[paranoia] DMA problem

[davidjoerg at web.de]

Hello,

here a quote that won't help you to fix the problem, but
explains why DMA won't always work. It's from 
"http://www.fokus.gmd.de/research/cc/glone/employees/
joerg.schilling/private/man/README/README.ATAPI"

[QUOTE]
People (with a Linux only background) often ask me why do you depend on 
"ATAPI-SCSI emulation", why don't you support generic IDE?

        Well first a statement: There is no single IDE burner out!
        Even a CD-ROM cannot be used decently if you use only IDE commands.
        Opening/closing the door, playing audio and similar things
        cannot be done using vanilla IDE commands - you will need SCSI commands
        to do this. But how do we do this with a drive that uses an IDE
        interface?

        The ATAPI standard describes method of sending SCSI commands over IDE
        transport with some small limitations to the "real" SCSI standard.
        SCSI commands are send via IDE transport using the 'ATA packet'
        command. There is no SCSI emulation - ATAPI drives include native
        SCSI command support. For this reason, sending SCSI commands to ATAPI
        drives is the native method of supporting ATAPI devices. Just imagine
        that IDE is one of many SCSI low level transport mechanisms.

        This is a list of some known SCSI transports:

        -	Good old Parallel SCSI 50/68 pin (what most people call SCSI)
        -	SCSI over fiber optics (e.g. FACL - there are others too)
        -	SCSI over a copper variant of FCAL (used in modern servers)
        -	SCSI over IEEE 1394 (Fire Wire)
        -	SCSI over USB
        -	SCSI over IDE (ATAPI)

        As you now see, the use of the naming convention "ATAPI-SCSI emulation"
        is a little bit misleading. It should rather be called:
                "IDE-SCSI host adapter emulation"

Some naming explanations:

        ATA	Attachment Adapter
        IDE	Integrated Drive Electronics (A Drive that includes ATA)
        ATAPI	ATA Packet Interface

When cdrecord has problems with ATAPI drives on Linux this usually is a Linux 
kernel problem. The Linux kernel maintainers unfortunately refuse to correct
their current IDE driver system setup which does not support ATAPI by default.
ATAPI _is_ SCSI over IDE transport. It is hard to understand why Linux still
uses a default driver setup that is designed for IDE CD-ROM drives made
before 1994 (using a IDE compat mode that only allows to use the drive
read-only) and does not handle to send SCSI commands to ATAPI drives by
default. This makes it hard for people who just started with Linux to do
CD-writing on Linux if they own an ATAPI drive.

[ ... ]

        All Linux ATAPI transport implementations do not support DMA.
        Current execptions are:

        -	ide-scsi with block size 2048 and if DMA has been enabled

        -	The new experimental ATAPI interface starting with Linux-2.5.45
                allows DMA if DMA has been enabled and the sector size is a
                multiple of 4. This allows to use DMA for audio CDs and
                when writing any type of CD in RAW mode.

        RAW mode is needed for many new and cheap drives that have bugs when
        writing in cooked mode. If there is no DMA, you cannot write faster
        than approx 16x.
[/QUOTE]

Problems with DMA and writing CDs or reading
audio CDs are getting at my nerves too :-(

Regards,
David
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