[PATCH v3 01/51] core: Introduce the scsi_cmd_to_rq() function

The 'request' member of struct scsi_cmnd is superfluous. The struct
request and struct scsi_cmnd data structures are adjacent and hence the
request pointer can be derived easily from a scsi_cmnd pointer. Introduce
a helper function that performs that conversion in a type-safe way. This
patch is the first step towards removing the request member from struct
scsi_cmnd. Making that change has the following advantages:
- This is a performance optimization since adding an offset to a pointer
  takes less time than dereferencing a pointer.
- struct scsi_cmnd becomes smaller.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Ming Lei <redacted>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 include/scsi/scsi_cmnd.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
index fed024f4c02a..6787670d0d16 100644
--- a/include/scsi/scsi_cmnd.h
+++ b/include/scsi/scsi_cmnd.h

@@ -146,6 +146,12 @@ struct scsi_cmnd {
 	unsigned int extra_len;	/* length of alignment and padding */
 };
 
+/* Variant of blk_mq_rq_from_pdu() that verifies the type of its argument. */
+static inline struct request *scsi_cmd_to_rq(struct scsi_cmnd *scmd)
+{
+	return blk_mq_rq_from_pdu(scmd);
+}
+
 /*
  * Return the driver private allocation behind the command.
  * Only works if cmd_size is set in the host template.