block: add a flag to identify PM request

Add a flag REQ_PM to identify the request is PM related, such requests
will not change the device request queue's runtime status. It is
intended to be used in driver's runtime PM callback, so that driver can
perform some IO to the device there with the queue's runtime status
unaffected. e.g. in SCSI disk's runtime suspend callback, the disk will
be put into stopped power state, and this require sending a command to
the device. Such command processing should not change the disk's runtime
status.

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
scsi: use REQ_PM in sd's runtime suspend operation

With the introduction of REQ_PM, modify sd's runtime suspend operation
functions to use that flag so that the operations to put the device into
runtime suspended state(i.e. sync cache and stop device) will not affect
its runtime PM status.

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
block: add runtime pm helpers

Add runtime pm helper functions:

void blk_pm_runtime_init(struct request_queue *q, struct device *dev)
  - Initialization function for drivers to call.

int blk_pre_runtime_suspend(struct request_queue *q)
  - If any requests are in the queue, mark last busy and return -EBUSY.
    Otherwise set q->rpm_status to RPM_SUSPENDING and return 0.

void blk_post_runtime_suspend(struct request_queue *q, int err)
  - If the suspend succeeded then set q->rpm_status to RPM_SUSPENDED.
    Otherwise set it to RPM_ACTIVE and mark last busy.

void blk_pre_runtime_resume(struct request_queue *q)
  - Set q->rpm_status to RPM_RESUMING.

void blk_post_runtime_resume(struct request_queue *q, int err)
  - If the resume succeeded then set q->rpm_status to RPM_ACTIVE
    and call __blk_run_queue, then mark last busy and autosuspend.
    Otherwise set q->rpm_status to RPM_SUSPENDED.

The idea and API is designed by Alan Stern and described here:
http://marc.info/?l=linux-scsi&m=133727953625963&w=2

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
block: implement runtime pm strategy

When a request is added:
    If device is suspended or is suspending and the request is not a
    PM request, resume the device.

When the last request finishes:
    Call pm_runtime_mark_last_busy().

When pick a request:
    If device is resuming/suspending, then only PM request is allowed
    to go.

The idea and API is designed by Alan Stern and described here:
http://marc.info/?l=linux-scsi&m=133727953625963&w=2

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
sd: change to auto suspend mode

Uses block layer runtime pm helper functions in
scsi_runtime_suspend/resume for devices that take advantage of it.

Remove scsi_autopm_* from sd open/release path and check_events path.

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>