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

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Message-ID: <bd5c1587-c830-40e6-90d5-be7fad215cef@baylibre.com>
Date: Mon, 21 Jul 2025 18:06:39 -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
Subject: Re: [PATCH v2] iio: temperature: maxim_thermocouple: use DMA-safe
 buffer for spi_read()

On 7/21/25 6:04 PM, David Lechner wrote:
> 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);

Compile check didn't catch this. Need to drop the &.
data->buffer.raw is already an array.


>  	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,