[PATCH] usb: storage: skip only when uas driver is loaded

[JC Kuo <jckuo@nvidia.com>]

When usb-storage driver detects a UAS capable device, it ignores the
device if CONFIG_USB_UAS is enabled. usb-storage driver assumes uas
driver certainly will be loaded. However, it's possible that uas
driver will not be loaded, for example, uas kernel module is not
installed properly or it is in modprobe blacklist.

In case of uas driver not being loaded, the UAS capable device will
not fallback to work at Bulk-only-transfer mode. The device just
disappears without any notification to user/userspace.

This commit changes usb-storage driver to skip UAS capable device
only when uas driver is already loaded to make sure the device will
at least work with Bulk protocol.

Signed-off-by: JC Kuo <jckuo@nvidia.com>
---
 drivers/usb/core/driver.c | 1 +
 drivers/usb/storage/usb.c | 5 +++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
index ebcadaad89d1..265c5dd490d2 100644
--- a/drivers/usb/core/driver.c
+++ b/drivers/usb/core/driver.c
@@ -1923,3 +1923,4 @@ struct bus_type usb_bus_type = {
 	.uevent =	usb_uevent,
 	.need_parent_lock =	true,
 };
+EXPORT_SYMBOL_GPL(usb_bus_type);
diff --git a/drivers/usb/storage/usb.c b/drivers/usb/storage/usb.c
index 9a79cd9762f3..d8f64e808783 100644
--- a/drivers/usb/storage/usb.c
+++ b/drivers/usb/storage/usb.c
@@ -1097,9 +1097,10 @@ static int storage_probe(struct usb_interface *intf,
 	int result;
 	int size;
 
-	/* If uas is enabled and this device can do uas then ignore it. */
+	/* If uas driver is loaded and this device can do uas then ignore it. */
 #if IS_ENABLED(CONFIG_USB_UAS)
-	if (uas_use_uas_driver(intf, id, NULL))
+	if (driver_find("uas", &usb_bus_type) &&
+		uas_use_uas_driver(intf, id, NULL))
 		return -ENXIO;
 #endif
 
-- 
2.17.1


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