[PATCH v3] SCSI/libsrp: document the libsrp source code

[Bart Van Assche <bart.vanassche-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Adds documentation to the libsrp source code.

Signed-off-by: Bart Van Assche <bart.vanassche-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: James E.J. Bottomley <James.Bottomley-l3A5Bk7waGM@public.gmane.org>
Cc: FUJITA Tomonori <fujita.tomonori-Zyj7fXuS5i5L9jVzuh4AOg@public.gmane.org>
Acked-by: Randy Dunlap <randy.dunlap-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>

----
This is the third version of this patch. Compared to the first version of this
patch, the patch does now adhere to the kernel-doc format and function
arguments that were not yet documented are now documented. Thanks to Randy
Dunlap for the reviews.


diff --git a/drivers/scsi/libsrp.c b/drivers/scsi/libsrp.c
index 9ad38e8..0eeeca2 100644
--- a/drivers/scsi/libsrp.c
+++ b/drivers/scsi/libsrp.c
@@ -29,6 +29,10 @@
 #include <scsi/srp.h>
 #include <scsi/libsrp.h>
 
+/*
+ * Allowed values for the 'TASK ATTRIBUTE' field of an SRP_CMD request. See
+ * also section 6.8 of the T10 SRP r16a document.
+ */
 enum srp_task_attributes {
 	SRP_SIMPLE_TASK = 0,
 	SRP_HEAD_TASK = 1,
@@ -44,6 +48,17 @@ do {								\
 /* #define dprintk eprintk */
 #define dprintk(fmt, args...)
 
+/**
+ * srp_iu_pool_alloc() - Allocate a pool of information units for use by an SRP
+ * target.
+ * @q: pointer to the pool structure that will be initialized.
+ * @max: number of information units the pool will contain.
+ * @ring: pointer to an array of 'max' SRP buffer pointers. Each information
+ *   unit allocated for the pool will be initialized such that it points to
+ *   the corresponding SRP buffer in array 'ring'.
+ *
+ * Returns zero upon success and a negative error code upon failure.
+ */
 static int srp_iu_pool_alloc(struct srp_queue *q, size_t max,
 			     struct srp_buf **ring)
 {
@@ -77,12 +92,28 @@ free_pool:
 	return -ENOMEM;
 }
 
+/**
+ * srp_iu_pool_free() - Free the memory allocated by srp_iu_pool_alloc().
+ * @q: SRP queue pointer.
+ *
+ * Note: The memory occupied by the struct srp_queue itself is not freed
+ * -- this is the responsibility of the caller.
+ */
 static void srp_iu_pool_free(struct srp_queue *q)
 {
 	kfree(q->items);
 	kfree(q->pool);
 }
 
+/**
+ * srp_ring_alloc() - Allocate a ring of SRP buffers and set up a coherent DMA
+ * mapping for each buffer.
+ * @dev: device context for DMA mapping.
+ * @max: number of elements the ring will contain.
+ * @size: size in bytes of one ring element.
+ *
+ * Returns a pointer to an array of 'max' pointers to SRP buffers.
+ */
 static struct srp_buf **srp_ring_alloc(struct device *dev,
 				       size_t max, size_t size)
 {
@@ -115,6 +146,13 @@ out:
 	return NULL;
 }
 
+/**
+ * srp_ring_free() - Free the memory allocated by srp_ring_alloc().
+ * @dev: device context for DMA unmapping.
+ * @ring: the buffer ring to be freed.
+ * @max: number of elements the ring contains.
+ * @size: size in bytes of one ring element.
+ */
 static void srp_ring_free(struct device *dev, struct srp_buf **ring, size_t max,
 			  size_t size)
 {
@@ -127,6 +165,16 @@ static void srp_ring_free(struct device *dev, struct srp_buf **ring, size_t max,
 	kfree(ring);
 }
 
