Commit 1bf2761c authored by Mathias Nyman's avatar Mathias Nyman Committed by Greg Kroah-Hartman
Browse files

usb: hub: Fix link power management max exit latency (MEL) calculations 



Maximum Exit Latency (MEL) value is used by host to know how much in
advance it needs to start waking up a U1/U2 suspended link in order to
service a periodic transfer in time.

Current MEL calculation only includes the time to wake up the path from
U1/U2 to U0. This is called tMEL1 in USB 3.1 section C 1.5.2

Total MEL = tMEL1 + tMEL2 +tMEL3 + tMEL4 which should additinally include:
- tMEL2 which is the time it takes for PING message to reach device
- tMEL3 time for device to process the PING and submit a PING_RESPONSE
- tMEL4 time for PING_RESPONSE to traverse back upstream to host.

Add the missing tMEL2, tMEL3 and tMEL4 to MEL calculation.

Cc: <stable@kernel.org> # v3.5
Signed-off-by: default avatarMathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20210715150122.1995966-1-mathias.nyman@linux.intel.com


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 57560ee9

drivers/usb/core/hub.c

+28 −24
Original line number Diff line number Diff line
@@ -48,6 +48,7 @@ 


#define USB_TP_TRANSMISSION_DELAY	40	/* ns */

#define USB_TP_TRANSMISSION_DELAY_MAX	65535	/* ns */

#define USB_PING_RESPONSE_TIME		400	/* ns */



/* Protect struct usb_device->state and ->children members

 * Note: Both are also protected by ->dev.sem, except that ->state can
@@ -182,8 +183,9 @@ int usb_device_supports_lpm(struct usb_device *udev) 
}



/*

 * Set the Maximum Exit Latency (MEL) for the host to initiate a transition from

 * either U1 or U2.

 * Set the Maximum Exit Latency (MEL) for the host to wakup up the path from

 * U1/U2, send a PING to the device and receive a PING_RESPONSE.

 * See USB 3.1 section C.1.5.2

 */

static void usb_set_lpm_mel(struct usb_device *udev,

		struct usb3_lpm_parameters *udev_lpm_params,
@@ -193,35 +195,37 @@ static void usb_set_lpm_mel(struct usb_device *udev, 
		unsigned int hub_exit_latency)

{

	unsigned int total_mel;

	unsigned int device_mel;

	unsigned int hub_mel;



	/*

	 * Calculate the time it takes to transition all links from the roothub

	 * to the parent hub into U0.  The parent hub must then decode the

	 * packet (hub header decode latency) to figure out which port it was

	 * bound for.

	 *

	 * The Hub Header decode latency is expressed in 0.1us intervals (0x1

	 * means 0.1us).  Multiply that by 100 to get nanoseconds.

	 * tMEL1. time to transition path from host to device into U0.

	 * MEL for parent already contains the delay up to parent, so only add

	 * the exit latency for the last link (pick the slower exit latency),

	 * and the hub header decode latency. See USB 3.1 section C 2.2.1

	 * Store MEL in nanoseconds

	 */

	total_mel = hub_lpm_params->mel +

		(hub->descriptor->u.ss.bHubHdrDecLat * 100);

		max(udev_exit_latency, hub_exit_latency) * 1000 +

		hub->descriptor->u.ss.bHubHdrDecLat * 100;



	/*

	 * How long will it take to transition the downstream hub's port into

	 * U0?  The greater of either the hub exit latency or the device exit

	 * latency.

	 *

	 * The BOS U1/U2 exit latencies are expressed in 1us intervals.

	 * Multiply that by 1000 to get nanoseconds.

	 * tMEL2. Time to submit PING packet. Sum of tTPTransmissionDelay for

	 * each link + wHubDelay for each hub. Add only for last link.

	 * tMEL4, the time for PING_RESPONSE to traverse upstream is similar.

	 * Multiply by 2 to include it as well.

	 */

	device_mel = udev_exit_latency * 1000;

	hub_mel = hub_exit_latency * 1000;

	if (device_mel > hub_mel)

		total_mel += device_mel;

	else

		total_mel += hub_mel;

	total_mel += (__le16_to_cpu(hub->descriptor->u.ss.wHubDelay) +

		      USB_TP_TRANSMISSION_DELAY) * 2;



	/*

	 * tMEL3, tPingResponse. Time taken by device to generate PING_RESPONSE

	 * after receiving PING. Also add 2100ns as stated in USB 3.1 C 1.5.2.4

	 * to cover the delay if the PING_RESPONSE is queued behind a Max Packet

	 * Size DP.

	 * Note these delays should be added only once for the entire path, so

	 * add them to the MEL of the device connected to the roothub.

	 */

	if (!hub->hdev->parent)

		total_mel += USB_PING_RESPONSE_TIME + 2100;



	udev_lpm_params->mel = total_mel;

}