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    SubjectRe: [PATCH 11/15] tty: msm_serial: use dmaengine_prep_slave_sg()
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    On 17.04.2024 12:50, Jiri Slaby wrote:
    > On 17. 04. 24, 12:15, Marek Szyprowski wrote:
    >> On 16.04.2024 12:23, Jiri Slaby wrote:
    >>> On 15. 04. 24, 23:17, Marek Szyprowski wrote:
    >>>> On 05.04.2024 08:08, Jiri Slaby (SUSE) wrote:
    >>>>> This is a preparatory for the serial-to-kfifo switch. kfifo
    >>>>> understands
    >>>>> only scatter-gatter approach, so switch to that.
    >>>>>
    >>>>> No functional change intended, it's just
    >>>>> dmaengine_prep_slave_single()
    >>>>> inline expanded.
    >>>>>
    >>>>> And in this case, switch from dma_map_single() to dma_map_sg() too.
    >>>>> This
    >>>>> needs struct msm_dma changes. I split the rx and tx parts into an
    >>>>> union.
    >>>>> TX is now struct scatterlist, RX remains the old good phys-virt-count
    >>>>> triple.
    >>>>>
    >>>>> Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
    >>>>> Cc: Bjorn Andersson <andersson@kernel.org>
    >>>>> Cc: Konrad Dybcio <konrad.dybcio@linaro.org>
    >>>>> Cc: linux-arm-msm@vger.kernel.org
    >>>>
    >>>> I've just found that this patch broke UART operation on DragonBoard
    >>>> 410c. I briefly checked and didn't notice anything obviously wrong
    >>>> here,
    >>>> but the board stops transmitting any data from its serial port
    >>>> after the
    >>>> first message. I will try to analyze this issue a bit more tomorrow.
    >>>
    >>> I double checked, but I see no immediate issues in the patch too. So
    >>> please, if you can analyze this more…
    >>
    >> I've spent some time digging into this issue and frankly speaking I
    >> still have no idea WHY it doesn't work (or I seriously mixed something
    >> in the scatterlist principles). However I found a workaround to make it
    >> working. Maybe it will help a bit guessing what happens there.
    > ...
    >> @@ -434,7 +436,7 @@ static void msm_start_tx(struct uart_port *port)
    >>           struct msm_dma *dma = &msm_port->tx_dma;
    >>
    >>           /* Already started in DMA mode */
    >> -       if (sg_dma_len(&dma->tx_sg))
    >> +       if (dma->mapped)
    >
    > Thanks for looking into this.
    >
    > I was hesitant if I should use a flag. I should have, apparently.
    >
    > Quick question:
    > What's value of CONFIG_NEED_SG_DMA_LENGTH in your .config?


    CONFIG_NEED_SG_DMA_LENGTH=y


    I alse tried to change the "if (dma->mapped)" check in msm_start_tx() to:

    1. if (dma->tx_sg.length)

    2. if (dma->tx_sg.page_link & ~SG_PAGE_LINK_MASK)

    but none of the above worked what is really strange and incomprehensible
    for me.


    Best regards
    --
    Marek Szyprowski, PhD
    Samsung R&D Institute Poland


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