On Tue, 22 May 2012, Lin Ming wrote: > > Next, you have to change the parts of the block layer responsible for > > taking a request from the queue and handing it to the lower-level > > driver (both peek and get). �If q->rpm_status is RPM_SUSPENDED, they > > shouldn't do anything -- act as though the queue is empty. �If > > q->rpm_status is RPM_SUSPENDING or RPM_RESUMING, they should hand over > > the request only if it has the REQ_PM flag set. I just realized this isn't quite right. In fact, when the status is RPM_SUSPENDING or RPM_RESUMING you need to search through the queue for the first request with REQ_PM set. Otherwise you can end up in a deadlock. For example, suppose the device is suspended and a normal I/O request arrives. It gets put on the queue and a runtime resume is issued. The sd resume method then submits a "spin-up drive" command to the queue with REQ_PM set, and it doesn't return until this command is finished. But the command won't even start, because it isn't at the head of the queue -- the I/O request is, and it prevents the "spin-up" command from running. There are two other issues you also need to address. The first is simple: Make sure your changes don't cause any harm if CONFIG_PM or CONFIG_PM_RUNTIME is set to N. Second, near the end of scsi_pm.c:scsi_bus_resume_common(), you can see that during resume from a system sleep, the device automatically gets set back to RPM_ACTIVE. You want the queue's runtime status to remain in sync with the device's status; therefore you need to call blk_pre_runtime_resume near the start of this function and blk_post_runtime_resume near the end. (This is the drawback of having separate rpm_status fields for the queue and the device.) Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
