scsi: target: target_core_user.[ch]: convert comments into DOC:

#include <linux/target_core_user.h>

/*

/**

 * DOC: Userspace I/O

 * Userspace I/O

 * -------------

 *

 * Define a shared-memory interface for LIO to pass SCSI commands and

 * data to userspace for processing. This is to allow backends that

 * are too complex for in-kernel support to be possible.

 * See the .h file for how the ring is laid out. Note that while the

 * command ring is defined, the particulars of the data area are

 * not. Offset values in the command entry point to other locations

 * internal to the mmap()ed area. There is separate space outside the

 * internal to the mmap-ed area. There is separate space outside the

 * command ring for data buffers. This leaves maximum flexibility for

 * moving buffer allocations, or even page flipping or other

 * allocation techniques, without altering the command ring layout.

#define TCMU_VERSION "2.0"

/*

/**

 * DOC: Ring Design

 * Ring Design

 * -----------

 *

 * The mmaped area is divided into three parts:

 * 1) The mailbox (struct tcmu_mailbox, below)

 * 2) The command ring

 * 3) Everything beyond the command ring (data)

 * 1) The mailbox (struct tcmu_mailbox, below);

 * 2) The command ring;

 * 3) Everything beyond the command ring (data).

 *

 * The mailbox tells userspace the offset of the command ring from the

 * start of the shared memory region, and how big the command ring is.

 *

 * The kernel passes SCSI commands to userspace by putting a struct

 * tcmu_cmd_entry in the ring, updating mailbox->cmd_head, and poking

 * userspace via uio's interrupt mechanism.

 * userspace via UIO's interrupt mechanism.

 *

 * tcmu_cmd_entry contains a header. If the header type is PAD,

 * userspace should skip hdr->length bytes (mod cmdr_size) to find the