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

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Message-ID: <pj2pzpd33covkhi2ho3nsoazqqqpfeiy6n42fkmada6drpr6bj@2dm4iqxdhotj>
Date: Tue, 22 Jul 2025 10:24:56 +0100
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Cameron <jic23@...nel.org>, 
	Nuno Sá <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>, 
	Matt Ranostay <mranostay@...il.com>, 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 Mon, Jul 21, 2025 at 06:06:39PM -0500, David Lechner wrote:
> 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.

Neither did I :). I should have looked into this email __before__ (and
not after) sending my tag. Anyways, consider the tag with the above
fixed.

- Nuno Sá

> 
> 
> >  	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,
>