iio: adc: ad7124: fix sample rate for multi-channel use

Change how the FS[10:0] field of the FILTER register is calculated to
get consistent sample rates when only one channel is enabled vs when
multiple channels are enabled in a buffered read.

By default, the AD7124 allows larger sampling frequencies when only one
channel is enabled. It assumes that you will discard the first sample or
so to allow for settling time and then no additional settling time is
needed between samples because there is no multiplexing due to only one
channel being enabled. The conversion formula to convert between the
sampling frequency and the FS[10:0] field is:

    fADC = fCLK / (FS[10:0] x 32)

which is what the driver has been using.

On the other hand, when multiple channels are enabled, there is
additional settling time needed when switching between channels so the
calculation to convert between becomes:

    fADC = fCLK / (FS[10:0] x 32 x N)

where N depends on if SINGLE_CYCLE is set, the selected filter type and,
in some cases, the power mode.

The FILTER register has a SINGLE_CYCLE bit that can be set to force the
single channel case to use the same timing as the multi-channel case.

Before this change, the first formula was always used, so if all of the
in_voltageY_sampling_frequency attributes were set to 10 Hz, then doing
a buffered read with 1 channel enabled would result in the requested
sampling frequency of 10 Hz. But when more than one channel was
enabled, the actual sampling frequency would be 2.5 Hz per channel,
which is 1/4 of the requested frequency.

After this change, the SINGLE_CYCLE flag is now always enabled and the
multi-channel formula is now always used. This causes the sampling
frequency to be consistent regardless of the number of channels enabled.

For now, we are hard-coding N = 4 since the driver doesn't yet support
other filter types other than the default sinc4 filter.

The AD7124_FILTER_FS define is moved while we are touching this to
keep the bit fields in descending order to be consistent with the rest
of the file.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

drivers/iio/adc/ad7124.c

@@ -93,10 +93,11 @@
 #define AD7124_CONFIG_PGA		GENMASK(2, 0)
 
 /* AD7124_FILTER_X */
-#define AD7124_FILTER_FS		GENMASK(10, 0)
 #define AD7124_FILTER_FILTER		GENMASK(23, 21)
 #define AD7124_FILTER_FILTER_SINC4		0
 #define AD7124_FILTER_FILTER_SINC3		2
+#define AD7124_FILTER_SINGLE_CYCLE	BIT(16)
+#define AD7124_FILTER_FS		GENMASK(10, 0)
 
 #define AD7124_MAX_CONFIGS	8
 #define AD7124_MAX_CHANNELS	16
@@ -285,18 +286,20 @@ static u32 ad7124_get_fclk_hz(struct ad7124_state *st)
 
 static void ad7124_set_channel_odr(struct ad7124_state *st, unsigned int channel, unsigned int odr)
 {
-	unsigned int fclk, odr_sel_bits;
+	unsigned int fclk, factor, odr_sel_bits;
 
 	fclk = ad7124_get_fclk_hz(st);
 
 	/*
-	 * FS[10:0] = fCLK / (fADC x 32) where:
+	 * FS[10:0] = fCLK / (fADC x 32 * N) where:
 	 * fADC is the output data rate
 	 * fCLK is the master clock frequency
+	 * N is number of conversions per sample (depends on filter type)
 	 * FS[10:0] are the bits in the filter register
 	 * FS[10:0] can have a value from 1 to 2047
 	 */
-	odr_sel_bits = DIV_ROUND_CLOSEST(fclk, odr * 32);
+	factor = 32 * 4; /* N = 4 for default sinc4 filter. */
+	odr_sel_bits = DIV_ROUND_CLOSEST(fclk, odr * factor);
 	if (odr_sel_bits < 1)
 		odr_sel_bits = 1;
 	else if (odr_sel_bits > 2047)
@@ -306,7 +309,8 @@ static void ad7124_set_channel_odr(struct ad7124_state *st, unsigned int channel
 		st->channels[channel].cfg.live = false;
 
 	/* fADC = fCLK / (FS[10:0] x 32) */
-	st->channels[channel].cfg.odr = DIV_ROUND_CLOSEST(fclk, odr_sel_bits * 32);
+	st->channels[channel].cfg.odr = DIV_ROUND_CLOSEST(fclk, odr_sel_bits *
+								factor);
 	st->channels[channel].cfg.odr_sel_bits = odr_sel_bits;
 }
 
@@ -439,10 +443,20 @@ static int ad7124_write_config(struct ad7124_state *st, struct ad7124_channel_co
 	if (ret < 0)
 		return ret;
 
+	/*
+	 * NB: AD7124_FILTER_SINGLE_CYCLE is always set so that we get the same
+	 * sampling frequency even when only one channel is enabled in a
+	 * buffered read. If it was not set, the N in ad7124_set_channel_odr()
+	 * would be 1 and we would get a faster sampling frequency than what
+	 * was requested.
+	 */
 	tmp = FIELD_PREP(AD7124_FILTER_FILTER, cfg->filter_type) |
+		AD7124_FILTER_SINGLE_CYCLE |
 		FIELD_PREP(AD7124_FILTER_FS, cfg->odr_sel_bits);
 	return ad7124_spi_write_mask(st, AD7124_FILTER(cfg->cfg_slot),
-				     AD7124_FILTER_FILTER | AD7124_FILTER_FS,
+				     AD7124_FILTER_FILTER |
+				     AD7124_FILTER_SINGLE_CYCLE |
+				     AD7124_FILTER_FS,
 				     tmp, 3);
 }