+/**
+ * srp_target_alloc() - Initialize an SRP target structure and allocate an SRP
+ * information unit ring.
+ * @target: pointer to the SRP target structure to be initialized.
+ * @dev: device to be associated with the SRP target.
+ * @nr: number of elements the SRP receive ring will contain.
+ * @iu_size: size in bytes of a single information unit.
+ *
+ * Returns zero upon success and a negative error code upon failure.
+ */
 int srp_target_alloc(struct srp_target *target, struct device *dev,
 		     size_t nr, size_t iu_size)
 {
@@ -155,6 +203,10 @@ free_ring:
 }
 EXPORT_SYMBOL_GPL(srp_target_alloc);
 
+/**
+ * srp_target_free() - Free the memory allocated by srp_target_alloc().
+ * @target: SRP target.
+ */
 void srp_target_free(struct srp_target *target)
 {
 	srp_ring_free(target->dev, target->rx_ring, target->rx_ring_size,
@@ -163,6 +215,13 @@ void srp_target_free(struct srp_target *target)
 }
 EXPORT_SYMBOL_GPL(srp_target_free);
 
+/**
+ * srp_iu_get() - Get an information unit that is not in use.
+ * @target: SRP target to receive an information unit from.
+ *
+ * Initialize the ilist and flags members of the information unit before
+ * returning.
+ */
 struct iu_entry *srp_iu_get(struct srp_target *target)
 {
 	struct iu_entry *iue = NULL;
@@ -177,12 +236,27 @@ struct iu_entry *srp_iu_get(struct srp_target *target)
 }
 EXPORT_SYMBOL_GPL(srp_iu_get);
 
+/**
+ * srp_iu_put() - Put an information unit back in the SRP target receive ring.
+ * @iue: information unit to be put back.
+ */
 void srp_iu_put(struct iu_entry *iue)
 {
 	kfifo_put(iue->target->iu_queue.queue, (void *) &iue, sizeof(void *));
 }
 EXPORT_SYMBOL_GPL(srp_iu_put);
 
+/**
+ * srp_direct_data() - Transfer the data referred to by an SRP direct data
+ * buffer descriptor via RDMA.
+ * @sc: SCSI command.
+ * @md: SRP direct data buffer descriptor.
+ * @dir: DMA data direction, the second last parameter passed to the function
+ *       'rdma_io'.
+ * @rdma_io: pointer to a callback function that performs the actual I/O.
+ * @dma_map: whether or not to map and unmap the scsi_sglist(sc).
+ * @ext_desc: ignored.
+ */
 static int srp_direct_data(struct scsi_cmnd *sc, struct srp_direct_buf *md,
 			   enum dma_data_direction dir, srp_rdma_t rdma_io,
 			   int dma_map, int ext_desc)
@@ -216,6 +290,18 @@ static int srp_direct_data(struct scsi_cmnd *sc, struct srp_direct_buf *md,
 	return err;
 }
 
+/**
+ * srp_indirect_data() - Transfer the data referred to by an SRP indirect data
+ * buffer descriptor via RDMA.
+ * @sc: SCSI command.
+ * @cmd: SRP command.
+ * @id: SRP indirect data buffer descriptor.
+ * @dir: DMA data direction, the second last parameter passed to the function
+ *       'rdma_io'.
+ * @rdma_io: pointer to a callback function that performs the actual I/O.
+ * @dma_map: whether or not to map and unmap the scsi_sglist(sc).
+ * @ext_desc: whether or not this command uses an external indirect buffer.
+ */
 static int srp_indirect_data(struct scsi_cmnd *sc, struct srp_cmd *cmd,
 			     struct srp_indirect_buf *id,
 			     enum dma_data_direction dir, srp_rdma_t rdma_io,
@@ -292,6 +378,11 @@ free_mem:
 	return err;
 }
 
