linux-kernel - Re: [PATCH 3/4] serial: qcom-geni: fix mapping of empty DMA buffer

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Message-ID: <CAD=FV=VR5oCThAuc29Bum-VHQUcH_H+s4nr55YpJk1aYaqZKTQ@mail.gmail.com>
Date:   Tue, 7 Mar 2023 10:41:46 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Johan Hovold <johan+linaro@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] serial: qcom-geni: fix mapping of empty DMA buffer

Hi,

On Tue, Mar 7, 2023 at 8:43 AM Johan Hovold <johan+linaro@...nel.org> wrote:
>
> Make sure that there is data in the ring buffer before trying to set up
> a zero-length DMA transfer.
>
> This specifically fixes the following warning when unmapping the empty
> buffer on the sc8280xp-crd:
>
>    WARNING: CPU: 0 PID: 138 at drivers/iommu/dma-iommu.c:1046 iommu_dma_unmap_page+0xbc/0xd8
>    ...
>    Call trace:
>     iommu_dma_unmap_page+0xbc/0xd8
>     dma_unmap_page_attrs+0x30/0x1c8
>     geni_se_tx_dma_unprep+0x28/0x38
>     qcom_geni_serial_isr+0x358/0x75c
>
> Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
> Cc: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> Signed-off-by: Johan Hovold <johan+linaro@...nel.org>
> ---
>  drivers/tty/serial/qcom_geni_serial.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index 2aa3872e6283..9871225b2f9b 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -631,6 +631,9 @@ static void qcom_geni_serial_start_tx_dma(struct uart_port *uport)
>         if (port->tx_dma_addr)
>                 return;
>
> +       if (uart_circ_empty(xmit))
> +               return;

I guess you could remove the uart_circ_empty() test in
qcom_geni_serial_handle_tx_dma() now? In any case, with or without
that:

Reviewed-by: Douglas Anderson <dianders@...omium.org>