Re: Suspending SCSI devices and buses

[Nathan Bryant <nbryant@optonline.net>]

Alan Stern wrote:

>Thanks.  Looking at your patch, I have a question.  It doesn't look like
>the resume path is careful to check for Unit Attention with Power On or
>Medium May Have Changed sense.  What happens if somebody changes the
>medium while the drive is suspended?  Or am I missing something?
>
You're not missing anything. :(

Thanks for the feedback, looks like you've found a real problem with the 
patch: that is, due to the unconditonal spinup call on resume, we clear 
any UNIT ATTENTION state before any of the upper layers ever see it, so 
nobody will notice a possible media change.

Unfortunately, I think that the current media change detection code in 
the Linux kernel can not distinguish power-on events from media change 
events. I'm not sure doing so is even possible for SCSI devices. 
(Comments on that?) Proposed solutions:

Approach #1:
* Continue to do the unconditional spinup, but only for devices that are 
already mounted.
This may miss some media change events, but if we really can't 
distinguish power-on from media change, maybe that's somebody else's 
problem if the device was already mounted. (Changing mounted media is 
the user's fault.)

(Hmm, what about devices that are opened for read/write but not mounted?)

Approach #2:
* Test for UNIT_ATTENTION before spinning up and report this as a media 
change.
Safer, but may report "false positive" media change events if the device 
was only powered down/up.


Nathan