diff options
Diffstat (limited to 'drivers')
-rw-r--r-- | drivers/block/pktcdvd.c | 48 |
1 files 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 @@ | |||
5 | * May be copied or modified under the terms of the GNU General Public | 5 | * May be copied or modified under the terms of the GNU General Public |
6 | * License. See linux/COPYING for more information. | 6 | * License. See linux/COPYING for more information. |
7 | * | 7 | * |
8 | * Packet writing layer for ATAPI and SCSI CD-R, CD-RW, DVD-R, and | 8 | * Packet writing layer for ATAPI and SCSI CD-RW, DVD+RW, DVD-RW and |
9 | * DVD-RW devices (aka an exercise in block layer masturbation) | 9 | * DVD-RAM devices. |
10 | * | 10 | * |
11 | * Theory of operation: | ||
11 | * | 12 | * |
12 | * TODO: (circa order of when I will fix it) | 13 | * At the lowest level, there is the standard driver for the CD/DVD device, |
13 | * - Only able to write on CD-RW media right now. | 14 | * typically ide-cd.c or sr.c. This driver can handle read and write requests, |
14 | * - check host application code on media and set it in write page | 15 | * but it doesn't know anything about the special restrictions that apply to |
15 | * - interface for UDF <-> packet to negotiate a new location when a write | 16 | * packet writing. One restriction is that write requests must be aligned to |
16 | * fails. | 17 | * packet boundaries on the physical media, and the size of a write request |
17 | * - handle OPC, especially for -RW media | 18 | * must be equal to the packet size. Another restriction is that a |
19 | * GPCMD_FLUSH_CACHE command has to be issued to the drive before a read | ||
20 | * command, if the previous command was a write. | ||
18 | * | 21 | * |
19 | * Theory of operation: | 22 | * The purpose of the packet writing driver is to hide these restrictions from |
23 | * higher layers, such as file systems, and present a block device that can be | ||
24 | * randomly read and written using 2kB-sized blocks. | ||
25 | * | ||
26 | * The lowest layer in the packet writing driver is the packet I/O scheduler. | ||
27 | * Its data is defined by the struct packet_iosched and includes two bio | ||
28 | * queues with pending read and write requests. These queues are processed | ||
29 | * by the pkt_iosched_process_queue() function. The write requests in this | ||
30 | * queue are already properly aligned and sized. This layer is responsible for | ||
31 | * issuing the flush cache commands and scheduling the I/O in a good order. | ||
20 | * | 32 | * |
21 | * We use a custom make_request_fn function that forwards reads directly to | 33 | * The next layer transforms unaligned write requests to aligned writes. This |
22 | * the underlying CD device. Write requests are either attached directly to | 34 | * transformation requires reading missing pieces of data from the underlying |
23 | * a live packet_data object, or simply stored sequentially in a list for | 35 | * block device, assembling the pieces to full packets and queuing them to the |
24 | * later processing by the kcdrwd kernel thread. This driver doesn't use | 36 | * packet I/O scheduler. |
25 | * any elevator functionally as defined by the elevator_s struct, but the | ||
26 | * underlying CD device uses a standard elevator. | ||
27 | * | 37 | * |
28 | * This strategy makes it possible to do very late merging of IO requests. | 38 | * At the top layer there is a custom make_request_fn function that forwards |
29 | * A new bio sent to pkt_make_request can be merged with a live packet_data | 39 | * read requests directly to the iosched queue and puts write requests in the |
30 | * object even if the object is in the data gathering state. | 40 | * unaligned write queue. A kernel thread performs the necessary read |
41 | * gathering to convert the unaligned writes to aligned writes and then feeds | ||
42 | * them to the packet I/O scheduler. | ||
31 | * | 43 | * |
32 | *************************************************************************/ | 44 | *************************************************************************/ |
33 | 45 | ||