[PATCH] iio: buffer: rework buffer attr read-only stat-flags with 'is_visible' hook

[Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>]

The kernel provides a facility for attribute groups to specify the stat
flags of a sysfs file with an 'is_visible()' hook. This mechanism is
usually more flexible than assigning read-only attributes at construction
time.
It's added-value is a bit more apparent when the number of attributes
grows, so for sysfs buffer attributes this change may not be that be useful
quite now.

It should become more useful as the sysfs structure for buffer attributes
gets changed a bit.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
---
 drivers/iio/industrialio-buffer.c | 48 ++++++++++++++++++++++---------
 1 file changed, 35 insertions(+), 13 deletions(-)

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index 221157136af6..60bb03e72afc 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -1214,24 +1214,50 @@ static ssize_t iio_dma_show_data_available(struct device *dev,
 	return sprintf(buf, "%zu\n", bytes);
 }
 
+enum {
+	IIO_BUFFER_ATTR_LENGTH,
+	IIO_BUFFER_ATTR_ENABLE,
+	IIO_BUFFER_ATTR_WATERMARK,
+	IIO_BUFFER_ATTR_DATA_AVAILABLE,
+	__IIO_BUFFER_ATTR_MAX
+};
+
+static umode_t iio_buffer_attr_group_is_visible(struct kobject *kobj,
+						struct attribute *attr,
+						int index)
+{
+	struct device *dev = container_of(kobj, struct device, kobj);
+	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
+	struct iio_buffer *buffer = indio_dev->buffer;
+
+	switch (index) {
+	case IIO_BUFFER_ATTR_LENGTH:
+		if (!buffer->access->set_length)
+			return S_IRUGO;
+		return attr->mode;
+	case IIO_BUFFER_ATTR_WATERMARK:
+		if (buffer->access->flags & INDIO_BUFFER_FLAG_FIXED_WATERMARK)
+			return S_IRUGO;
+		return attr->mode;
+	default:
+		return attr->mode;
+	}
+}
+
 static DEVICE_ATTR(length, S_IRUGO | S_IWUSR, iio_buffer_read_length,
 		   iio_buffer_write_length);
-static struct device_attribute dev_attr_length_ro = __ATTR(length,
-	S_IRUGO, iio_buffer_read_length, NULL);
 static DEVICE_ATTR(enable, S_IRUGO | S_IWUSR,
 		   iio_buffer_show_enable, iio_buffer_store_enable);
 static DEVICE_ATTR(watermark, S_IRUGO | S_IWUSR,
 		   iio_buffer_show_watermark, iio_buffer_store_watermark);
-static struct device_attribute dev_attr_watermark_ro = __ATTR(watermark,
-	S_IRUGO, iio_buffer_show_watermark, NULL);
 static DEVICE_ATTR(data_available, S_IRUGO,
 		iio_dma_show_data_available, NULL);
 
 static struct attribute *iio_buffer_attrs[] = {
-	&dev_attr_length.attr,
-	&dev_attr_enable.attr,
-	&dev_attr_watermark.attr,
-	&dev_attr_data_available.attr,
+	[IIO_BUFFER_ATTR_LENGTH] = &dev_attr_length.attr,
+	[IIO_BUFFER_ATTR_ENABLE] = &dev_attr_enable.attr,
+	[IIO_BUFFER_ATTR_WATERMARK] = &dev_attr_watermark.attr,
+	[IIO_BUFFER_ATTR_DATA_AVAILABLE] = &dev_attr_data_available.attr,
 };
 
 int iio_buffer_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
@@ -1266,11 +1292,6 @@ int iio_buffer_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
 		return -ENOMEM;
 
 	memcpy(attr, iio_buffer_attrs, sizeof(iio_buffer_attrs));
-	if (!buffer->access->set_length)
-		attr[0] = &dev_attr_length_ro.attr;
-
-	if (buffer->access->flags & INDIO_BUFFER_FLAG_FIXED_WATERMARK)
-		attr[2] = &dev_attr_watermark_ro.attr;
 
 	if (buffer->attrs)
 		memcpy(&attr[ARRAY_SIZE(iio_buffer_attrs)], buffer->attrs,
@@ -1279,6 +1300,7 @@ int iio_buffer_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
 	attr[attrcount + ARRAY_SIZE(iio_buffer_attrs)] = NULL;
 
 	buffer->buffer_group.name = "buffer";
+	buffer->buffer_group.is_visible = iio_buffer_attr_group_is_visible;
 	buffer->buffer_group.attrs = attr;
 
 	indio_dev->groups[indio_dev->groupcounter++] = &buffer->buffer_group;
-- 
2.17.1