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

[Jiri Slaby]

On 17. 04. 24, 14:45, Marek Szyprowski wrote:
> 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@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?
>
>
> 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.

Thanks. Neither for me. Could you add:
        {
                static DEFINE_RATELIMIT_STATE(rs, 
DEFAULT_RATELIMIT_INTERVAL, DEFAULT_RATELIMIT_BURST);
                if (dma->mapped != !!sg_dma_len(&dma->tx_sg) && 
__ratelimit(&rs))
                        printk_deferred(KERN_DEBUG "%s (%d): mapped=%u 
dma_len=%u\n",
                                        __func__, __LINE__, 
dma->mapped, sg_dma_len(&dma->tx_sg));
        }

before each of your 'if (dma->mapped)' to see where sg_dma_len() is 
wrong and what is its value in the bad case. I hope I did the logic right.

thanks,
--
js
suse labs