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-rw-r--r--include/linux/device-mapper.h15
1 files changed, 15 insertions, 0 deletions
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 1e483fa7afb..3cd32478f2f 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -79,11 +79,26 @@ typedef int (*dm_ioctl_fn) (struct dm_target *ti, unsigned int cmd,
typedef int (*dm_merge_fn) (struct dm_target *ti, struct bvec_merge_data *bvm,
struct bio_vec *biovec, int max_size);
+/*
+ * These iteration functions are typically used to check (and combine)
+ * properties of underlying devices.
+ * E.g. Does at least one underlying device support flush?
+ * Does any underlying device not support WRITE_SAME?
+ *
+ * The callout function is called once for each contiguous section of
+ * an underlying device. State can be maintained in *data.
+ * Return non-zero to stop iterating through any further devices.
+ */
typedef int (*iterate_devices_callout_fn) (struct dm_target *ti,
struct dm_dev *dev,
sector_t start, sector_t len,
void *data);
+/*
+ * This function must iterate through each section of device used by the
+ * target until it encounters a non-zero return code, which it then returns.
+ * Returns zero if no callout returned non-zero.
+ */
typedef int (*dm_iterate_devices_fn) (struct dm_target *ti,
iterate_devices_callout_fn fn,
void *data);