+/**
+ * data_out_desc_size() - Compute the number of bytes occupied by the DATA-OUT
+ * descriptor.
+ * @cmd: SRP_CMD request.
+ */
 static int data_out_desc_size(struct srp_cmd *cmd)
 {
 	int size = 0;
@@ -314,8 +405,17 @@ static int data_out_desc_size(struct srp_cmd *cmd)
 	return size;
 }
 
-/*
- * TODO: this can be called multiple times for a single command if it
+/**
+ * srp_transfer_data() - Transfer the data associated with an SRP_CMD request.
+ * @sc: SCSI command.
+ * @cmd: SRP_CMD request.
+ * @rdma_io: callback function for performing RDMA I/O.
+ * @dma_map: second-last parameter passed to rdma_io.
+ * @ext_desc: whether this command uses an external indirect buffer.
+ *
+ * Note: Does not support bidirectional commands.
+ *
+ * To Do: This function can be called multiple times for a single command if it
  * has very long data.
  */
 int srp_transfer_data(struct scsi_cmnd *sc, struct srp_cmd *cmd,
@@ -327,10 +427,23 @@ int srp_transfer_data(struct scsi_cmnd *sc, struct srp_cmd *cmd,
 	int offset, err = 0;
 	u8 format;
 
+	/*
+	 * 'offset' is the offset in bytes of the data buffer descriptor in
+	 * an SRP_CMD request. For an SRP request that transfers data from
+	 * initiator to target, 'offset' is the offset of the data-out buffer
+	 * descriptor. For an SRP request that transfers data from target to
+	 * initiator, 'offset' is the offset of the data-in buffer descriptor.
+	 * In an SRP_CMD request the following descriptors are present starting
+	 * from cmd->add_data: additional CDB, data-out buffer descriptor and
+	 * data-in buffer descriptor.
+	 */
+
+	/* Skip over 'additional CDB' to start of data-out descriptor. */
 	offset = cmd->add_cdb_len * 4;
 
 	dir = srp_cmd_direction(cmd);
 	if (dir == DMA_FROM_DEVICE)
+		/* Skip over data-out descriptor to start of data-in. */
 		offset += data_out_desc_size(cmd);
 
 	if (dir == DMA_TO_DEVICE)
@@ -361,6 +474,13 @@ int srp_transfer_data(struct scsi_cmnd *sc, struct srp_cmd *cmd,
 }
 EXPORT_SYMBOL_GPL(srp_transfer_data);
 
+/**
+ * vscsis_data_length() - Compute the size in bytes of one of the two data
+ * descriptors at the end of an SRP_CMD request.
+ * @cmd: SRP_CMD request.
+ * @dir: DMA_TO_DEVICE for the size of the data-out buffer descriptor;
+ * DMA_FROM_DEVICE for the size of the data-in buffer descriptor.
+ */
 static int vscsis_data_length(struct srp_cmd *cmd, enum dma_data_direction dir)
 {
 	struct srp_direct_buf *md;
@@ -393,6 +513,17 @@ static int vscsis_data_length(struct srp_cmd *cmd, enum dma_data_direction dir)
 	return len;
 }
 
+/**
+ * srp_cmd_queue() - Queue an SRP_CMD request for processing by appropriate
+ * target software, e.g. the user space process tgtd.
+ * @shost: pointer to SCSI host.
+ * @cmd: pointer to SRP_CMD request.
+ * @info: data that is copied to the SCp field of the generated SCSI command.
+ * @itn_id: ID that is passed verbatim to the callback functions registered
+ *   via struct srp_template (see also <scsi/scsi_transport.h>).
+ * @addr: scatter/gather list (struct scatterlist *) to be included in the
+ *   generated SCSI command.
+ */
 int srp_cmd_queue(struct Scsi_Host *shost, struct srp_cmd *cmd, void *info,
 		  u64 itn_id, u64 addr)
 {
diff --git a/include/scsi/libsrp.h b/include/scsi/libsrp.h
index ba615e4..e11408f 100644
--- a/include/scsi/libsrp.h
+++ b/include/scsi/libsrp.h
@@ -1,11 +1,17 @@
 #ifndef __LIBSRP_H__
 #define __LIBSRP_H__
 
