[PATCH 2/2] iio: iio-mux: kzalloc instead of devm_kzalloc to ensure page alignment

[Matteo Martelli <matteomartelli3@gmail.com>]

During channel configuration, the iio-mux driver allocates a page with
devm_kzalloc(PAGE_SIZE) to read channel ext_info. However, the resulting
buffer points to an offset of the page due to the devres header sitting
at the beginning of the allocated area. This leads to failure in the
provider driver when sysfs_emit* helpers are used to format the ext_info
attributes.

Switch to plain kzalloc version. The devres version is not strictly
necessary as the buffer is only accessed during the channel
configuration phase. Rely on __free cleanup to deallocate the buffer.
Also, move the ext_info handling into a new function to have the page
buffer definition and assignment in one statement as suggested by
cleanup documentation.

Signed-off-by: Matteo Martelli <matteomartelli3@gmail.com>
---
 drivers/iio/multiplexer/iio-mux.c | 84 +++++++++++++++++++++------------------
 1 file changed, 46 insertions(+), 38 deletions(-)

diff --git a/drivers/iio/multiplexer/iio-mux.c b/drivers/iio/multiplexer/iio-mux.c
index 2953403bef53bbe47a97a8ab1c475ed88d7f86d2..c309d991490c63ba4299f1cda7102f10dcf54982 100644
--- a/drivers/iio/multiplexer/iio-mux.c
+++ b/drivers/iio/multiplexer/iio-mux.c
@@ -7,6 +7,7 @@
  * Author: Peter Rosin <peda@axentia.se>
  */
 
+#include <linux/cleanup.h>
 #include <linux/err.h>
 #include <linux/iio/consumer.h>
 #include <linux/iio/iio.h>
@@ -237,49 +238,18 @@ static ssize_t mux_write_ext_info(struct iio_dev *indio_dev, uintptr_t private,
 	return ret;
 }
 
-static int mux_configure_channel(struct device *dev, struct mux *mux,
-				 u32 state, const char *label, int idx)
+static int mux_configure_chan_ext_info(struct device *dev, struct mux *mux,
+				       int idx, int num_ext_info)
 {
 	struct mux_child *child = &mux->child[idx];
-	struct iio_chan_spec *chan = &mux->chan[idx];
 	struct iio_chan_spec const *pchan = mux->parent->channel;
-	char *page = NULL;
-	int num_ext_info;
 	int i;
 	int ret;
 
-	chan->indexed = 1;
-	chan->output = pchan->output;
-	chan->datasheet_name = label;
-	chan->ext_info = mux->ext_info;
-
-	ret = iio_get_channel_type(mux->parent, &chan->type);
-	if (ret < 0) {
-		dev_err(dev, "failed to get parent channel type\n");
-		return ret;
-	}
-
-	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_RAW))
-		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_RAW);
-	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_SCALE))
-		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_SCALE);
-
-	if (iio_channel_has_available(pchan, IIO_CHAN_INFO_RAW))
-		chan->info_mask_separate_available |= BIT(IIO_CHAN_INFO_RAW);
-
-	if (state >= mux_control_states(mux->control)) {
-		dev_err(dev, "too many channels\n");
-		return -EINVAL;
-	}
-
-	chan->channel = state;
+	char *page __free(kfree) = kzalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!page)
+		return -ENOMEM;
 
-	num_ext_info = iio_get_channel_ext_info_count(mux->parent);
-	if (num_ext_info) {
-		page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
-		if (!page)
-			return -ENOMEM;
-	}
 	child->ext_info_cache = devm_kcalloc(dev,
 					     num_ext_info,
 					     sizeof(*child->ext_info_cache),
@@ -318,8 +288,46 @@ static int mux_configure_channel(struct device *dev, struct mux *mux,
 		child->ext_info_cache[i].size = ret;
 	}
 
-	if (page)
-		devm_kfree(dev, page);
+	return 0;
+}
+
+static int mux_configure_channel(struct device *dev, struct mux *mux, u32 state,
+				 const char *label, int idx)
+{
+	struct iio_chan_spec *chan = &mux->chan[idx];
+	struct iio_chan_spec const *pchan = mux->parent->channel;
+	int num_ext_info;
+	int ret;
+
+	chan->indexed = 1;
+	chan->output = pchan->output;
+	chan->datasheet_name = label;
+	chan->ext_info = mux->ext_info;
+
+	ret = iio_get_channel_type(mux->parent, &chan->type);
+	if (ret < 0) {
+		dev_err(dev, "failed to get parent channel type\n");
+		return ret;
+	}
+
+	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_RAW))
+		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_RAW);
+	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_SCALE))
+		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_SCALE);
+
+	if (iio_channel_has_available(pchan, IIO_CHAN_INFO_RAW))
+		chan->info_mask_separate_available |= BIT(IIO_CHAN_INFO_RAW);
+
+	if (state >= mux_control_states(mux->control)) {
+		dev_err(dev, "too many channels\n");
+		return -EINVAL;
+	}
+
+	chan->channel = state;
+
+	num_ext_info = iio_get_channel_ext_info_count(mux->parent);
+	if (num_ext_info)
+		return mux_configure_chan_ext_info(dev, mux, idx, num_ext_info);
 
 	return 0;
 }

-- 
2.47.0