Keenel 2.6.11 程序代码的疑问

请问一下有没有高手可以解释一下这 function 是做什么的。
static ide_startstop_t idedisk_start_power_step (ide_drive_t *drive, struct request *rq)
{
ide_task_t *args = rq->special;

memset(args, 0, sizeof(*args));

switch (rq->pm->pm_step) {
case idedisk_pm_flush_cache: /* Suspend step 1 (flush cache) */
/* Not supported? Switch to next step now. */
if (!drive->wcache || !ide_id_has_flush_cache(drive->id)) {
idedisk_complete_power_step(drive, rq, 0, 0);
return ide_stopped;
}
if (ide_id_has_flush_cache_ext(drive->id))
args->tfRegister[IDE_COMMAND_OFFSET] = WIN_FLUSH_CACHE_EXT;
else
args->tfRegister[IDE_COMMAND_OFFSET] = WIN_FLUSH_CACHE;
args->command_type = IDE_DRIVE_TASK_NO_DATA;
args->handler   = &task_no_data_intr;
return do_rw_taskfile(drive, args);

case idedisk_pm_standby: /* Suspend step 2 (standby) */
args->tfRegister[IDE_COMMAND_OFFSET] = WIN_STANDBYNOW1;
args->command_type = IDE_DRIVE_TASK_NO_DATA;
args->handler   = &task_no_data_intr;
return do_rw_taskfile(drive, args);

case idedisk_pm_idle: /* Resume step 1 (idle) */
args->tfRegister[IDE_COMMAND_OFFSET] = WIN_IDLEIMMEDIATE;
args->command_type = IDE_DRIVE_TASK_NO_DATA;
args->handler = task_no_data_intr;
return do_rw_taskfile(drive, args);

case idedisk_pm_restore_dma: /* Resume step 2 (restore DMA) */
/*
* Right now, all we do is call hwif->ide_dma_check(drive),
* we could be smarter and check for current xfer_speed
* in struct drive etc...
* Also, this step could be implemented as a generic helper
* as most subdrivers will use it
*/
if ((drive->id->capability & 1) == 0)
break;
if (HWIF(drive)->ide_dma_check == NULL)
break;
HWIF(drive)->ide_dma_check(drive);
break;
}
rq->pm->pm_step = ide_pm_state_completed;
return ide_stopped;
}