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Date: Wed, 17 Apr 2024 12:50:48 +0200
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Subject: Re: [PATCH 11/15] tty: msm_serial: use dmaengine_prep_slave_sg()
To: Marek Szyprowski <m.szyprowski@samsung.com>, gregkh@linuxfoundation.org
Cc: linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
 Bjorn Andersson <andersson@kernel.org>,
 Konrad Dybcio <konrad.dybcio@linaro.org>, linux-arm-msm@vger.kernel.org,
 Anders Roxell <anders.roxell@linaro.org>
References: <20240405060826.2521-1-jirislaby@kernel.org>
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From: Jiri Slaby <jirislaby@kernel.org>
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In-Reply-To: <0335b679-da36-42c1-a1ba-8affb7a98d44@samsung.com>
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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@kernel.org>
>>>> Cc: Bjorn Andersson <andersson@kernel.org>
>>>> Cc: Konrad Dybcio <konrad.dybcio@linaro.org>
>>>> Cc: linux-arm-msm@vger.kernel.org
>>>
>>> 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