Re: [PATCH 11/15] tty: msm_serial: use dmaengine_prep_slave_sg()

[Jiri Slaby]

On 17. 04. 24, 12:15, Marek Szyprowski wrote:
> Hi Jiri,
>
> 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@xxxxxxxxxx>
> > > > Cc: Bjorn Andersson <andersson@xxxxxxxxxx>
> > > > Cc: Konrad Dybcio <konrad.dybcio@xxxxxxxxxx>
> > > > Cc: linux-arm-msm@xxxxxxxxxxxxxxx
> > >
> > > 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?

thanks,
--
js
suse labs