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Date: Wed, 21 Nov 2001 23:58:25 +0200 (EET)
From: Kai Makisara <Kai.Makisara@kolumbus.fi>
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To: "Jeff V. Merkey" <jmerkey@vger.timpanogas.org>
cc: <linux-kernel@vger.kernel.org>
Subject: Re: st.c SCSI Tape ioctl() bug
In-Reply-To: <20011118204819.A968@vger.timpanogas.org>
Message-ID: <Pine.LNX.4.33.0111212349540.5548-100000@kai.makisara.local>
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On Sun, 18 Nov 2001, Jeff V. Merkey wrote:

>
>
> Kai/Linux,
>
> The ioctl() function to enable/disable code 15 comrpession has
> some problems.  I have a fix to the code, but it does not
> always seem to work properly, so I think you should do this
> review.
>
> If you call the ioctl() tape command from kernel space to
> enable and disable **DEFAULT** compression (not MTCOMPRESSION
> ioctl, the MT_ST_DEF_COMPRESSION code path) there is a case
> where the default_compression/compression_changed flags
> can horribly out of sync.
>
> Please take a look at this code.  We have gotten around it
> by simply calling MTCOMPRESSION everytime we need to use it,
> however, but the other path seems busted, and it would be
> nice for it to work properly.
>
The default compression is meant to be set only at system
startup/module loading. If the user wants to change the compression at any
other time, using MTCOMPRESSION is the correct way.

However, I agree that setting the default compression at any other time
should also give the expected result. Currently, the compression default
will be enforced when a new tape is inserted. It might be better to change
the compression immediately after the default has been changed if the
drive is ready. This should also synchronize the compression_changed flag.
I will think about this a little more and then make the changes.

	Kai


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