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    SubjectRe: [Discussion] USB: musb-gadget: how to fix ZLP issue in musb_g_tx
    Ming Lei wrote:
    > Hi All,

    > In another thread, Sergei pointed out there is a ZLP issue in musb_g_tx:

    > Sergei Shtylyov <sshtylyov@mvista.com> wrote:
    >> Hello.

    >> On 15-09-2010 14:05, Felipe Balbi wrote:

    >>>> I didn't say it was duplicate for DMA, just too late.

    >>> how come ? we need to send ZLP before giving back the request.

    >> Well, look at the code ionce again. We need to send ZLP *after*
    >> request->actual == request->length, but as the check is inserted
    >> after the ZLP send, ZLP *may* be sent once the first DMA completes,
    >> not the last.

    >> WBR, Sergei

    > balbi also confirmed that is is really a problem.

    > I also have two related questions below for the problem:

    > 1), why is the check for "is_dma" needed here?

    > if (is_dma || request->actual == request->length) {
    > ....
    > }

    I'm not sure -- it seems erratic.

    > 2), why is a zlp needed in the case below?
    >
    > #ifdef CONFIG_USB_INVENTRA_DMA
    > || (is_dma && (!dma->desired_mode ||
    > (request->actual & (musb_ep->packet_sz - 1))))
    > #endif

    > Seems no request->zero is set to ask for zlp explicitly in
    > the case above.

    This is not for ZLP -- this is here to set TXPktRdy for the last short
    packet in the Inventra DMA mode 0 that doesn't set TXPktRdy in such case.

    > IMO, it is not difficult to give a good fix for the ZLP problem
    > if the two questions are clear.

    WBR, Sergei



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