[PATCH v2 2/2] usb: uas: add US_FL_QDEPTH_075 quirk to cap queue depth at probe time

[Luca Cecchi <luca.cecchi.info@gmail.com>]

The previous patch in this series ("usb: uas: implement
.change_queue_depth to allow per-device queue depth override") lets
userspace lower a device's queue depth via the standard sysfs
attribute, e.g. through a udev rule matching idVendor/idProduct.

That is not sufficient on its own: relying on udev to act after the
device is already probed and in use leaves a window, right after
probe, during which the device is already live with the unmodified
(and, for some bridges, unsafe) default depth. A lockup can happen in
that window before udev has a chance to write the reduced value.

This adds US_FL_QDEPTH_075, a quirk flag that caps can_queue to 3/4 of
the device's reported maximum directly in uas_probe(), before the
device is ever added to the SCSI layer. This closes the race window
entirely for devices known to need it, while leaving the
.change_queue_depth path available for anyone who wants to tune
further.

Applied to the Lexar ES3 enclosure (21c4:0003) already described in
the previous patch: qdepth=32, so this caps can_queue to 24 - exactly
the value empirically found stable during that investigation (zero
lockups across multiple 360s runs and a 1-hour soak test at that
depth, vs. lockups on every run at the unmodified qdepth-2 default).

Signed-off-by: Luca Cecchi <luca.cecchi.info@gmail.com>
---
 drivers/usb/storage/uas.c         | 10 ++++++++++
 drivers/usb/storage/unusual_uas.h |  7 +++++++
 include/linux/usb_usual.h         |  2 ++
 3 files changed, 19 insertions(+)

diff --git a/drivers/usb/storage/uas.c b/drivers/usb/storage/uas.c
index 971a47f..95374dc 100644
--- a/drivers/usb/storage/uas.c
+++ b/drivers/usb/storage/uas.c
@@ -1085,6 +1085,16 @@ static int uas_probe(struct usb_interface *intf, const struct usb_device_id *id)
 	 */
 	shost->can_queue = devinfo->qdepth - 2;
 
+	/*
+	 * Some bridge chips lock up under sustained deep command queueing
+	 * even within the margin above. For those, US_FL_QDEPTH_075 caps
+	 * the depth to 3/4 of the device's reported maximum at probe time,
+	 * closing the window a udev-triggered .change_queue_depth write
+	 * would otherwise leave open between probe and udev acting.
+	 */
+	if (devinfo->flags & US_FL_QDEPTH_075)
+		shost->can_queue = (devinfo->qdepth * 3) / 4;
+
 	usb_set_intfdata(intf, shost);
 	result = scsi_add_host(shost, &intf->dev);
 	if (result)
diff --git a/drivers/usb/storage/unusual_uas.h b/drivers/usb/storage/unusual_uas.h
index d6f86d5..d41f206 100644
--- a/drivers/usb/storage/unusual_uas.h
+++ b/drivers/usb/storage/unusual_uas.h
@@ -167,6 +167,13 @@ UNUSUAL_DEV(0x2109, 0x0711, 0x0000, 0x9999,
 		USB_SC_DEVICE, USB_PR_DEVICE, NULL,
 		US_FL_NO_ATA_1X),
 
+/* Reported-by: Luca Cecchi <luca.cecchi.info@gmail.com> */
+UNUSUAL_DEV(0x21c4, 0x0003, 0x0000, 0x9999,
+		"Lexar",
+		"ES3",
+		USB_SC_DEVICE, USB_PR_DEVICE, NULL,
+		US_FL_QDEPTH_075),
+
 /* Reported-by: Icenowy Zheng <icenowy@aosc.io> */
 UNUSUAL_DEV(0x2537, 0x1068, 0x0000, 0x9999,
 		"Norelsys",
diff --git a/include/linux/usb_usual.h b/include/linux/usb_usual.h
index 712363c..b436c34 100644
--- a/include/linux/usb_usual.h
+++ b/include/linux/usb_usual.h
@@ -88,6 +88,8 @@
 		/* Cannot handle WRITE_SAME */			\
 	US_FLAG(SENSE_AFTER_SYNC, 0x80000000)			\
 		/* Do REQUEST_SENSE after SYNCHRONIZE_CACHE */	\
+	US_FLAG(QDEPTH_075,	0x100000000ULL)			\
+		/* Cap UAS queue depth to 3/4 of device max at probe time */
 
 #define US_FLAG(name, value)	US_FL_##name = value ,
 enum { US_DO_ALL_FLAGS };
-- 
2.55.0