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    SubjectRe: IDE vs. SCSI CDROM last block handling
      From: "Larry M. Augustin" <lma@varesearch.com>
    Date: Mon, 29 Dec 1997 13:48:49 -0800

    In kernel 2.0.32 the IDE and SCSI CDROM drivers behave differently at
    the end of data on a CDROM. The IDE driver returns an I/O error. The
    SCSI driver returns the end of file.

    For example, on IDE this fails with an I/O error:

    10:18:33 [3] ~ root-l29$ md5sum /dev/hda
    md5sum: /dev/hda: I/O error

    While it will complete normally on SCSI:

    [root@cray /root]# md5sum /dev/scd0
    f474d26e7bac5b755b4ac261947bd89c /dev/scd0

    [root@cray /root]# dd if=/dev/scd0 | md5sum
    1275728+0 records in
    1275728+0 records out
    f474d26e7bac5b755b4ac261947bd89c -

    Of course, it's easy enough to buffer programs from the I/O error on
    IDE devices using dd:

    11:01:38 [5] ~ root-l29$ dd if=/dev/hda | md5sum
    dd: /dev/hda: I/O error
    1275724+0 records in
    1275724+0 records out
    0aa766e51134fe1cc1750380e45ae32d -

    Also, note that the number of records read is slightly different. The
    SCSI driver successfully reads 318932 2K blocks and the IDE driver
    reads 318931 2K blocks (the I/O error presumably coming on block
    318932).

    Shouldn't the SCSI and IDE drivers be consistent in handling this?

    That would make sense, if possible.

    There's logic in the SCSI CD-ROM driver to catch this case and return EOF
    rather than an I/O error (though a CD-ROM I/O error is still logged by the
    kernel). According to the SCSI specification, the results of a READ_CAPACITY
    command is allowed to be over by up to 75 2KB blocks, so the size saved from
    READ_CAPACITY may be wrong. So if an error occurs within 75 blocks of the end,
    it is assumed that the size is wrong, and we correct the size and return EOF.
    See sr.c for the details. I have no idea what the specs are for IDE/ATAPI
    CD-ROMs.

    Leonard

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