On Mon, 6 Oct 2014, Mark Knibbs wrote:
> Hi,
>
> This is about USB mass storage devices.
>
> The max_lun field in the scsi_host/us_data structure is the maximum
> possible LUN value *plus 1*.
You are confusing two different fields. The max_lun field in scsi_host
is different from the max_lun field in us_data.
The field in us_data is the maximum possible LUN value, _not_ plus 1.
See this code in usb_stor_control_thread():
else if (us->srb->device->lun > us->max_lun) {
usb_stor_dbg(us, "Bad LUN (%d:%llu)\n",
us->srb->device->id,
us->srb->device->lun);
us->srb->result = DID_BAD_TARGET << 16;
}
> It's initially set to 8 by scsi_host_alloc()
> which is called by usb_stor_probe1().
scsi_host_alloc() doesn't touch anything in the us_data structure.
> usb_stor_scan_dwork() in drivers/usb/storage/usb.c contains this:
>
> /* For bulk-only devices, determine the max LUN value */
> if (us->protocol == USB_PR_BULK && !(us->fflags & US_FL_SINGLE_LUN)) {
> mutex_lock(&us->dev_mutex);
> us->max_lun = usb_stor_Bulk_max_lun(us);
> mutex_unlock(&us->dev_mutex);
> }
> scsi_scan_host(us_to_host(us));
>
> usb_stor_Bulk_max_lun() in drivers/usb/storage/transport.c contains this:
>
> usb_stor_dbg(us, "GetMaxLUN command result is %d, data is %d\n",
> result, us->iobuf[0]);
>
> /* if we have a successful request, return the result */
> if (result > 0)
> return us->iobuf[0];
>
> /*
> * Some devices don't like GetMaxLUN. They may STALL the control
> * pipe, they may return a zero-length result, they may do nothing at
> * all and timeout, or they may fail in even more bizarrely creative
> * ways. In these cases the best approach is to use the default
> * value: only one LUN.
> */
> return 0;
>
> The USB Mass Storage Class document specifies that Get Max LUN returns the
> maximum LUN value (*not* plus 1). So for a single-LUN device or if
> GetMaxLUN fails, usb_stor_Bulk_max_lun() returns 0. In view of that,
> shouldn't the above code in usb_stor_scan_dwork() be changed to:
>
> /* For bulk-only devices, determine the max LUN value */
> if (us->protocol == USB_PR_BULK && !(us->fflags & US_FL_SINGLE_LUN)) {
> mutex_lock(&us->dev_mutex);
> us->max_lun = usb_stor_Bulk_max_lun(us) + 1;
> mutex_unlock(&us->dev_mutex);
> }
> scsi_scan_host(us_to_host(us));
No, it is correct as it stands.
Alan Stern
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