Re: st driver runs async only

[hgweigand@wiesbaden.netsurf.de (Hans-Guenter Weigand)]

> > ncr0 at pci0 dev 12 function 0 "Symbios Logic 53c810" rev 0x02: ncr
> > 53c810 fast10 scsi, irq 11
> > scsibus1 at ncr0: 8 targets
> > st0 at scsibus1 targ 1 lun 0: <Quantum, DLT4000, D991> SCSI2
> > 1/sequential removable
> > st0: st0(ncr0:1:0): 10.0 MB/s (100 ns, offset 8)
>                                  ^^^^^^^^^^^^^^^^
>   This indicates the driver negotiated Fast SCSI synchronous transfers.

I think this just means that the driver has detected that the drive
_supports_ synchronous transfers. This does not say anything about the
transfer mode when the tape is actually used. If st(4) would use
synchronous mode, the transfer rate would easily exceed 3.5 MB/sec, but
it doesn't.

According to the DLT documentation the tape drive does not request
synchronous transfers, in contrast to disks.

from the manual:

"3.5.5.2 Synchronous Data Transfer
Synchronous Data Transfer is optional and is only used in DATA phases
and only if a synchronous data transfer agreement is established."

"4.2.14 Synchronous Data Transfer Request Message (01h)
This extended message allows the target and initiator to agree on the
values of the parameters relevant to synchronous transfers. The tape
drive will not initiate the Synchronous Data Transfer Request Message;
it relies on the initiator to do so."

I imagine that there might be just a single call added to st(4) to get
the host initiating synchronous transfers.

-hgw