scsi: sg: Enable runtime power management

In 2010, runtime power management support was implemented in the SCSI
core.  The description of patch "[SCSI] implement runtime Power
Management" mentions that the sg driver is skipped but not why. This
patch enables runtime power management even if an instance of the sg
driver is held open.  Enabling runtime PM for the sg driver is safe
because all interactions of the sg driver with the SCSI device pass
through the block layer (blk_execute_rq_nowait()) and the block layer
already supports runtime PM.

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Fixes: bc4f240 ("[SCSI] implement runtime Power Management")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20241030220310.1373569-1-bvanassche@acm.org
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sg.c

@@ -307,18 +307,14 @@ sg_open(struct inode *inode, struct file *filp)
 	if (retval)
 		goto sg_put;
 
-	retval = scsi_autopm_get_device(device);
-	if (retval)
-		goto sdp_put;
-
 	/* scsi_block_when_processing_errors() may block so bypass
 	 * check if O_NONBLOCK. Permits SCSI commands to be issued
 	 * during error recovery. Tread carefully. */
 	if (!((flags & O_NONBLOCK) ||
 	      scsi_block_when_processing_errors(device))) {
 		retval = -ENXIO;
 		/* we are in error recovery for this device */
-		goto error_out;
+		goto sdp_put;
 	}
 
 	mutex_lock(&sdp->open_rel_lock);
@@ -371,8 +367,6 @@ sg_open(struct inode *inode, struct file *filp)
 	}
 error_mutex_locked:
 	mutex_unlock(&sdp->open_rel_lock);
-error_out:
-	scsi_autopm_put_device(device);
 sdp_put:
 	kref_put(&sdp->d_ref, sg_device_destroy);
 	scsi_device_put(device);
@@ -392,7 +386,6 @@ sg_release(struct inode *inode, struct file *filp)
 	SCSI_LOG_TIMEOUT(3, sg_printk(KERN_INFO, sdp, "sg_release\n"));
 
 	mutex_lock(&sdp->open_rel_lock);
-	scsi_autopm_put_device(sdp->device);
 	kref_put(&sfp->f_ref, sg_remove_sfp);
 	sdp->open_cnt--;