usb-storage: enable multi-LUN scanning when needed

People sometimes create their own custom-configured kernels and forget
to enable CONFIG_SCSI_MULTI_LUN.  This causes problems when they plug
in a USB storage device (such as a card reader) with more than one
LUN.

Fortunately, we can tell fairly easily when a storage device claims to
have more than one LUN.  When that happens, this patch asks the SCSI
layer to probe all the LUNs automatically, regardless of the config
setting.

The patch also updates the Kconfig help text for usb-storage,
explaining that CONFIG_SCSI_MULTI_LUN may be necessary.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Thomas Raschbacher <lordvan@lordvan.com>
CC: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
CC: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/storage/Kconfig

@@ -18,7 +18,9 @@ config USB_STORAGE
 
 	  This option depends on 'SCSI' support being enabled, but you
 	  probably also need 'SCSI device support: SCSI disk support'
-	  (BLK_DEV_SD) for most USB storage devices.
+	  (BLK_DEV_SD) for most USB storage devices.  Some devices also
+	  will require 'Probe all LUNs on each SCSI device'
+	  (SCSI_MULTI_LUN).
 
 	  To compile this driver as a module, choose M here: the
 	  module will be called usb-storage.

drivers/usb/storage/scsiglue.c

@@ -78,6 +78,8 @@ static const char* host_info(struct Scsi_Host *host)
 
 static int slave_alloc (struct scsi_device *sdev)
 {
+	struct us_data *us = host_to_us(sdev->host);
+
 	/*
 	 * Set the INQUIRY transfer length to 36.  We don't use any of
 	 * the extra data and many devices choke if asked for more or
@@ -102,6 +104,10 @@ static int slave_alloc (struct scsi_device *sdev)
 	 */
 	blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));
 
+	/* Tell the SCSI layer if we know there is more than one LUN */
+	if (us->protocol == USB_PR_BULK && us->max_lun > 0)
+		sdev->sdev_bflags |= BLIST_FORCELUN;
+
 	return 0;
 }