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Date: Mon, 15 Apr 2024 15:28:43 +0200
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Subject: Re: [PATCH 12/15] tty: serial: switch from circ_buf to kfifo
To: Marek Szyprowski <m.szyprowski@samsung.com>, gregkh@linuxfoundation.org,
 linux-amlogic@lists.infradead.org,
 "linux-arm-msm@vger.kernel.org" <linux-arm-msm@vger.kernel.org>
Cc: linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
 Bjorn Andersson <andersson@kernel.org>,
 Konrad Dybcio <konrad.dybcio@linaro.org>,
 Neil Armstrong <neil.armstrong@linaro.org>,
 Kevin Hilman <khilman@baylibre.com>, Jerome Brunet <jbrunet@baylibre.com>,
 Martin Blumenstingl <martin.blumenstingl@googlemail.com>
References: <20240405060826.2521-1-jirislaby@kernel.org>
 <20240405060826.2521-13-jirislaby@kernel.org>
 <CGME20240415125847eucas1p2bc180c35f40f9c490c713679871af9ae@eucas1p2.samsung.com>
 <91ac609b-0fae-4856-a2a6-636908d7ad3c@samsung.com>
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From: Jiri Slaby <jirislaby@kernel.org>
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In-Reply-To: <91ac609b-0fae-4856-a2a6-636908d7ad3c@samsung.com>
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On 15. 04. 24, 14:58, Marek Szyprowski wrote:
> Dear All,
> 
> On 05.04.2024 08:08, Jiri Slaby (SUSE) wrote:
>> Switch from struct circ_buf to proper kfifo. kfifo provides much better
>> API, esp. when wrap-around of the buffer needs to be taken into account.
>> Look at pl011_dma_tx_refill() or cpm_uart_tx_pump() changes for example.
>>
>> Kfifo API can also fill in scatter-gather DMA structures, so it easier
>> for that use case too. Look at lpuart_dma_tx() for example. Note that
>> not all drivers can be converted to that (like atmel_serial), they
>> handle DMA specially.
>>
>> Note that usb-serial uses kfifo for TX for ages.
>>
>> omap needed a bit more care as it needs to put a char into FIFO to start
>> the DMA transfer when OMAP_DMA_TX_KICK is set. In that case, we have to
>> do kfifo_dma_out_prepare twice: once to find out the tx_size (to find
>> out if it is worths to do DMA at all -- size >= 4), the second time for
>> the actual transfer.
>>
>> All traces of circ_buf are removed from serial_core.h (and its struct
>> uart_state).
>>
>> Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
>> ...
> 
> This patch landed in linux-next as commit 1788cf6a91d9 ("tty: serial:
> switch from circ_buf to kfifo"). Unfortunately it breaks UART operation
> on thr Amlogic Meson based boards (drivers/tty/serial/meson_uart.c
> driver) and Qualcomm RB5 board (drivers/tty/serial/qcom_geni_serial.c).
> Once the init process is started, a complete garbage is printed to the
> serial console. Here is an example how it looks:

Oh my!

Both drivers move the tail using both kfifo and uart_xmit_advance() 
interfaces. Bah. Does it help to remove that uart_xmit_advance() for 
both of them? (TX stats will be broken.)

Users of uart_port_tx() are not affected.

This is my fault when merging uart_xmit_advance() with this series.

thanks,
-- 
js
suse labs