usb: hub: Per-port setting to reduce TRSTRCY to 10 ms

Currently, the USB hub core waits for 50 ms after enumerating the
device. This was added to help "some high speed devices" to
enumerate (b789696af8 "[PATCH] USB: relax usbcore reset timings").

On some devices, the time-to-active is important, so we provide
a per-port option to reduce the time to what the USB specification
requires: 10 ms.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-usb

@@ -206,6 +206,10 @@ Description:
 		   using /sys/module/usbcore/parameters/old_scheme_first, but
 		   it is often not desirable as the new scheme was introduced to
 		   increase compatibility with more devices.
+		 - Bit 1 reduces TRSTRCY to the 10 ms that are required by the
+		   USB 2.0 specification, instead of the 50 ms that are normally
+		   used to help make enumeration work better on some high speed
+		   devices.
 
 What:		/sys/bus/usb/devices/.../(hub interface)/portX/over_current_count
 Date:		February 2018

drivers/usb/core/hub.c

@@ -2879,7 +2879,11 @@ static int hub_port_reset(struct usb_hub *hub, int port1,
 done:
 	if (status == 0) {
 		/* TRSTRCY = 10 ms; plus some extra */
-		msleep(10 + 40);
+		if (port_dev->quirks & USB_PORT_QUIRK_FAST_ENUM)
+			usleep_range(10000, 12000);
+		else
+			msleep(10 + 40);
+
 		if (udev) {
 			struct usb_hcd *hcd = bus_to_hcd(udev->bus);
 

include/linux/usb.h

@@ -496,6 +496,9 @@ enum usb_port_connect_type {
 /* For the given port, prefer the old (faster) enumeration scheme. */
 #define USB_PORT_QUIRK_OLD_SCHEME	BIT(0)
 
+/* Decrease TRSTRCY to 10ms during device enumeration. */
+#define USB_PORT_QUIRK_FAST_ENUM	BIT(1)
+
 /*
  * USB 2.0 Link Power Management (LPM) parameters.
  */