SCSI passthrough ioctl calls vs. sd/sg devices

SCSI passthrough ioctl calls vs. sd/sg devices

[Lee Xing]

Hi,

I would appreciate it if some one could clarify this.  It's a bit confusing.

<<The Linux SCSI Generic (sg) HOWTO>> page 2 says "The sg driver permits user applications to send SCSI commands to devices that understand them.  SCSI commands are 6, 10, 12 or 16 bytes long.  The SCSI disk driver (sd), once device initialization is complete, only sends SCSI READ and WRITE commands."

<<The Linux 2.4 SCSI subsystem HOWTO>> page 3 says "Sg is a SCSI command pass through device that uses a char device interface.  General purpose Linux commands should not be used on sg devices."

My questions are:

- looks like SCSI passthrough ioctl calls can only apply 
  to sg devices.  Is it right, and can we apply SCSI
  passthrough ioctl calls to sd devices as well?
- what are the so-called "General purpose Linux commands"?
  Are they read()/write() or 'mount' / 'dd' or something else?

What should we do if we need to 'mount' a file system on a disk and also need to issue SCSI passthrough ioctl calls to the same disk?  Maybe we should use both sd and sg (i.e. mount the file system to /dev/sda, and issue SCSI passthrough ioctl calls to /dev/sg0 that maps to the same physical disk)?  'mount' doesn't work on sg devices as I was told earlier.

Many thanks,


Lee

Re: SCSI passthrough ioctl calls vs. sd/sg devices

[Douglas Gilbert]

Lee Xing wrote:
> Hi,
> 
> I would appreciate it if some one could clarify this.  It's a bit confusing.
> 
> <<The Linux SCSI Generic (sg) HOWTO>> page 2 says "The sg driver permits user applications to send SCSI commands to devices that understand them.  SCSI commands are 6, 10, 12 or 16 bytes long.  The SCSI disk driver (sd), once device initialization is complete, only sends SCSI READ and WRITE commands."
> 
> <<The Linux 2.4 SCSI subsystem HOWTO>> page 3 says "Sg is a SCSI command pass through device that uses a char device interface.  General purpose Linux commands should not be used on sg devices."
> 
> My questions are:
> 
> - looks like SCSI passthrough ioctl calls can only apply 
>   to sg devices.  Is it right, and can we apply SCSI
>   passthrough ioctl calls to sd devices as well?

True in the lk 2.4 series. When all else fails you can still
use the SCSI_IOCTL_SEND_COMMAND ioctl() [which is what is
used by smartmontools currently].

In the lk 2.6 series the SG_IO ioctl is available for block
devices which include SCSI disks and ATAPI dvd/cd drives.
The SG_IO ioctl is currently not available for SCSI char
device drivers (e.g. st and osst) other than sg itself.
BTW ATA disks don't understand SCSI commands so the SG_IO
ioctl is of little use with them.

It the next week or so I hope to propose a SG_IO ioctl
for the scsi mid level for consideration in lk 2.6.
That will make the SG_IO ioctl available to st and osst
as well.

> - what are the so-called "General purpose Linux commands"?
>   Are they read()/write() or 'mount' / 'dd' or something else?

mount, dd, cp, mv, rm (etc).

The sg3_utils package has a dd variant called sg_dd which can use
sg devices (as well as those that the standard dd can handle).

> What should we do if we need to 'mount' a file system on a disk and 
> also need to issue SCSI passthrough ioctl calls to the same disk?  
 > Maybe we should use both sd and sg (i.e. mount the file system to
 > /dev/sda, and issue SCSI passthrough ioctl calls to /dev/sg0 that
> maps to the same physical disk)?  

yes

> 'mount' doesn't work on sg devices as I was told earlier.

true.

Doug Gilbert

Re: SCSI passthrough ioctl calls vs. sd/sg devices

[Kai Makisara]

On Sat, 24 Apr 2004, Douglas Gilbert wrote:

...
> In the lk 2.6 series the SG_IO ioctl is available for block
> devices which include SCSI disks and ATAPI dvd/cd drives.
> The SG_IO ioctl is currently not available for SCSI char
> device drivers (e.g. st and osst) other than sg itself.

