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: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20210715150122.1995966-1-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
index d1efc71..a35d0be 100644 (file)
--- 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;
 }