usb/uas: only bind if the hcd supports SG

The UAS driver requires SG support by the HCD operating the device. This
patch stops UAS from operating on a HCD without sg support and prints a
message to let him know.

The spec says:
|For [USB2] backward compatibility, the device shall present [BOT] as
|alternate interface zero (primary) and [UAS] as alternate interface one
|(secondary). A device which does not need backward compatibility with
|[BOT] shall present [UAS] as alternate interface zero. In [USB2]
|systems, the [BOT] driver or an associated filter driver may need to
|issue a SET INTERFACE request for alternate interface one and then allow
|the [UAS] driver to load.

If the user used usb_modeswitch to switch to UAS then he can go back to
BOT or use a different HCD. In case UAS is the only interface then there
is currently no way out.
In future usb_sg_wait() should be extended to provide a non-blocking
interface so it can work with the UAS driver.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

1 files changed, 19 insertions, 6 deletions

diff --git a/drivers/usb/storage/uas.c b/drivers/usb/storage/uas.c
index e0133c9ab0bf..e34c75970ed1 100644
--- a/drivers/usb/storage/uas.c
+++ b/drivers/usb/storage/uas.c
@@ -13,6 +13,7 @@
 #include <linux/types.h>
 #include <linux/module.h>
 #include <linux/usb.h>
+#include <linux/usb/hcd.h>
 #include <linux/usb/storage.h>
 
 #include <scsi/scsi.h>
@@ -621,22 +622,34 @@ static int uas_is_interface(struct usb_host_interface *intf)
                 intf->desc.bInterfaceProtocol == USB_PR_UAS);
 }
 
+static int uas_isnt_supported(struct usb_device *udev)
+{
+        struct usb_hcd *hcd = bus_to_hcd(udev->bus);
+
+        dev_warn(&udev->dev, "The driver for the USB controller %s does not "
+                        "support scatter-gather which is\n",
+                        hcd->driver->description);
+        dev_warn(&udev->dev, "required by the UAS driver. Please try an"
+                        "alternative USB controller if you wish to use UAS.\n");
+        return -ENODEV;
+}
+
 static int uas_switch_interface(struct usb_device *udev,
                                                 struct usb_interface *intf)
 {
         int i;
-
-        if (uas_is_interface(intf->cur_altsetting))
-                return 0;
+        int sg_supported = udev->bus->sg_tablesize != 0;
 
         for (i = 0; i < intf->num_altsetting; i++) {
                 struct usb_host_interface *alt = &intf->altsetting[i];
-                if (alt == intf->cur_altsetting)
-                        continue;
-                if (uas_is_interface(alt))
+
+                if (uas_is_interface(alt)) {
+                        if (!sg_supported)
+                                return uas_isnt_supported(udev);
                         return usb_set_interface(udev,
                                                 alt->desc.bInterfaceNumber,
                                                 alt->desc.bAlternateSetting);
+                }
         }
 
         return -ENODEV;