iio: accel: adxl345: initialize FIFO delay value for SPI

Add the possibility to delay FIFO access when SPI is used. According to
the datasheet this is needed for the adxl345. When initialization
happens over SPI the need for delay is to be signalized, and the delay
will be used.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20241228232949.72487-3-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: Rubusch

drivers/iio/accel/adxl345.h

@@ -62,6 +62,7 @@ struct adxl345_chip_info {
 };
 
 int adxl345_core_probe(struct device *dev, struct regmap *regmap,
+		       bool fifo_delay_default,
 		       int (*setup)(struct device*, struct regmap*));
 
 #endif /* _ADXL345_H_ */

drivers/iio/accel/adxl345_core.c

@@ -25,6 +25,7 @@
 struct adxl345_state {
 	const struct adxl345_chip_info *info;
 	struct regmap *regmap;
+	bool fifo_delay; /* delay: delay is needed for SPI */
 	int irq;
 	u8 intio;
 };
@@ -196,12 +197,21 @@ static const struct iio_info adxl345_info = {
  * adxl345_core_probe() - Probe and setup for the accelerometer.
  * @dev:	Driver model representation of the device
  * @regmap:	Regmap instance for the device
+ * @fifo_delay_default: Using FIFO with SPI needs delay
  * @setup:	Setup routine to be executed right before the standard device
  *		setup
  *
+ * For SPI operation greater than 1.6 MHz, it is necessary to deassert the CS
+ * pin to ensure a total delay of 5 us; otherwise, the delay is not sufficient.
+ * The total delay necessary for 5 MHz operation is at most 3.4 us. This is not
+ * a concern when using I2C mode because the communication rate is low enough
+ * to ensure a sufficient delay between FIFO reads.
+ * Ref: "Retrieving Data from FIFO", p. 21 of 36, Data Sheet ADXL345 Rev. G
+ *
  * Return: 0 on success, negative errno on error
  */
 int adxl345_core_probe(struct device *dev, struct regmap *regmap,
+		       bool fifo_delay_default,
 		       int (*setup)(struct device*, struct regmap*))
 {
 	struct adxl345_state *st;
@@ -222,6 +232,7 @@ int adxl345_core_probe(struct device *dev, struct regmap *regmap,
 	st->info = device_get_match_data(dev);
 	if (!st->info)
 		return -ENODEV;
+	st->fifo_delay = fifo_delay_default;
 
 	indio_dev->name = st->info->name;
 	indio_dev->info = &adxl345_info;

drivers/iio/accel/adxl345_i2c.c

@@ -27,7 +27,7 @@ static int adxl345_i2c_probe(struct i2c_client *client)
 	if (IS_ERR(regmap))
 		return dev_err_probe(&client->dev, PTR_ERR(regmap), "Error initializing regmap\n");
 
-	return adxl345_core_probe(&client->dev, regmap, NULL);
+	return adxl345_core_probe(&client->dev, regmap, false, NULL);
 }
 
 static const struct adxl345_chip_info adxl345_i2c_info = {

drivers/iio/accel/adxl345_spi.c

@@ -12,6 +12,7 @@
 #include "adxl345.h"
 
 #define ADXL345_MAX_SPI_FREQ_HZ		5000000
+#define ADXL345_MAX_FREQ_NO_FIFO_DELAY	1500000
 
 static const struct regmap_config adxl345_spi_regmap_config = {
 	.reg_bits = 8,
@@ -28,6 +29,7 @@ static int adxl345_spi_setup(struct device *dev, struct regmap *regmap)
 static int adxl345_spi_probe(struct spi_device *spi)
 {
 	struct regmap *regmap;
+	bool needs_delay;
 
 	/* Bail out if max_speed_hz exceeds 5 MHz */
 	if (spi->max_speed_hz > ADXL345_MAX_SPI_FREQ_HZ)
@@ -38,10 +40,11 @@ static int adxl345_spi_probe(struct spi_device *spi)
 	if (IS_ERR(regmap))
 		return dev_err_probe(&spi->dev, PTR_ERR(regmap), "Error initializing regmap\n");
 
+	needs_delay = spi->max_speed_hz > ADXL345_MAX_FREQ_NO_FIFO_DELAY;
 	if (spi->mode & SPI_3WIRE)
-		return adxl345_core_probe(&spi->dev, regmap, adxl345_spi_setup);
+		return adxl345_core_probe(&spi->dev, regmap, needs_delay, adxl345_spi_setup);
 	else
-		return adxl345_core_probe(&spi->dev, regmap, NULL);
+		return adxl345_core_probe(&spi->dev, regmap, needs_delay, NULL);
 }
 
 static const struct adxl345_chip_info adxl345_spi_info = {