On 07/07/2014 02:37 PM, Michael Grzeschik wrote:
On Sun, Jul 06, 2014 at 11:47:50PM +0500, Abbas Raza wrote:
On 07/06/2014 11:01 PM, Greg KH wrote:
On Sun, Jul 06, 2014 at 07:52:41PM +0200, Michael Grzeschik wrote:
Hi,
On Sun, Jul 06, 2014 at 09:26:02PM +0500, Abbas Raza wrote:
From: Abbas Raza <Abbas_Raza@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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 7a12d13..2857dff 100644
--- a/drivers/usb/chipidea/udc.c
+++ b/drivers/usb/chipidea/udc.c
@@ -1169,8 +1169,8 @@ 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
--
1.8.3.2
How about an module parameter for this?
If you want me to reject the patch, sure, do that :)
Hint, never add module parameters for a driver, that is not acceptable.
greg k-h
Hi Michael,
Could you please clarify why do you want to make it configurable?
Can the proposed change cause problems in some scenario?
The problem you are talking about is not the candidate for an module
parameter. In fact I was thinking/refering to the hardcoded ZLT bit.
Were setting it is not correct for every case.
Why setting it doesn't work in every case and software based ZLP generation is not enough?
The information who is supposed to generate the zero package should come
from the upper layers, or at least from the kernel paramter. The latter
is probably not the nicest way.
Upper layer is handling it already by setting req->zero field. Right?
Regards,
Michael
Thanks .
Abbas
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