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

[Johan Hovold <johan@kernel.org>]

On Tue, Mar 07, 2023 at 10:41:46AM -0800, Doug Anderson wrote:
> Hi,
> 
> On Tue, Mar 7, 2023 at 8:43 AM Johan Hovold <johan+linaro@kernel.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@linaro.org>
> > Signed-off-by: Johan Hovold <johan+linaro@kernel.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?

I considered that, but decided to leave it in as it makes the flow in
qcom_geni_serial_handle_tx_dma() a bit more obvious (and that function
already handles the related uart_write_wakeup() which the check could
potentially be combined with).

> In any case, with or without that:
> 
> Reviewed-by: Douglas Anderson <dianders@chromium.org>

Thanks for reviewing.

Johan