iio: ad_sigma_delta: Properly handle SPI bus locking vs CS assertion

For devices from the SigmaDelta family we need to keep CS low when doing a
conversion, since the device will use the MISO line as a interrupt to
indicate that the conversion is complete.

This is why the driver locks the SPI bus and when the SPI bus is locked
keeps as long as a conversion is going on. The current implementation gets
one small detail wrong though. CS is only de-asserted after the SPI bus is
unlocked. This means it is possible for a different SPI device on the same
bus to send a message which would be wrongfully be addressed to the
SigmaDelta device as well. Make sure that the last SPI transfer that is
done while holding the SPI bus lock de-asserts the CS signal.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <Alexandru.Ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad_sigma_delta.c

@@ -62,7 +62,7 @@ int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,
 	struct spi_transfer t = {
 		.tx_buf		= data,
 		.len		= size + 1,
-		.cs_change	= sigma_delta->bus_locked,
+		.cs_change	= sigma_delta->keep_cs_asserted,
 	};
 	struct spi_message m;
 	int ret;
@@ -217,6 +217,7 @@ static int ad_sd_calibrate(struct ad_sigma_delta *sigma_delta,
 
 	spi_bus_lock(sigma_delta->spi->master);
 	sigma_delta->bus_locked = true;
+	sigma_delta->keep_cs_asserted = true;
 	reinit_completion(&sigma_delta->completion);
 
 	ret = ad_sigma_delta_set_mode(sigma_delta, mode);
@@ -234,9 +235,10 @@ static int ad_sd_calibrate(struct ad_sigma_delta *sigma_delta,
 		ret = 0;
 	}
 out:
+	sigma_delta->keep_cs_asserted = false;
+	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
 	sigma_delta->bus_locked = false;
 	spi_bus_unlock(sigma_delta->spi->master);
-	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
 
 	return ret;
 }
@@ -289,6 +291,7 @@ int ad_sigma_delta_single_conversion(struct iio_dev *indio_dev,
 
 	spi_bus_lock(sigma_delta->spi->master);
 	sigma_delta->bus_locked = true;
+	sigma_delta->keep_cs_asserted = true;
 	reinit_completion(&sigma_delta->completion);
 
 	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_SINGLE);
@@ -298,9 +301,6 @@ int ad_sigma_delta_single_conversion(struct iio_dev *indio_dev,
 	ret = wait_for_completion_interruptible_timeout(
 			&sigma_delta->completion, HZ);
 
-	sigma_delta->bus_locked = false;
-	spi_bus_unlock(sigma_delta->spi->master);
-
 	if (ret == 0)
 		ret = -EIO;
 	if (ret < 0)
@@ -321,7 +321,10 @@ int ad_sigma_delta_single_conversion(struct iio_dev *indio_dev,
 		sigma_delta->irq_dis = true;
 	}
 
+	sigma_delta->keep_cs_asserted = false;
 	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
+	sigma_delta->bus_locked = false;
+	spi_bus_unlock(sigma_delta->spi->master);
 	mutex_unlock(&indio_dev->mlock);
 
 	if (ret)
@@ -358,6 +361,8 @@ static int ad_sd_buffer_postenable(struct iio_dev *indio_dev)
 
 	spi_bus_lock(sigma_delta->spi->master);
 	sigma_delta->bus_locked = true;
+	sigma_delta->keep_cs_asserted = true;
+
 	ret = ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_CONTINUOUS);
 	if (ret)
 		goto err_unlock;
@@ -386,6 +391,7 @@ static int ad_sd_buffer_postdisable(struct iio_dev *indio_dev)
 		sigma_delta->irq_dis = true;
 	}
 
+	sigma_delta->keep_cs_asserted = false;
 	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
 
 	sigma_delta->bus_locked = false;

include/linux/iio/adc/ad_sigma_delta.h

@@ -69,6 +69,7 @@ struct ad_sigma_delta {
 	bool			irq_dis;
 
 	bool			bus_locked;
+	bool			keep_cs_asserted;
 
 	uint8_t			comm;