SG_IO is available for st in current 2.6 (Patrick Mansfield added a few 
lines of code to st.c). Similar changes could be probably done to osst.

-- 
Kai

Re: SCSI passthrough ioctl calls vs. sd/sg devices

[Willem Riede]

On 2004.04.24 02:58, Kai Makisara wrote:
> On Sat, 24 Apr 2004, Douglas Gilbert wrote:
> 
> ...
> > In the lk 2.6 series the SG_IO ioctl is available for block
> > devices which include SCSI disks and ATAPI dvd/cd drives.
> > The SG_IO ioctl is currently not available for SCSI char
> > device drivers (e.g. st and osst) other than sg itself.
> 
> SG_IO is available for st in current 2.6 (Patrick Mansfield added a few 
> lines of code to st.c). Similar changes could be probably done to osst.

I'll be happy to do that (make the equivalent change to osst) if that's 
the best way to complete the SG_IO support.

Regards, Willem Riede.

Re: SCSI passthrough ioctl calls vs. sd/sg devices

[Douglas Gilbert]

Willem Riede wrote:
> On 2004.04.24 02:58, Kai Makisara wrote:
> 
>>On Sat, 24 Apr 2004, Douglas Gilbert wrote:
>>
>>...
>>
>>>In the lk 2.6 series the SG_IO ioctl is available for block
>>>devices which include SCSI disks and ATAPI dvd/cd drives.
>>>The SG_IO ioctl is currently not available for SCSI char
>>>device drivers (e.g. st and osst) other than sg itself.
>>
>>SG_IO is available for st in current 2.6 (Patrick Mansfield added a few 
>>lines of code to st.c). Similar changes could be probably done to osst.
> 
> 
> I'll be happy to do that (make the equivalent change to osst) if that's 
> the best way to complete the SG_IO support.

Willem,
The st mods for SG_IO seem to be working fine in lk 2.6.6-rc2.
See st's call to scsi_cmd_ioctl(). [As well as calling the SG_IO
ioctl, CDROM_SEND_PACKET is also available?! ]

So a recent version of sg3_utils (like version 1.06 beta on
http://www.torque.net/sg ) can know use /dev/st* device
names. For example:

# sg_modes -6 /dev/st0
     SONY      SDT-7000          0192   peripheral_type: tape [0x1]
Mode parameter header from 6 byte MODE SENSE:
   Mode data length=72, medium type=0x00, specific param=0x10, longlba=0
   Block descriptor length=8
 > General mode parameter block descriptors:
    Density code=0x13
  00     13 00 00 00 00 00 02 00

 >> page_code: Disconnect-Reconnect, page_control: current
  00     02 0e 00 00 00 00 00 00  00 00 01 ee 00 00 00 00
 >> page_code: Data Compression, page_control: current
  00     0f 0e c0 80 00 00 00 20  00 00 00 20 00 00 00 00
 >> page_code: Device config, page_control: current
  00     10 0e 08 00 00 00 00 32  60 00 18 00 00 00 00 00
 >> page_code: Medium Partition [1], page_control: current
  00     11 0a 01 00 10 03 00 00  0e 73 00 00

Also on that site is a beta of lsscsi version 0.12 for
lk 2.6.6 :

# lsscsi
[3:0:6:0]    tape    SONY     SDT-7000         0192  /dev/st0
[root@dink devices]# lsscsi -g
[3:0:6:0]    tape    SONY     SDT-7000         0192  /dev/st0  /dev/sg0
# lsscsi -l
[3:0:6:0]    tape    SONY     SDT-7000         0192  /dev/st0
   state=running queue_depth=1 scsi_level=3 type=1 device_blocked=0

# lsscsi -H
[3]    aha152x
# lsscsi -Hv
  sysfsroot: /sys
[3]    aha152x
   dir: /sys/class/scsi_host/host3
   device dir:/sys/devices/platform/host3
# lsscsi -Hl
[3]    aha152x
   cmd_per_lun=1    host_busy=0    sg_tablesize=255  unchecked_isa_dma=0



Doug Gilbert