK(29, 27), 27 },
+ /* STM32H7_ADC_SMPR2, smpr[] index, mask, shift for SMP10 to SMP19 */
+ { 1, GENMASK(2, 0), 0 },
+ { 1, GENMASK(5, 3), 3 },
+ { 1, GENMASK(8, 6), 6 },
+ { 1, GENMASK(11, 9), 9 },
+ { 1, GENMASK(14, 12), 12 },
+ { 1, GENMASK(17, 15), 15 },
+ { 1, GENMASK(20, 18), 18 },
+ { 1, GENMASK(23, 21), 21 },
+ { 1, GENMASK(26, 24), 24 },
+ { 1, GENMASK(29, 27), 27 },
+};
+
+/* STM32H7 programmable sampling time (ADC clock cycles, rounded down) */
+static const unsigned int stm32h7_adc_smp_cycles[STM32_ADC_MAX_SMP + 1] = {
+ 1, 2, 8, 16, 32, 64, 387, 810,
+};
+
static const struct stm32_adc_regspec stm32h7_adc_regspec = {
.dr = STM32H7_ADC_DR,
.ier_eoc = { STM32H7_ADC_IER, STM32H7_EOCIE },
@@ -492,6 +572,8 @@ struct stm32_adc_info {
.extsel = { STM32H7_ADC_CFGR, STM32H7_EXTSEL_MASK,
STM32H7_EXTSEL_SHIFT },
.res = { STM32H7_ADC_CFGR, STM32H7_RES_MASK, STM32H7_RES_SHIFT },
+ .smpr = { STM32H7_ADC_SMPR1, STM32H7_ADC_SMPR2 },
+ .smp_bits = stm32h7_smp_bits,
};

/**
@@ -933,6 +1015,7 @@ static void stm32h7_adc_unprepare(struct stm32_adc *adc)
* @scan_mask: channels to be converted
*
* Conversion sequence :
+ * Apply sampling time settings for all channels.
* Configure ADC scan sequence based on selected channels in scan_mask.
* Add channels to SQR registers, from scan_mask LSB to MSB, then
* program sequence len.
@@ -946,6 +1029,10 @@ static int stm32_adc_conf_scan_seq(struct iio_dev *indio_dev,
u32 val, bit;
int i = 0;

+ /* Apply sampling time settings */
+ stm32_adc_writel(adc, adc->cfg->regs->smpr[0], adc->smpr_val[0]);
+ stm32_adc_writel(adc, adc->cfg->regs->smpr[1], adc->smpr_val[1]);
+
for_each_set_bit(bit, scan_mask, indio_dev->masklength) {
chan = indio_dev->channels + bit;
/*
@@ -1079,6 +1166,7 @@ static int stm32_adc_get_trig_pol(struct iio_dev *indio_dev,
* @res: conversion result
*
* The function performs a single conversion on a given channel:
+ * - Apply sampling time settings
* - Program sequencer with one channel (e.g. in SQ1 with len = 1)
* - Use SW trigger
* - Start conversion, then wait for interrupt completion.
@@ -1103,6 +1191,10 @@ static int stm32_adc_single_conv(struct iio_dev *indio_dev,
return ret;
}

+ /* Apply sampling time settings */
+ stm32_adc_writel(adc, regs->smpr[0], adc->smpr_val[0]);
+ stm32_adc_writel(adc, regs->smpr[1], adc->smpr_val[1]);
+
/* Program chan number in regular sequence (SQ1) */
val = stm32_adc_readl(adc, regs->sqr[1].reg);
val &= ~regs->sqr[1].mask;
@@ -1507,10 +1599,28 @@ static int stm32_adc_of_get_resolution(struct iio_dev *indio_dev)
return 0;
}

+static void stm32_adc_smpr_init(struct stm32_adc *adc, int channel, u32 smp_ns)
+{
+ const struct stm32_adc_regs *smpr = &adc->cfg->regs->smp_bits[channel];
+ u32 period_ns, shift = smpr->shift, mask = smpr->mask;
+ unsigned int smp, r = smpr->reg;
+
+ /* Determine sampling time (ADC clock cycles) */
+ period_ns = NSEC_PER_SEC / adc->common->rate;
+ for (smp = 0; smp <= STM32_ADC_MAX_SMP; smp++)
+ if ((period_ns * adc->cfg->smp_cycles[smp]) >= smp_ns)
+ break;
+ if (smp > STM32_ADC_MAX_SMP)
+ smp = STM32_ADC_MAX_SMP;
+
+ /* pre-build sampling time registers (e.g. smpr1, smpr2) */
+ adc->smpr_val[r] = (adc->smpr_val[r] & ~mask) | (smp << shift);
+}
+
static void stm32_adc_chan_init_one(struct iio_dev *indio_dev,
struct iio_chan_spec *chan,
const struct stm32_adc_chan_spec *channel,
- int scan_index)
+ int scan_index, u32 smp)
{
struct stm32_adc *adc = iio_priv(indio_dev);

@@ -1526,6 +1636,9 @@ static void stm32_adc_chan_init_one(struct iio_dev *indio_dev,
chan->scan_type.storagebits = 16;
chan->ext_info = stm32_adc_ext_info;

+ /* Prepare sampling time settings */
+ stm32_adc_smpr_init(adc, chan->channel, smp);
+
/* pre-build selected channels mask */
adc->pcsel |= BIT(chan->channel);
}
@@ -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);
+
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[] = {
--
1.9.1