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Message-ID: <20250724121546.72d82f80@jic23-huawei>
Date: Thu, 24 Jul 2025 12:15:46 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: Nuno Sá <noname.nuno@...il.com>
Cc: David Lechner <dlechner@...libre.com>, 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 Tue, 22 Jul 2025 10:24:56 +0100
Nuno Sá <noname.nuno@...il.com> wrote:
> 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
>
Applied to the fixes-togreg-for-6.17 branch + marked for stable with:
diff --git a/drivers/iio/temperature/maxim_thermocouple.c b/drivers/iio/temperature/maxim_thermocouple.c
index fa648a6542a4..205939680fd4 100644
--- a/drivers/iio/temperature/maxim_thermocouple.c
+++ b/drivers/iio/temperature/maxim_thermocouple.c
@@ -172,7 +172,7 @@ 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.raw, 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,
sizeof(data->buffer),
Quicker than going around again :)
> >
> >
> > > 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,
> >
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