On Tue, Oct 19, 2021 at 12:15:09PM +0200, mark_k@xxxxxxxxx wrote: > Hi, > > I have a Core Micro Systems USB2 Link USB-SCSI converter (07B4:0380). > > Adding an entry to unusual_devs.h should get it to work, just needing > USB_PR_BULK. That should at least allow the connected device with SCSI ID 0 > to be accessed. > > However in order to access multiple targets and LUNs, the USB2 Link uses > byte 13 of the command block wrapper in a special way. > > Normally CBW byte 13 has bCBWLUN in bits [3:0] with bits [7:4] reserved. > The USB2 Link expects the target ID in bits [3:0] and LUN in bits [7:4]. Why on earth does it use such a non-standard protocol? After all, the USB mass-storage specifications have been available since 1999 or earlier! And they haven't exactly been kept secret during all that time. > The advantage of that is, it should be possible to access multiple targets > without needing to modify the USB mass storage driver. (It returns 0x06 to > a Get Max LUN request since its SCSI ID is hard-coded to 7.) > > Being able to access non-zero actual LUNs would of course require changes > to the driver. > > I'm just wondering, how does the usb-storage driver handle these cases: > > - (What it thinks are) LUNs are not contiguous. Suppose the user has two > SCSI devices in the chain, one with ID 0 the other with ID 3. Would it > scan LUNs (which map to separate targets) 1, 2, 4, 5 and 6? Or would it > give up on getting no response from LUN 1? This is not handled by usb-storage at all; it is handled by the SCSI core. My memory of what the SCSI core does is old (so a little unreliable) and possibly out of date, but here it is: The handling of non-contiguous LUNs may depend on which SCSI level the device claims to support. However, in many cases the LUN scan does stop when no response is received. > - "LUN" 0 is not present. E.g. where the connected SCSI devices have IDs 1 > and 3. If there's no LUN 0 then the SCSI layer will give up on the target entirely. > - When different "LUNs" are completely different devices (e.g. one a > CD-ROM, another a hard disk, another a tape drive). No matter. Each LUN can have its own individual device type. Alan Stern
