Re: [PATCH 1/3] block layer: early detection of medium not present

[Jens Axboe]

On Fri, Jun 09 2006, Alan Stern wrote:
> On Thu, 8 Jun 2006, James Bottomley wrote:
> 
> > On Tue, 2006-06-06 at 11:26 -0400, Alan Stern wrote:
> > > When the block layer checks for new media in a drive, it uses a two-step 
> > > procedure: First it checks for media change and then it revalidates the 
> > > disk.  When no medium is present the second step fails.
> > > 
> > > However some drivers (such as the SCSI disk driver) are capable of
> > > detecting medium-not-present as part of the media-changed check.  Doing so
> > > will reduce by a factor of 2 or more the amount of work done by tasks
> > > which, like hald, constantly poll empty drives.
> > > 
> > > This patch (as694) changes the block layer core to make it recognize a 
> > > -ENOMEDIUM error return from the media_changed method.  A follow-on patch 
> > > makes the sd driver return this code when no medium is present.
> > 
> > I'm not sure there's enough buy in to make this change yet ... our media
> > change handling is incredibly (and quite possibly far too) complex.
> > 
> > As documented in Documentation/cdrom/cdrom-standard.tex, the return
> > codes for media change are either 0 or 1.
> 
> I can change the documentation, if necessary.  On the other hand, I don't 
> want to embark on a global alteration of the media-change handling 
> throughout the entire kernel!  :-)
> 
> > Personally, I can't see a problem with overloading the true return to
> > have more information that the error codes provide, but before we do
> > this we need the buy in of the cdrom layer, since that's where this
> > handling came from, and we need to update the documents to reflect the
> > new behaviour ... someone also needs to consider what changes should be
> > made in the cdrom layer for this (and whether this is actually the
> > correct way to do this from the point of view of CDs).
> 
> Agreed.  That's why I cc'ed Jens.  Is there anyone else I should also ask
> about this change?

I'll be gone over the weekend, I can take a look next week.

-- 
Jens Axboe

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