+/*
+ * Structures and constants exported by libsrp, a library for implementing
+ * SRP initiators and targets.
+ */
+
 #include <linux/list.h>
 #include <scsi/scsi_cmnd.h>
 #include <scsi/scsi_host.h>
 #include <scsi/srp.h>
 
+/* Names assigned to the bits of iu_entry::flags by the ibmvstgt driver. */
 enum iue_flags {
 	V_DIOVER,
 	V_WRITE,
@@ -13,11 +19,19 @@ enum iue_flags {
 	V_FLYING,
 };
 
+/* SRP data buffer as managed by libsrp. */
 struct srp_buf {
 	dma_addr_t dma;
 	void *buf;
 };
 
+/**
+ * struct srp_queue - Pool of SRP information units.
+ * @pool: kfifo storage; an array of n struct iu_entry* items.
+ * @items: information unit storage; an array of n struct iu_entry items.
+ * @queue: kfifo containing pointers to iu_entry items currently not in use.
+ * @lock: spinlock associated with the above kfifo.
+ */
 struct srp_queue {
 	void *pool;
 	void *items;
@@ -25,6 +39,18 @@ struct srp_queue {
 	spinlock_t lock;
 };
 
+/**
+ * struct srp_target - SRP target information.
+ * @shost: Pointer to the SCSI host associated with the SRP target.
+ * @dev: Pointer to the device node associated with the SRP target.
+ * @lock: Initialized but not used otherwise inside libsrp.
+ * @cmd_queue: Initialized but not used otherwise inside libsrp.
+ * @srp_iu_size: Size in bytes of a single information unit.
+ * @iu_queue: Information unit receive ring.
+ * @rx_ring_size: Number of elements in the information unit receive ring.
+ * @rx_ring: Pointer to an array of SRP buffers associated with the rx ring.
+ * @ldata: Pointer to target-private data.
+ */
 struct srp_target {
 	struct Scsi_Host *shost;
 	struct device *dev;
@@ -40,6 +66,14 @@ struct srp_target {
 	void *ldata;
 };
 
+/**
+ * struct iu_entry - Information unit as processed by an SRP target.
+ * @target: Backpointer to the SRP target processing this information unit.
+ * @ilist: Used by ibmvscsi for insertion in the srp_target::cmd_queue list.
+ * @remote_token: Used by the ibmvscsi driver but not by libsrp.
+ * @flags: Initialized by libsrp but not used otherwise by libsrp.
+ * @sbuf: SRP buffer associated with this information unit.
+ */
 struct iu_entry {
 	struct srp_target *target;
 
@@ -50,6 +84,14 @@ struct iu_entry {
 	struct srp_buf *sbuf;
 };
 
+/*
+ * Pointer to a callback function that performs RDMA I/O.
+ *
+ * Arguments (in order): SCSI command pointer, scatterlist pointer,
+ * number of entries in the scatterlist, pointer to an array of SRP
+ * memory descriptor pointers, number of elements in this array, DMA
+ * data direction, maximum number of bytes to transfer.
+ */
 typedef int (srp_rdma_t)(struct scsi_cmnd *, struct scatterlist *, int,
 			 struct srp_direct_buf *, int,
 			 enum dma_data_direction, unsigned int);
@@ -69,6 +111,16 @@ static inline struct srp_target *host_to_srp_target(struct Scsi_Host *host)
 	return (struct srp_target *) host->hostdata;
 }
 
