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

[Alan Stern]

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?

Alan Stern

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