linux-kernel - [PATCH v2] iio: temperature: maxim_thermocouple: use DMA-safe buffer for spi

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Message-Id: <20250721-iio-use-more-iio_declare_buffer_with_ts-3-v2-1-0c68d41ccf6c@baylibre.com>
Date: Mon, 21 Jul 2025 18:04:04 -0500
From: David Lechner <dlechner@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>, 
 Nuno Sá <nuno.sa@...log.com>, 
 Andy Shevchenko <andy@...nel.org>, Matt Ranostay <mranostay@...il.com>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
 David Lechner <dlechner@...libre.com>
Subject: [PATCH v2] 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 <dlechner@...libre.com>
---
Changes in v2:
- This is a new patch since when looking at it again, I noticed a bug
  with passing stack-allocated memory to spi_read(). So now the primary
  purpose is a fix and converting the array to a struct comes free with
  it.
- Link to v1: https://lore.kernel.org/r/20250711-iio-use-more-iio_declare_buffer_with_ts-3-v1-1-f6dd3363fd85@baylibre.com
---
 drivers/iio/temperature/maxim_thermocouple.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/drivers/iio/temperature/maxim_thermocouple.c b/drivers/iio/temperature/maxim_thermocouple.c
index cae8e84821d7fd521d59432580d51def939fa4d1..fa648a6542a4e2f08adb556c776b68331ae69631 100644
--- a/drivers/iio/temperature/maxim_thermocouple.c
+++ b/drivers/iio/temperature/maxim_thermocouple.c
@@ -11,6 +11,7 @@
 #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>
@@ -121,8 +122,15 @@ struct maxim_thermocouple_data {
 	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,
@@ -130,18 +138,16 @@ 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;
@@ -166,9 +172,9 @@ static irqreturn_t maxim_thermocouple_trigger_handler(int irq, void *private)
 	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));
 	}

---
base-commit: cd2731444ee4e35db76f4fb587f12d327eec5446
change-id: 20250711-iio-use-more-iio_declare_buffer_with_ts-3-2cc387a66bdc

Best regards,
-- 
David Lechner <dlechner@...libre.com>