+/**
+ * srp_cmd_direction() - Find out data transfer direction.
+ * @cmd: pointer to an SRP_CMD request.
+ *
+ * Returns DMA_TO_DEVICE when transferring data from initiator to target and
+ * DMA_FROM_DEVICE when transferring data from target to initiator. This
+ * reflects the point of view of an SRP initiator implementation.
+ *
+ * Note: Do not use this function for bidirectional commands.
+ */
 static inline int srp_cmd_direction(struct srp_cmd *cmd)
 {
 	return (cmd->buf_fmt >> 4) ? DMA_TO_DEVICE : DMA_FROM_DEVICE;
diff --git a/include/scsi/srp.h b/include/scsi/srp.h
index ad178fa..ebb6096 100644
--- a/include/scsi/srp.h
+++ b/include/scsi/srp.h
@@ -38,11 +38,13 @@
 /*
  * Structures and constants for the SCSI RDMA Protocol (SRP) as
  * defined by the INCITS T10 committee.  This file was written using
- * draft Revision 16a of the SRP standard.
+ * draft Revision 16a of the SRP standard. See also
+ * http://www.t10.org/drafts.htm#SCSI3_SRP.
  */
 
 #include <linux/types.h>
 
+/* SRP information unit types. */
 enum {
 	SRP_LOGIN_REQ	= 0x00,
 	SRP_TSK_MGMT	= 0x01,
@@ -58,17 +60,31 @@ enum {
 	SRP_AER_RSP	= 0x42
 };
 
+/*
+ * Flags for the 'REQUIRED BUFFER FORMATS' field of the SRP_LOGIN_REQ request
+ * (srp_login_req::req_buf_fmt) or the 'SUPPORTED BUFFER FORMATS' field of the
+ * SRP_LOGIN_RSP response (srp_login_rsp::buf_fmt). See also section 5.6.2.2
+ * in the T10 SRP r16a document.
+ */
 enum {
 	SRP_BUF_FORMAT_DIRECT	= 1 << 1,
 	SRP_BUF_FORMAT_INDIRECT	= 1 << 2
 };
 
+/*
+ * Format of data-in and data-out buffer descriptors in an SRP_CMD
+ * request. See also section 5.6.2.1 in the T10 SRP r16a document.
+ */
 enum {
 	SRP_NO_DATA_DESC	= 0,
 	SRP_DATA_DESC_DIRECT	= 1,
 	SRP_DATA_DESC_INDIRECT	= 2
 };
 
+/*
+ * Allowed values for the 'TASK MANAGEMENT FUNCTION' field of the SRP_TSK_MGMT
+ * request. See also section 6.7 in the T10 SRP r16a document.
+ */
 enum {
 	SRP_TSK_ABORT_TASK	= 0x01,
 	SRP_TSK_ABORT_TASK_SET	= 0x02,
@@ -77,6 +93,10 @@ enum {
 	SRP_TSK_CLEAR_ACA	= 0x40
 };
 
+/*
+ * Allowed values for the 'REASON' field of the SRP_LOGIN_REJ response. See
+ * also section 6.4 in the T10 SRP r16a document.
+ */
 enum srp_login_rej_reason {
 	SRP_LOGIN_REJ_UNABLE_ESTABLISH_CHANNEL		= 0x00010000,
 	SRP_LOGIN_REJ_INSUFFICIENT_RESOURCES		= 0x00010001,
@@ -87,11 +107,20 @@ enum srp_login_rej_reason {
 	SRP_LOGIN_REJ_CHANNEL_LIMIT_REACHED		= 0x00010006
 };
 
+/*
+ * IB I/O class assigned to revisions 10 and r16a of the SRP protocol.
+ * See also section B.7 in the T10 SRP r16a document.
+ */
 enum {
 	SRP_REV10_IB_IO_CLASS	= 0xff00,
 	SRP_REV16A_IB_IO_CLASS	= 0x0100
 };
 
+/*
+ * Direct data buffer descriptor as defined in section 5.6.2.4 of the T10 SRP
+ * r16a document. Each direct data buffer descriptor consists of a virtual
+ * address, a memory handle and a data length.
+ */
 struct srp_direct_buf {
 	__be64	va;
 	__be32	key;
@@ -99,6 +128,9 @@ struct srp_direct_buf {
 };
 
