linux-kernel - Re: [PATCH] scsi: sd: add runtime pm to open / release

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Message-ID: <1804723c-4aaf-a820-d3ef-e70125017cad@acm.org>
Date:   Tue, 30 Jun 2020 12:23:12 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Martin Kepplinger <martin.kepplinger@...i.sm>, jejb@...ux.ibm.com,
        Can Guo <cang@...eaurora.org>, martin.petersen@...cle.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...i.sm
Subject: Re: [PATCH] scsi: sd: add runtime pm to open / release

On 2020-06-30 11:02, Alan Stern wrote:
> Right now there doesn't seem to be any mechanism for resuming the queue 
> if an REQ_PREEMPT request is added while the queue is suspended.

I do not agree with the above statement. My understanding is that resuming
happens as follows if a request is submitted against a runtime suspended
queue owned by a SCSI LLD:

blk_queue_enter()
  -> blk_pm_request_resume()
    -> pm_request_resume(dev)
      -> __pm_runtime_resume(dev, RPM_ASYNC)
        -> rpm_resume(dev, RPM_ASYNC)
          -> dev->power.request = RPM_REQ_RESUME;
          -> queue_work(pm_wq, &dev->power.work)
            -> pm_runtime_work()
              -> rpm_resume(dev, RPM_NOWAIT)
                -> callback = scsi_runtime_resume;
                -> rpm_callback(callback, dev);
                  -> scsi_runtime_resume(dev);
                    -> sdev_runtime_resume(dev);
                      -> blk_pre_runtime_resume(sdev->request_queue);
                        -> q->rpm_status = RPM_RESUMING;
                      -> sd_resume(dev);
                        -> sd_start_stop_device(sdkp);
                          -> sd_pm_ops.runtime_resume == scsi_execute(sdp, START);
                            -> blk_get_request(..., ..., BLK_MQ_REQ_PREEMPT)
                              -> blk_mq_alloc_request()
                                -> blk_queue_enter()
                                -> __blk_mq_alloc_request()
                            -> blk_execute_rq()
                            -> blk_put_request()
                      -> blk_post_runtime_resume(sdev->request_queue);
                        -> q->rpm_status = RPM_ACTIVE;
                        -> pm_runtime_mark_last_busy(q->dev);
                        -> pm_request_autosuspend(q->dev);

Bart.