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Commit ded155b5 authored by Finn Thain's avatar Finn Thain Committed by Martin K. Petersen
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atari_scsi: Allow can_queue to be increased for Falcon 

The benefit of limiting can_queue to 1 is that atari_scsi shares the
ST DMA chip more fairly with other drivers (e.g. falcon-ide).

Unfortunately, this can limit SCSI bus utilization. On systems without
IDE, atari_scsi should issue SCSI commands whenever it can arbitrate for
the bus. Make that possible by making can_queue configurable.
Signed-off-by: default avatarFinn Thain <fthain@telegraphics.com.au>
Reviewed-by: default avatarHannes Reinecke <hare@suse.com>
Tested-by: default avatarMichael Schmitz <schmitzmic@gmail.com>
Signed-off-by: default avatarMartin K. Petersen <martin.petersen@oracle.com>
parent a5217a86
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Showing with 22 additions and 61 deletions
drivers/scsi/atari_scsi.c
View file @ ded155b5
......@@ -14,55 +14,23 @@
*
*/
/**************************************************************************/
/* */
/* Notes for Falcon SCSI: */
/* ---------------------- */
/* */
/* Since the Falcon SCSI uses the ST-DMA chip, that is shared among */
/* several device drivers, locking and unlocking the access to this */
/* chip is required. But locking is not possible from an interrupt, */
/* since it puts the process to sleep if the lock is not available. */
/* This prevents "late" locking of the DMA chip, i.e. locking it just */
/* before using it, since in case of disconnection-reconnection */
/* commands, the DMA is started from the reselection interrupt. */
/* */
/* Two possible schemes for ST-DMA-locking would be: */
/* 1) The lock is taken for each command separately and disconnecting */
/* is forbidden (i.e. can_queue = 1). */
/* 2) The DMA chip is locked when the first command comes in and */
/* released when the last command is finished and all queues are */
/* empty. */
/* The first alternative would result in bad performance, since the */
/* interleaving of commands would not be used. The second is unfair to */
/* other drivers using the ST-DMA, because the queues will seldom be */
/* totally empty if there is a lot of disk traffic. */
/* */
/* For this reasons I decided to employ a more elaborate scheme: */
/* - First, we give up the lock every time we can (for fairness), this */
/* means every time a command finishes and there are no other commands */
/* on the disconnected queue. */
/* - If there are others waiting to lock the DMA chip, we stop */
/* issuing commands, i.e. moving them onto the issue queue. */
/* Because of that, the disconnected queue will run empty in a */
/* while. Instead we go to sleep on a 'fairness_queue'. */
/* - If the lock is released, all processes waiting on the fairness */
/* queue will be woken. The first of them tries to re-lock the DMA, */
/* the others wait for the first to finish this task. After that, */
/* they can all run on and do their commands... */
/* This sounds complicated (and it is it :-(), but it seems to be a */
/* good compromise between fairness and performance: As long as no one */
/* else wants to work with the ST-DMA chip, SCSI can go along as */
/* usual. If now someone else comes, this behaviour is changed to a */
/* "fairness mode": just already initiated commands are finished and */
/* then the lock is released. The other one waiting will probably win */
/* the race for locking the DMA, since it was waiting for longer. And */
/* after it has finished, SCSI can go ahead again. Finally: I hope I */
/* have not produced any deadlock possibilities! */
/* */
/**************************************************************************/
/*
* Notes for Falcon SCSI DMA
*
* The 5380 device is one of several that all share the DMA chip. Hence
* "locking" and "unlocking" access to this chip is required.
*
* Two possible schemes for ST DMA acquisition by atari_scsi are:
* 1) The lock is taken for each command separately (i.e. can_queue == 1).
* 2) The lock is taken when the first command arrives and released
* when the last command is finished (i.e. can_queue > 1).
*
* The first alternative limits SCSI bus utilization, since interleaving
* commands is not possible. The second gives better performance but is
* unfair to other drivers needing to use the ST DMA chip. In order to
* allow the IDE and floppy drivers equal access to the ST DMA chip
* the default is can_queue == 1.
*/
#include <linux/module.h>
#include <linux/types.h>
......@@ -443,6 +411,10 @@ static int falcon_get_lock(struct Scsi_Host *instance)
if (IS_A_TT())
return 1;
if (stdma_is_locked_by(scsi_falcon_intr) &&
instance->hostt->can_queue > 1)
return 1;
if (in_interrupt())
return stdma_try_lock(scsi_falcon_intr, instance);
......@@ -776,22 +748,11 @@ static int __init atari_scsi_probe(struct platform_device *pdev)
atari_scsi_reg_write = atari_scsi_falcon_reg_write;
}
/* The values for CMD_PER_LUN and CAN_QUEUE are somehow arbitrary.
* Higher values should work, too; try it!
* (But cmd_per_lun costs memory!)
*
* But there seems to be a bug somewhere that requires CAN_QUEUE to be
* 2*CMD_PER_LUN. At least on a TT, no spurious timeouts seen since
* changed CMD_PER_LUN...
*
* Note: The Falcon currently uses 8/1 setting due to unsolved problems
* with cmd_per_lun != 1
*/
if (ATARIHW_PRESENT(TT_SCSI)) {
atari_scsi_template.can_queue = 16;
atari_scsi_template.sg_tablesize = SG_ALL;
} else {
atari_scsi_template.can_queue = 8;
atari_scsi_template.can_queue = 1;
atari_scsi_template.sg_tablesize = SG_NONE;
}
......
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