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

[Marek Szyprowski]

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.


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