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.
Signed-off-by: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
---
drivers/usb/core/hub.c | 52 +++++++++++++++++++++++-------------------
1 file changed, 28 insertions(+), 24 deletions(-)
diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
index d1efc7141333..a35d0bedafa3 100644
--- a/drivers/usb/core/hub.c
+++ b/drivers/usb/core/hub.c
@@ -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;
}
--
2.25.1
