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

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Message-ID: <20200729182515.GB1580638@rowland.harvard.edu>
Date:   Wed, 29 Jul 2020 14:25:15 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Martin Kepplinger <martin.kepplinger@...i.sm>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        Bart Van Assche <bvanassche@....org>,
        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 Wed, Jul 29, 2020 at 06:43:48PM +0200, Martin Kepplinger wrote:
> 
> 
> Am 29. Juli 2020 17:44:42 MESZ schrieb James Bottomley <James.Bottomley@...senPartnership.com>:
> >On Wed, 2020-07-29 at 17:40 +0200, Martin Kepplinger wrote:
> >> On 29.07.20 16:53, James Bottomley wrote:
> >> > On Wed, 2020-07-29 at 07:46 -0700, James Bottomley wrote:
> >> > > On Wed, 2020-07-29 at 10:32 -0400, Alan Stern wrote:
> >[...]
> >> > > > This error report comes from the SCSI layer, not the block
> >> > > > layer.
> >> > > 
> >> > > That sense code means "NOT READY TO READY CHANGE, MEDIUM MAY HAVE
> >> > > CHANGED" so it sounds like it something we should be
> >> > > ignoring.  Usually this signals a problem, like you changed the
> >> > > medium manually (ejected the CD).  But in this case you can tell
> >> > > us to expect this by setting
> >> > > 
> >> > > sdev->expecting_cc_ua
> >> > > 
> >> > > And we'll retry.  I think you need to set this on all resumed
> >> > > devices.
> >> > 
> >> > Actually, it's not quite that easy, we filter out this ASC/ASCQ
> >> > combination from the check because we should never ignore medium
> >> > might have changed events on running devices.  We could ignore it
> >> > if we had a flag to say the power has been yanked (perhaps an
> >> > additional sdev flag you set on resume) but we would still miss the
> >> > case where you really had powered off the drive and then changed
> >> > the media ... if you can regard this as the user's problem, then we
> >> > might have a solution.
> >> > 
> >> > James
> >> >  
> >> 
> >> oh I see what you mean now, thanks for the ellaboration.
> >> 
> >> if I do the following change, things all look normal and runtime pm
> >> works. I'm not 100% sure if just setting expecting_cc_ua in resume()
> >> is "correct" but that looks like it is what you're talking about:
> >> 
> >> (note that this is of course with the one block layer diff applied
> >> that Alan posted a few emails back)
> >> 
> >> 
> >> --- a/drivers/scsi/scsi_error.c
> >> +++ b/drivers/scsi/scsi_error.c
> >> @@ -554,16 +554,8 @@ int scsi_check_sense(struct scsi_cmnd *scmd)
> >>                  * so that we can deal with it there.
> >>                  */
> >>                 if (scmd->device->expecting_cc_ua) {
> >> -                       /*
> >> -                        * Because some device does not queue unit
> >> -                        * attentions correctly, we carefully check
> >> -                        * additional sense code and qualifier so as
> >> -                        * not to squash media change unit attention.
> >> -                        */
> >> -                       if (sshdr.asc != 0x28 || sshdr.ascq != 0x00)
> >> {
> >> -                               scmd->device->expecting_cc_ua = 0;
> >> -                               return NEEDS_RETRY;
> >> -                       }
> >> +                       scmd->device->expecting_cc_ua = 0;
> >> +                       return NEEDS_RETRY;
> >
> >Well, yes, but you can't do this because it would lose us media change
> >events in the non-suspend/resume case which we really don't want. 
> >That's why I was suggesting a new flag.
> >
> >James
> 
> also if I set expecting_cc_ua in resume() only, like I did?

That wouldn't make any difference.  The information sent by your card 
reader has sshdr.asc == 0x28 and sshdr.ascq == 0x00 (you can see it in 
the log).  So because of the code here in scsi_check_sense(), which you 
can't change, the Unit Attention sent by the card reader would not be 
retried even if you do set the flag in resume().

Alan Stern