• Abbas Raza's avatar
    usb: chipidea: udc: Disable auto ZLP generation on ep0 · 953c6646
    Abbas Raza authored
    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.
    
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarAbbas Raza <Abbas_Raza@mentor.com>
    Signed-off-by: default avatarPeter Chen <peter.chen@freescale.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    953c6646
udc.c 45 KB