iio: adc: ad7944: don't use storagebits for sizing

Replace use of storagebits with realbits for determining the number of
bytes needed for SPI transfers.

When adding SPI offload support, storagebits will always be 32 rather
than 16 for 16-bit 16-bit chips so we can no longer rely on storagebits
being the correct size expected by the SPI framework (it always uses
4 bytes for > 16-bit xfers and 2 bytes for > 8-bit xfers). Instead,
derive the correct size from realbits since it will always be correct
even when SPI offloading is used.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-vy: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-10-e48a489be48c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

drivers/iio/adc/ad7944.c

@@ -98,6 +98,9 @@ struct ad7944_chip_info {
 	const struct iio_chan_spec channels[2];
 };
 
+/* get number of bytes for SPI xfer */
+#define AD7944_SPI_BYTES(scan_type) ((scan_type).realbits > 16 ? 4 : 2)
+
 /*
  * AD7944_DEFINE_CHIP_INFO - Define a chip info structure for a specific chip
  * @_name: The name of the chip
@@ -164,7 +167,7 @@ static int ad7944_3wire_cs_mode_init_msg(struct device *dev, struct ad7944_adc *
 
 	/* Then we can read the data during the acquisition phase */
 	xfers[2].rx_buf = &adc->sample.raw;
-	xfers[2].len = BITS_TO_BYTES(chan->scan_type.storagebits);
+	xfers[2].len = AD7944_SPI_BYTES(chan->scan_type);
 	xfers[2].bits_per_word = chan->scan_type.realbits;
 
 	spi_message_init_with_transfers(&adc->msg, xfers, 3);
@@ -193,7 +196,7 @@ static int ad7944_4wire_mode_init_msg(struct device *dev, struct ad7944_adc *adc
 	xfers[0].delay.unit = SPI_DELAY_UNIT_NSECS;
 
 	xfers[1].rx_buf = &adc->sample.raw;
-	xfers[1].len = BITS_TO_BYTES(chan->scan_type.storagebits);
+	xfers[1].len = AD7944_SPI_BYTES(chan->scan_type);
 	xfers[1].bits_per_word = chan->scan_type.realbits;
 
 	spi_message_init_with_transfers(&adc->msg, xfers, 2);
@@ -228,7 +231,7 @@ static int ad7944_chain_mode_init_msg(struct device *dev, struct ad7944_adc *adc
 	xfers[0].delay.unit = SPI_DELAY_UNIT_NSECS;
 
 	xfers[1].rx_buf = adc->chain_mode_buf;
-	xfers[1].len = BITS_TO_BYTES(chan->scan_type.storagebits) * n_chain_dev;
+	xfers[1].len = AD7944_SPI_BYTES(chan->scan_type) * n_chain_dev;
 	xfers[1].bits_per_word = chan->scan_type.realbits;
 
 	spi_message_init_with_transfers(&adc->msg, xfers, 2);
@@ -274,12 +277,12 @@ static int ad7944_single_conversion(struct ad7944_adc *adc,
 		return ret;
 
 	if (adc->spi_mode == AD7944_SPI_MODE_CHAIN) {
-		if (chan->scan_type.storagebits > 16)
+		if (chan->scan_type.realbits > 16)
 			*val = ((u32 *)adc->chain_mode_buf)[chan->scan_index];
 		else
 			*val = ((u16 *)adc->chain_mode_buf)[chan->scan_index];
 	} else {
-		if (chan->scan_type.storagebits > 16)
+		if (chan->scan_type.realbits > 16)
 			*val = adc->sample.raw.u32;
 		else
 			*val = adc->sample.raw.u16;
@@ -409,8 +412,7 @@ static int ad7944_chain_mode_alloc(struct device *dev,
 	/* 1 word for each voltage channel + aligned u64 for timestamp */
 
 	chain_mode_buf_size = ALIGN(n_chain_dev *
-		BITS_TO_BYTES(chan[0].scan_type.storagebits), sizeof(u64))
-		+ sizeof(u64);
+		AD7944_SPI_BYTES(chan[0].scan_type), sizeof(u64)) + sizeof(u64);
 	buf = devm_kzalloc(dev, chain_mode_buf_size, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;