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

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Date:   Wed, 29 Jul 2020 08:44:42 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Martin Kepplinger <martin.kepplinger@...i.sm>,
        Alan Stern <stern@...land.harvard.edu>
Cc:     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, 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