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    Subject[PATCH 5.5 072/189] usb: dwc3: gadget: Update chain bit correctly when using sg list
    Date
    From: Pratham Pratap <prathampratap@codeaurora.org>

    commit dad2aff3e827b112f27fa5e6f2bf87a110067c3f upstream.

    If scatter-gather operation is allowed, a large USB request is split
    into multiple TRBs. For preparing TRBs for sg list, driver iterates
    over the list and creates TRB for each sg and mark the chain bit to
    false for the last sg. The current IOMMU driver is clubbing the list
    of sgs which shares a page boundary into one and giving it to USB driver.
    With this the number of sgs mapped it not equal to the the number of sgs
    passed. Because of this USB driver is not marking the chain bit to false
    since it couldn't iterate to the last sg. This patch addresses this issue
    by marking the chain bit to false if it is the last mapped sg.

    At a practical level, this patch resolves USB transfer stalls
    seen with adb on dwc3 based db845c, pixel3 and other qcom
    hardware after functionfs gadget added scatter-gather support
    around v4.20.

    Credit also to Anurag Kumar Vulisha <anurag.kumar.vulisha@xilinx.com>
    who implemented a very similar fix to this issue.

    Cc: Felipe Balbi <balbi@kernel.org>
    Cc: Yang Fei <fei.yang@intel.com>
    Cc: Thinh Nguyen <thinhn@synopsys.com>
    Cc: Tejas Joglekar <tejas.joglekar@synopsys.com>
    Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
    Cc: Jack Pham <jackp@codeaurora.org>
    Cc: Todd Kjos <tkjos@google.com>
    Cc: Greg KH <gregkh@linuxfoundation.org>
    Cc: Linux USB List <linux-usb@vger.kernel.org>
    Cc: stable <stable@vger.kernel.org> #4.20+
    Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
    [jstultz: Slight tweak to remove sg_is_last() usage, reworked
    commit message, minor comment tweak]
    Signed-off-by: John Stultz <john.stultz@linaro.org>
    Link: https://lore.kernel.org/r/20200302214443.55783-1-john.stultz@linaro.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/usb/dwc3/gadget.c | 9 ++++++++-
    1 file changed, 8 insertions(+), 1 deletion(-)

    --- a/drivers/usb/dwc3/gadget.c
    +++ b/drivers/usb/dwc3/gadget.c
    @@ -1068,7 +1068,14 @@ static void dwc3_prepare_one_trb_sg(stru
    unsigned int rem = length % maxp;
    unsigned chain = true;

    - if (sg_is_last(s))
    + /*
    + * IOMMU driver is coalescing the list of sgs which shares a
    + * page boundary into one and giving it to USB driver. With
    + * this the number of sgs mapped is not equal to the number of
    + * sgs passed. So mark the chain bit to false if it isthe last
    + * mapped sg.
    + */
    + if (i == remaining - 1)
    chain = false;

    if (rem && usb_endpoint_dir_out(dep->endpoint.desc) && !chain) {

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