[PATCH v2 1/2] iio: adc: ltc2309: add read delay for ltc2305

[Carlos Jones Jr]

The LTC2305 requires a minimum 1.6μs delay between the I2C write
operation (channel selection) and the subsequent read operation to
allow the chip to process the command and prepare the result. While
not explicitly documented in the datasheet, this timing requirement
was identified by the hardware designer as necessary for reliable
operation.

Add a read_delay_us field to both the ltc2309_chip_info and ltc2309
device structures to support chip-specific timing requirements. Use
fsleep() to implement the delay when non-zero, with LTC2305 set to
2μs (1.6μs requirement rounded up). LTC2309 does not require
additional delay beyond inherent I2C bus timing.

This extends the existing LTC2305 support added in
(commit 8625d418d24b ("iio: adc: ltc2309: add support for ltc2305"))
with the missing inter-transaction delay.

Signed-off-by: Carlos Jones Jr <carlosjr.jones@xxxxxxxxxx>
---
 drivers/iio/adc/ltc2309.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/iio/adc/ltc2309.c b/drivers/iio/adc/ltc2309.c
index 316256edf150..094966289922 100644
--- a/drivers/iio/adc/ltc2309.c
+++ b/drivers/iio/adc/ltc2309.c
@@ -9,7 +9,9 @@
  *
  * Copyright (c) 2023, Liam Beguin <liambeguin@xxxxxxxxx>
  */
+#include <linux/array_size.h>
 #include <linux/bitfield.h>
+#include <linux/delay.h>
 #include <linux/i2c.h>
 #include <linux/iio/iio.h>
 #include <linux/kernel.h>
@@ -34,12 +36,14 @@
  * @client:	I2C reference
  * @lock:	Lock to serialize data access
  * @vref_mv:	Internal voltage reference
+ * @read_delay_us:	Chip-specific read delay in microseconds
  */
 struct ltc2309 {
 	struct device		*dev;
 	struct i2c_client	*client;
 	struct mutex		lock; /* serialize data access */
 	int			vref_mv;
+	unsigned int		read_delay_us;
 };
 
 /* Order matches expected channel address, See datasheet Table 1. */
@@ -118,12 +122,14 @@ static const struct iio_chan_spec ltc2309_channels[] = {
 struct ltc2309_chip_info {
 	const char *name;
 	const struct iio_chan_spec *channels;
+	unsigned int read_delay_us;
 	int num_channels;
 };
 
 static const struct ltc2309_chip_info ltc2305_chip_info = {
 	.name = "ltc2305",
 	.channels = ltc2305_channels,
+	.read_delay_us = 2,
 	.num_channels = ARRAY_SIZE(ltc2305_channels),
 };
 
@@ -151,6 +157,9 @@ static int ltc2309_read_raw_channel(struct ltc2309 *ltc2309,
 		return ret;
 	}
 
+	if (ltc2309->read_delay_us)
+		fsleep(ltc2309->read_delay_us);
+
 	ret = i2c_master_recv(ltc2309->client, (char *)&buf, 2);
 	if (ret < 0) {
 		dev_err(ltc2309->dev, "i2c read failed: %pe\n", ERR_PTR(ret));
@@ -206,6 +215,7 @@ static int ltc2309_probe(struct i2c_client *client)
 
 	ltc2309->dev = &indio_dev->dev;
 	ltc2309->client = client;
+	ltc2309->read_delay_us = chip_info->read_delay_us;
 
 	indio_dev->name = chip_info->name;
 	indio_dev->modes = INDIO_DIRECT_MODE;
-- 
2.43.0