linux-kernel - Re: [PATCH] Block layer: separate out queue-oriented ioctls

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Message-ID: <Pine.LNX.4.44L0.0702192238110.4237-100000@netrider.rowland.org>
Date:	Mon, 19 Feb 2007 22:48:08 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Douglas Gilbert <dougg@...que.net>
cc:	Joerg Schilling <Joerg.Schilling@...us.fraunhofer.de>,
	<linux-kernel@...r.kernel.org>, <jens.axboe@...cle.com>,
	<James.Bottomley@...eleye.com>
Subject: Re: [PATCH] Block layer: separate out queue-oriented ioctls

On Mon, 19 Feb 2007, Douglas Gilbert wrote:

> Alan,
> The SG_GET_RESERVED_SIZE ioctl is also defined in
> the block layer, see block/scsi_ioctl.c .

Ah, I didn't know that.  (Or more likely, I used to know and have since 
forgotten.)  Thanks for pointing it out.

> I suspect it is just a kludge to fool cdrecord that it
> is talking to a sg device. [One of many kludges in the
> block SG_IO ioctl implementation to that end.]
> So perhaps the block layer versions of SG_SET_RESERVED_SIZE
> and SG_GET_RESERVED_SIZE need to be similarly capped.

Yes.  In fact one of them already is, but the other should be too.

> Actually I think that I would default SG_GET_RESERVED_SIZE to
> request_queue->max_sectors * 512 in the block layer
> implementation (as there is no "reserve buffer" associated
> with a block device).

Okay.

Come to think of it, the reserved_size value used when a new sg device is
created should also be capped at max_sectors * 512.  Agreed?  I can't see
any reason for ever having a larger buffer -- it would be impossible to
make use of the extra space.

Alan Stern

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