[PATCH 1/5] HID: logitech-dj: Fix USB 3.0 issue

[Benjamin Tissoires <benjamin.tissoires@redhat.com>]

From: Benjamin Tisssoires <benjamin.tissoires@redhat.com>

This fix (not very clean though) should fix the long time USB3
issue that was spotted last year. The rational has been given by
Hans de Goede:

 ----

I think the most likely cause for this is a firmware bug
in the unifying receiver, likely a race condition.

The most prominent difference between having a USB-2 device
plugged into an EHCI (so USB-2 only) port versus an XHCI
port will be inter packet timing. Specifically if you
send packets (ie hid reports) one at a time, then with
the EHCI controller their will be a significant pause
between them, where with XHCI they will be very close
together in time.

The reason for this is the difference in EHCI / XHCI
controller OS <-> driver interfaces.

For non periodic endpoints (control, bulk) the EHCI uses a
circular linked-list of commands in dma-memory, which it
follows to execute commands, if the list is empty, it
will go into an idle state and re-check periodically.

The XHCI uses a ring of commands per endpoint, and if the OS
places anything new on the ring it will do an ioport write,
waking up the XHCI making it send the new packet immediately.

For periodic transfers (isoc, interrupt) the delay between
packets when sending one at a time (rather then queuing them
up) will be even larger, because they need to be inserted into
the EHCI schedule 2 ms in the future so the OS driver can be
sure that the EHCI driver does not try to start executing the
time slot in question before the insertion has completed.

So a possible fix may be to insert a delay between packets
being send to the receiver.

 ----

I tested this on a buggy Haswell USB 3.0 motherboard, and I always
get the notification after adding the msleep.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
---
 drivers/hid/hid-logitech-dj.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/hid/hid-logitech-dj.c b/drivers/hid/hid-logitech-dj.c
index a7947d8..f45279c 100644
--- a/drivers/hid/hid-logitech-dj.c
+++ b/drivers/hid/hid-logitech-dj.c
@@ -516,6 +516,14 @@ static int logi_dj_recv_switch_to_dj_mode(struct dj_receiver_dev *djrcv_dev,
 	dj_report->report_params[CMD_SWITCH_PARAM_TIMEOUT_SECONDS] = (u8)timeout;
 	retval = logi_dj_recv_send_report(djrcv_dev, dj_report);
 	kfree(dj_report);
+
+	/*
+	 * Ugly sleep to work around a USB 3.0 bug when the receiver is still
+	 * processing the "switch-to-dj" command while we send an other command.
+	 * 50 msec should gives enough time to the receiver to be ready.
+	 */
+	msleep(50);
+
 	return retval;
 }
 
-- 
1.8.4.2