Re: [PATCH 12/15] tty: serial: switch from circ_buf to kfifo

[Jiri Slaby]

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@xxxxxxxxxx>
> > ...
>
> 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