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04-27-2012 09:31 AM

usb/uas: only bind if the hcd supports SG
From: Sebastian Andrzej Siewior <>

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 <>
Signed-off-by: Sarah Sharp <>
drivers/usb/storage/uas.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/storage/uas.c b/drivers/usb/storage/uas.c
index e0133c9..e34c759 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
+ hcd->driver->description);
+ dev_warn(&udev->dev, "required by the UAS driver. Please try an"
+ "alternative USB controller if you wish to use UAS.
+ 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,
+ }

return -ENODEV;

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