From a676f8d092f2a3aff419cacae79c80c3b7f6c0f5 Mon Sep 17 00:00:00 2001 From: Peter Osterlund Date: Tue, 13 Sep 2005 01:25:27 -0700 Subject: [PATCH] [PATCH] pktcdvd: documentation update Update the "theory of operation" description. Signed-off-by: Peter Osterlund Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/block/pktcdvd.c | 48 +++++++++++++++++++++++++---------------- 1 file changed, 30 insertions(+), 18 deletions(-) diff --git a/drivers/block/pktcdvd.c b/drivers/block/pktcdvd.c index 58d01c820799..aacf5cfccada 100644 --- a/drivers/block/pktcdvd.c +++ b/drivers/block/pktcdvd.c @@ -5,29 +5,41 @@ * May be copied or modified under the terms of the GNU General Public * License. See linux/COPYING for more information. * - * Packet writing layer for ATAPI and SCSI CD-R, CD-RW, DVD-R, and - * DVD-RW devices (aka an exercise in block layer masturbation) + * Packet writing layer for ATAPI and SCSI CD-RW, DVD+RW, DVD-RW and + * DVD-RAM devices. * + * Theory of operation: * - * TODO: (circa order of when I will fix it) - * - Only able to write on CD-RW media right now. - * - check host application code on media and set it in write page - * - interface for UDF <-> packet to negotiate a new location when a write - * fails. - * - handle OPC, especially for -RW media + * At the lowest level, there is the standard driver for the CD/DVD device, + * typically ide-cd.c or sr.c. This driver can handle read and write requests, + * but it doesn't know anything about the special restrictions that apply to + * packet writing. One restriction is that write requests must be aligned to + * packet boundaries on the physical media, and the size of a write request + * must be equal to the packet size. Another restriction is that a + * GPCMD_FLUSH_CACHE command has to be issued to the drive before a read + * command, if the previous command was a write. * - * Theory of operation: + * The purpose of the packet writing driver is to hide these restrictions from + * higher layers, such as file systems, and present a block device that can be + * randomly read and written using 2kB-sized blocks. + * + * The lowest layer in the packet writing driver is the packet I/O scheduler. + * Its data is defined by the struct packet_iosched and includes two bio + * queues with pending read and write requests. These queues are processed + * by the pkt_iosched_process_queue() function. The write requests in this + * queue are already properly aligned and sized. This layer is responsible for + * issuing the flush cache commands and scheduling the I/O in a good order. * - * We use a custom make_request_fn function that forwards reads directly to - * the underlying CD device. Write requests are either attached directly to - * a live packet_data object, or simply stored sequentially in a list for - * later processing by the kcdrwd kernel thread. This driver doesn't use - * any elevator functionally as defined by the elevator_s struct, but the - * underlying CD device uses a standard elevator. + * The next layer transforms unaligned write requests to aligned writes. This + * transformation requires reading missing pieces of data from the underlying + * block device, assembling the pieces to full packets and queuing them to the + * packet I/O scheduler. * - * This strategy makes it possible to do very late merging of IO requests. - * A new bio sent to pkt_make_request can be merged with a live packet_data - * object even if the object is in the data gathering state. + * At the top layer there is a custom make_request_fn function that forwards + * read requests directly to the iosched queue and puts write requests in the + * unaligned write queue. A kernel thread performs the necessary read + * gathering to convert the unaligned writes to aligned writes and then feeds + * them to the packet I/O scheduler. * *************************************************************************/ -- 2.20.1