iio: temperature: maxim_thermocouple: use DMA-safe buffer for spi_read()

Replace using stack-allocated buffers with a DMA-safe buffer for use
with spi_read(). This allows the driver to be safely used with
DMA-enabled SPI controllers.

The buffer array is also converted to a struct with a union to make the
usage of the memory in the buffer more clear and ensure proper alignment.

Fixes: 1f25ca11d84a ("iio: temperature: add support for Maxim thermocouple chips")
Signed-off-by: David Lechner <[e-mail]>
Reviewed-by: Nuno Sá <[e-mail]>
Link: https://patch.msgid.link/2025[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/temperature/maxim_thermocouple.c

···
 #include <linux/module.h>
 #include <linux/err.h>
 #include <linux/spi/spi.h>
+#include <linux/types.h>
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
 #include <linux/iio/trigger.h>
···
 	struct spi_device *spi;
 	const struct maxim_thermocouple_chip *chip;
 	char tc_type;
-
-	u8 buffer[16] __aligned(IIO_DMA_MINALIGN);
+	/* Buffer for reading up to 2 hardware channels. */
+	struct {
+		union {
+			__be16 raw16;
+			__be32 raw32;
+			__be16 raw[2];
+		};
+		aligned_s64 timestamp;
+	} buffer __aligned(IIO_DMA_MINALIGN);
 };
 
 static int maxim_thermocouple_read(struct maxim_thermocouple_data *data,
···
 {
 	unsigned int storage_bytes = data->chip->read_size;
 	unsigned int shift = chan->scan_type.shift + (chan->address * 8);
-	__be16 buf16;
-	__be32 buf32;
 	int ret;
 
 	switch (storage_bytes) {
 	case 2:
-		ret = spi_read(data->spi, (void *)&buf16, storage_bytes);
-		*val = be16_to_cpu(buf16);
+		ret = spi_read(data->spi, &data->buffer.raw16, storage_bytes);
+		*val = be16_to_cpu(data->buffer.raw16);
 		break;
 	case 4:
-		ret = spi_read(data->spi, (void *)&buf32, storage_bytes);
-		*val = be32_to_cpu(buf32);
+		ret = spi_read(data->spi, &data->buffer.raw32, storage_bytes);
+		*val = be32_to_cpu(data->buffer.raw32);
 		break;
 	default:
 		ret = -EINVAL;
···
 	struct maxim_thermocouple_data *data = iio_priv(indio_dev);
 	int ret;
 
-	ret = spi_read(data->spi, data->buffer, data->chip->read_size);
+	ret = spi_read(data->spi, data->buffer.raw, data->chip->read_size);
 	if (!ret) {
-		iio_push_to_buffers_with_ts(indio_dev, data->buffer,
+		iio_push_to_buffers_with_ts(indio_dev, &data->buffer,
 					    sizeof(data->buffer),
 					    iio_get_time_ns(indio_dev));
 	}