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    SubjectRe: CD writing in future Linux (stirring up a hornets' nest)
    On Tue, 14 Feb 2006, Rob Landley wrote:

    > With mkisofs I can just start from the spec, reverse engineer a few existing
    > ISOs, or grab the really old code from before Joerg got ahold of it (back
    > when it was still readable). That's no problem. But for cdrecord, I can't
    > find documentation on what the kernel expects.

    That's mostly the sg <http://sg.torque.net/sg/p/sg_v3_ho.html> interface
    that matters, and of that mostly the open and SG_IO parts. cdrecord is
    severely bound to talking SCSI.

    > I'm only interested in supporting ATA cd burners under a 2.6 or newer kernel,
    > using the DMA method. (SCSI is dead, I honestly don't care.)

    SCSI being dead for writing is actually a pity because SCSI was all in
    all so much smoother. More devices on the same cable (which was a real
    bus), no hassles with b0rked "IDE" interfaces that only work for hard
    disks but not ATAPI devices and more. Everything SCSI has had for more
    than a decade is slowly retrofitted into ATA(PI), removed if not good
    enough (TCQ), and reinvented (NCQ) when in fact SCSI had it right for
    aeons.

    The good thing is ATAPI via ide-cd vs. SCSI does not matter any more,
    and SCSI vs. parallel matters very little (but that's just as dead as
    SCSI for CD writing). If you don't care to enumerate devices or obtain
    b,t,l, you just take the device name, open it and do some sanity checks
    to see if you're talking to a CD-ROM.

    The downside is, and here an abstraction layer has a point, just this
    simple won't be portable. SG_IO is Linux-specific.

    Jörg's complaints about ide-cd being different, layer violations and
    else are entirely artificially constructed complaints, at least he
    hasn't been able to document real bugs in ide-cd in the course of this
    thread, but holding on to ide-scsi which is known to have severe bugs.
    He was under some miscomprehension of the Linux internals and split
    ATA: and SCSI namespaces and added some more artificial complaints about
    non-existent problems.

    One question I do have is if SG_IO would work on /dev/sr* as well. I
    don't know the answer and don't have time to dig through the relevant
    code now.

    > I was hoping I could just open the /dev/cdrom and call the appropriate
    > ioctls on it, but reading the cdrecord source proved enough of an
    > exercise in masochism that I always give up after the first hour and
    > put it back on the todo list.

    Perhaps reading a late MMC draft from t10.org is more useful as a
    starting point, if you want the real thing, you'll have to get an
    official standard. And perhaps retrofitting CD support into growisofs
    (from the dvd+rw-tools) might be another idea.

    > I suppose I should say "screw the source code" and just run the cdrecord
    > binary under strace to see what it's doing,

    You'd have to enable strace to actually unravel SG_IO contents, else
    you're only getting a useless pointer - unless you trust cdrecord -V.

    > * How bad? Random example of ignoring how the rest of the world works is that
    > it runs autoconf from within make, meaning there's no obvious way to specify
    > "./configure --prefix=/mypath", so the last time I played with it (which was
    > a while ago), I wound up doing this:

    To be fair, installation into specific paths is documented in 2.01 and
    newer alphas. Quoting INSTALL, section "Using a different installation
    directory[...]":

    "If your make program supports to propagate make macros to sub make
    programs which is the case for recent smake releases as well as for a
    recent gnumake:

    smake INS_BASE=/usr/local install
    or
    gmake INS_BASE=/usr/local install"

    --
    Matthias Andree
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