I've been looking to find the reason why CDIOCPLAYTRACKS, etc. fail to
work on some CD players (and cause a bus reset), and work but cause an
error on others.  I think I know the reason but I don't have a solution yet.

The reason is that the audio commands assume that the CD player has four
channels.  All of the CD players I tested have 2 channels.  The error
occurs because cd_scsibus_setvol() in /usr/src/sys/dev/scsipi/cd_scsi.c
has :

		data.page.audio.port[2].volume = arg->vol[2];
		data.page.audio.port[3].volume = arg->vol[3]; 

and cd_scsibus_setvol() is called as a result of a CDIOCPLAYTRACKS ioctl.
(I see that cd_scsibus_setchan() also sets channels 2 and 3.)

The simple solution is just to comment out the code that refers to these
channels, but the SCSI-2 spec mentions that the number of audio channels
is returned in control bit 3 of a READ SUB-CHANNEL call on sub-channel
Q.  Also, I see that struct scsi_cd_write_params_page{} in scsi_cd.h has a
reference to TRACK_MODE_4CHAN, but I don't see what this is used for.

I must admit that this is the first time I've looked at the SCSI code and
spec in detail, so a few pointers would be helpful, e.g. some code calling
CDIOCREADSUBCHANNEL.  I wonder if it would make sense to call this at attach
time, but I'm not sure what happens if you call READ SUB-CHANNEL without a
CD in the player or with a data only CD.  Any insights appreciated.

Thanks,

J

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