 /*
+ * Indirect data buffer descriptor as defined in section 5.6.2.5 of the T10
+ * SRP r16a document.
+ *
  * We need the packed attribute because the SRP spec puts the list of
  * descriptors at an offset of 20, which is not aligned to the size of
  * struct srp_direct_buf.  The whole structure must be packed to avoid
@@ -110,11 +142,20 @@ struct srp_indirect_buf {
 	struct srp_direct_buf	desc_list[0];
 } __attribute__((packed));
 
+/*
+ * Allowed values for the 'MULTI-CHANNEL ACTION CODE' field of the
+ * SRP_LOGIN_REQ request, as defined in section 6.2 of the T10 SRP r16a
+ * document.
+ */
 enum {
 	SRP_MULTICHAN_SINGLE = 0,
 	SRP_MULTICHAN_MULTI  = 1
 };
 
+/*
+ * SRP_LOGIN_REQ request, also defined in section 6.2 of the T10 SRP r16a
+ * document.
+ */
 struct srp_login_req {
 	u8	opcode;
 	u8	reserved1[7];
@@ -129,6 +170,9 @@ struct srp_login_req {
 };
 
 /*
+ * SRP_LOGIN_RSP response, as defined in section 6.3 of the T10 SRP r16a
+ * document.
+ *
  * The SRP spec defines the size of the LOGIN_RSP structure to be 52
  * bytes, so it needs to be packed to avoid having it padded to 56
  * bytes on 64-bit architectures.
@@ -145,6 +189,10 @@ struct srp_login_rsp {
 	u8	reserved2[25];
 } __attribute__((packed));
 
+/*
+ * SRP_LOGIN_REJ response, as defined in section 6.4 of the T10 SRP r16a
+ * document.
+ */
 struct srp_login_rej {
 	u8	opcode;
 	u8	reserved1[3];
@@ -155,12 +203,19 @@ struct srp_login_rej {
 	u8	reserved3[6];
 };
 
+/*
+ * SRP_I_LOGOUT request, as defined in section 6.5 of the T10 SRP r16a
+ * document.
+ */
 struct srp_i_logout {
 	u8	opcode;
 	u8	reserved[7];
 	u64	tag;
 };
 
+/*
+ * SRP_T_LOGOUT request, as defined in section 6.6 of the T10 SRP r16a document.
+ */
 struct srp_t_logout {
 	u8	opcode;
 	u8	sol_not;
@@ -170,6 +225,8 @@ struct srp_t_logout {
 };
 
 /*
+ * SRP_TSK_MGMT request, as defined in section 6.7 of the T10 SRP r16a document.
+ *
  * We need the packed attribute because the SRP spec only aligns the
  * 8-byte LUN field to 4 bytes.
  */
@@ -188,6 +245,9 @@ struct srp_tsk_mgmt {
 };
 
 /*
+ * SRP_CMD request with a single CDB as defined in section 6.8 of the T10 SRP
+ * r16a document.
+ *
  * We need the packed attribute because the SRP spec only aligns the
  * 8-byte LUN field to 4 bytes.
  */
@@ -209,6 +269,10 @@ struct srp_cmd {
 	u8	add_data[0];
 };
 
+/*
+ * Allowed flags for the flags field of the SRP_RSP response. See also
+ * section 6.9 of the T10 SRP r16a document.
+ */
 enum {
 	SRP_RSP_FLAG_RSPVALID = 1 << 0,
 	SRP_RSP_FLAG_SNSVALID = 1 << 1,
@@ -219,6 +283,8 @@ enum {
 };
 
 /*
+ * SRP_RSP response, as defined in section 6.9 of the T10 SRP r16a document.
+ *
  * The SRP spec defines the size of the RSP structure to be 36 bytes,
  * so it needs to be packed to avoid having it padded to 40 bytes on
  * 64-bit architectures.
--
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