[3.13.y.z extended stable] Patch "usb: chipidea: udc: Disable auto ZLP generation on ep0" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    usb: chipidea: udc: Disable auto ZLP generation on ep0

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.6.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From f485a7c29cc26d192f019dba7d2d76c528cda66f Mon Sep 17 00:00:00 2001
From: Abbas Raza <Abbas_Raza at mentor.com>
Date: Thu, 17 Jul 2014 19:34:31 +0800
Subject: usb: chipidea: udc: Disable auto ZLP generation on ep0

commit 953c66469735aed8d2ada639a72b150f01dae605 upstream.

There are 2 methods for ZLP (zero-length packet) generation:
1) In software
2) Automatic generation by device controller

1) is implemented in UDC driver and it attaches ZLP to IN packet if
   descriptor->size < wLength
2) can be enabled/disabled by setting ZLT bit in the QH

When gadget ffs is connected to ubuntu host, the host sends
get descriptor request and wLength in setup packet is 255 while the
size of descriptor which will be sent by gadget in IN packet is
64 byte. So the composite driver sets req->zero = 1.
In UDC driver following code will be executed then

        if (hwreq->req.zero && hwreq->req.length
            && (hwreq->req.length % hwep->ep.maxpacket == 0))
                add_td_to_list(hwep, hwreq, 0);

Case-A:
So in case of ubuntu host, UDC driver will attach a ZLP to the IN packet.
ubuntu host will request 255 byte in IN request, gadget will send 64 byte
with ZLP and host will come to know that there is no more data.
But hold on, by default ZLT=0 for endpoint 0 so hardware also tries to
automatically generate the ZLP which blocks enumeration for ~6 seconds due
to endpoint 0 STALL, NAKs are sent to host for any requests (OUT/PING)

Case-B:
In case when gadget ffs is connected to Apple device, Apple device sends
setup packet with wLength=64. So descriptor->size = 64 and wLength=64
therefore req->zero = 0 and UDC driver will not attach any ZLP to the
IN packet. Apple device requests 64 bytes, gets 64 bytes and doesn't
further request for IN data. But ZLT=0 by default for endpoint 0 so
hardware tries to automatically generate the ZLP which blocks enumeration
for ~6 seconds due to endpoint 0 STALL, NAKs are sent to host for any
requests (OUT/PING)

According to USB2.0 specs:

    8.5.3.2 Variable-length Data Stage
    A control pipe may have a variable-length data phase in which the
    host requests more data than is contained in the specified data
    structure. When all of the data structure is returned to the host,
    the function should indicate that the Data stage is ended by
    returning a packet that is shorter than the MaxPacketSize for the
    pipe. If the data structure is an exact multiple of wMaxPacketSize
    for the pipe, the function will return a zero-length packet to indicate
    the end of the Data stage.

In Case-A mentioned above:
If we disable software ZLP generation & ZLT=0 for endpoint 0 OR if software
ZLP generation is not disabled but we set ZLT=1 for endpoint 0 then
enumeration doesn't block for 6 seconds.

In Case-B mentioned above:
If we disable software ZLP generation & ZLT=0 for endpoint then enumeration
still blocks due to ZLP automatically generated by hardware and host not needing
it. But if we keep software ZLP generation enabled but we set ZLT=1 for
endpoint 0 then enumeration doesn't block for 6 seconds.

So the proper solution for this issue seems to disable automatic ZLP generation
by hardware (i.e by setting ZLT=1 for endpoint 0) and let software (UDC driver)
handle the ZLP generation based on req->zero field.

Signed-off-by: Abbas Raza <Abbas_Raza at mentor.com>
Signed-off-by: Peter Chen <peter.chen at freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/usb/chipidea/udc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/chipidea/udc.c b/drivers/usb/chipidea/udc.c
index 71d0d35..6c8b198 100644
--- a/drivers/usb/chipidea/udc.c
+++ b/drivers/usb/chipidea/udc.c
@@ -1179,8 +1179,8 @@ static int ep_enable(struct usb_ep *ep,

 	if (hwep->type == USB_ENDPOINT_XFER_CONTROL)
 		cap |= QH_IOS;
-	if (hwep->num)
-		cap |= QH_ZLT;
+
+	cap |= QH_ZLT;
 	cap |= (hwep->ep.maxpacket << __ffs(QH_MAX_PKT)) & QH_MAX_PKT;
 	/*
 	 * For ISO-TX, we set mult at QH as the largest value, and use
--
1.9.1