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Date:   Thu, 30 Jul 2020 11:10:30 -0400
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Martin Kepplinger <martin.kepplinger@puri.sm>
Cc:     James Bottomley <James.Bottomley@hansenpartnership.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Can Guo <cang@codeaurora.org>, martin.petersen@oracle.com,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@puri.sm
Subject: Re: [PATCH] scsi: sd: add runtime pm to open / release
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On Thu, Jul 30, 2020 at 10:52:14AM +0200, Martin Kepplinger wrote:
> Maybe I should just start a new discussion with a patch, but the below
> is what makes sense to me (when I understand you correctly) and seems to
> work. I basically add a new flag, so that the old flags behave unchanged
> and only call it during *runtime* resume for SD cards:
> 
> 
> --- a/drivers/scsi/scsi_error.c
> +++ b/drivers/scsi/scsi_error.c
> @@ -553,15 +553,21 @@ int scsi_check_sense(struct scsi_cmnd *scmd)
>                  * information that we should pass up to the upper-level
> driver
>                  * so that we can deal with it there.
>                  */
> -               if (scmd->device->expecting_cc_ua) {
> +               if (scmd->device->expecting_cc_ua ||
> +                   scmd->device->expecting_media_change) {
>                         /*
>                          * 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.
> +                        * not to squash media change unit attention;
> +                        * unless expecting_media_change is set, indicating
> +                        * that the media (most likely) didn't change
> +                        * but a device only believes so (for example
> +                        * because of suspend/resume).
>                          */
> -                       if (sshdr.asc != 0x28 || sshdr.ascq != 0x00) {
> -                               scmd->device->expecting_cc_ua = 0;
> +                       if ((sshdr.asc != 0x28 || sshdr.ascq != 0x00) ||
> +                           scmd->device->expecting_media_change) {
> +                               scmd->device->expecting_media_change = 0;
>                                 return NEEDS_RETRY;
>                         }
>                 }
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index d90fefffe31b..b647fab2b663 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -114,6 +114,7 @@ static void sd_shutdown(struct device *);
>  static int sd_suspend_system(struct device *);
>  static int sd_suspend_runtime(struct device *);
>  static int sd_resume(struct device *);
> +static int sd_resume_runtime(struct device *);
>  static void sd_rescan(struct device *);
>  static blk_status_t sd_init_command(struct scsi_cmnd *SCpnt);
>  static void sd_uninit_command(struct scsi_cmnd *SCpnt);
> @@ -574,7 +575,7 @@ static const struct dev_pm_ops sd_pm_ops = {
>         .poweroff               = sd_suspend_system,
>         .restore                = sd_resume,
>         .runtime_suspend        = sd_suspend_runtime,
> -       .runtime_resume         = sd_resume,
> +       .runtime_resume         = sd_resume_runtime,
>  };
> 
>  static struct scsi_driver sd_template = {
> @@ -3652,6 +3653,21 @@ static int sd_resume(struct device *dev)
>         return ret;
>  }
> 
> +static int sd_resume_runtime(struct device *dev)
> +{
> +       struct scsi_disk *sdkp = dev_get_drvdata(dev);
> +
> +       /* Some SD cardreaders report media change when resuming from
> suspend
> +        * because they can't keep track during suspend. */
> +
> +       /* XXX This is not unproblematic though: We won't notice when a card
> +        * was really changed during runtime suspend! We basically rely
> on users
> +        * to unmount or suspend before doing so. */
> +       sdkp->device->expecting_media_change = 1;
> +
> +       return sd_resume(dev);
> +}
> +
>  /**
>   *     init_sd - entry point for this driver (both when built in or when
>   *     a module).
> diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
> index bc5909033d13..8c8f053f71c8 100644
> --- a/include/scsi/scsi_device.h
> +++ b/include/scsi/scsi_device.h
> @@ -169,6 +169,8 @@ struct scsi_device {
>                                  * this device */
>         unsigned expecting_cc_ua:1; /* Expecting a CHECK_CONDITION/UNIT_ATTN
>                                      * because we did a bus reset. */
> +       unsigned expecting_media_change:1; /* Expecting media change
> ASC/ASCQ
> +                                             when it actually doesn't
> change */
>         unsigned use_10_for_rw:1; /* first try 10-byte read / write */
>         unsigned use_10_for_ms:1; /* first try 10-byte mode sense/select */
>         unsigned set_dbd_for_ms:1; /* Set "DBD" field in mode sense */

That's pretty much what James was suggesting, except for one thing: You 
must not set sdkp->device->expecting_media_change to 1 for all devices 
in sd_runtime_resume().  Only for devices which may generate a spurious 
Unit Attention following runtime resume -- and maybe not even for all of 
them, depending on what the user wants.

Alan Stern