iio: buffer: group attr count and attr alloc

If we want to merge the attributes of the buffer/ and scan_elements/
directories, we'll need to count all attributes first, then (depending on
the attribute group) either allocate 2 attribute groups, or a single one.
Historically an IIO buffer was described by 2 subdirectories under
/sys/bus/iio/iio:devicesX (i.e. buffer/ and scan_elements/); these subdirs
were actually 2 separate attribute groups on the iio_buffer object.

Moving forward, if we want to allow more than one buffer per IIO device,
keeping 2 subdirectories for each IIO buffer is a bit cumbersome
(especially for userpace ABI). So, we will merge the attributes of these 2
subdirs under a /sys/bus/iio/iio:devicesX/bufferY subdirectory. To do this,
we need to count all attributes first, and then distribute them based on
which buffer this is. For the first buffer, we'll need to also allocate the
legacy 2 attribute groups (for buffer/ and scan_elements/), and also a
/sys/bus/iio/iio:devicesX/buffer0 attribute group.

For buffer1 and above, just a single attribute group will be allocated (the
merged one).
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-12-alexandru.ardelean@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -1257,41 +1257,16 @@ static int __iio_buffer_alloc_sysfs_and_mask(struct iio_buffer *buffer,
 {
 	struct iio_dev_attr *p;
 	struct attribute **attr;
-	int ret, i, attrn, attrcount;
+	int ret, i, attrn, scan_el_attrcount, buffer_attrcount;
 	const struct iio_chan_spec *channels;
 
-	attrcount = 0;
+	buffer_attrcount = 0;
 	if (buffer->attrs) {
-		while (buffer->attrs[attrcount] != NULL)
-			attrcount++;
+		while (buffer->attrs[buffer_attrcount] != NULL)
+			buffer_attrcount++;
 	}
 
-	attr = kcalloc(attrcount + ARRAY_SIZE(iio_buffer_attrs) + 1,
-		       sizeof(struct attribute *), GFP_KERNEL);
-	if (!attr)
-		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,
-		       sizeof(struct attribute *) * attrcount);
-
-	attr[attrcount + ARRAY_SIZE(iio_buffer_attrs)] = NULL;
-
-	buffer->buffer_group.name = "buffer";
-	buffer->buffer_group.attrs = attr;
-
-	ret = iio_device_register_sysfs_group(indio_dev, &buffer->buffer_group);
-	if (ret)
-		goto error_free_buffer_attrs;
-
-	attrcount = 0;
+	scan_el_attrcount = 0;
 	INIT_LIST_HEAD(&buffer->scan_el_dev_attr_list);
 	channels = indio_dev->channels;
 	if (channels) {
@@ -1304,7 +1279,7 @@ static int __iio_buffer_alloc_sysfs_and_mask(struct iio_buffer *buffer,
 							 &channels[i]);
 			if (ret < 0)
 				goto error_cleanup_dynamic;
-			attrcount += ret;
+			scan_el_attrcount += ret;
 			if (channels[i].type == IIO_TIMESTAMP)
 				indio_dev->scan_index_timestamp =
 					channels[i].scan_index;
@@ -1319,9 +1294,37 @@ static int __iio_buffer_alloc_sysfs_and_mask(struct iio_buffer *buffer,
 		}
 	}
 
+	attr = kcalloc(buffer_attrcount + ARRAY_SIZE(iio_buffer_attrs) + 1,
+		       sizeof(*attr), GFP_KERNEL);
+	if (!attr) {
+		ret = -ENOMEM;
+		goto error_free_scan_mask;
+	}
+
+	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,
+		       sizeof(struct attribute *) * buffer_attrcount);
+
+	buffer_attrcount += ARRAY_SIZE(iio_buffer_attrs);
+	attr[buffer_attrcount] = NULL;
+
+	buffer->buffer_group.name = "buffer";
+	buffer->buffer_group.attrs = attr;
+
+	ret = iio_device_register_sysfs_group(indio_dev, &buffer->buffer_group);
+	if (ret)
+		goto error_free_buffer_attrs;
+
 	buffer->scan_el_group.name = iio_scan_elements_group_name;
 
-	buffer->scan_el_group.attrs = kcalloc(attrcount + 1,
+	buffer->scan_el_group.attrs = kcalloc(scan_el_attrcount + 1,
 					      sizeof(buffer->scan_el_group.attrs[0]),
 					      GFP_KERNEL);
 	if (buffer->scan_el_group.attrs == NULL) {
@@ -1341,12 +1344,12 @@ static int __iio_buffer_alloc_sysfs_and_mask(struct iio_buffer *buffer,
 
 error_free_scan_el_attrs:
 	kfree(buffer->scan_el_group.attrs);
+error_free_buffer_attrs:
+	kfree(buffer->buffer_group.attrs);
 error_free_scan_mask:
 	bitmap_free(buffer->scan_mask);
 error_cleanup_dynamic:
 	iio_free_chan_devattr_list(&buffer->scan_el_dev_attr_list);
-error_free_buffer_attrs:
-	kfree(buffer->buffer_group.attrs);
 
 	return ret;
 }