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

[Marek Szyprowski]

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 at 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:

[    8.763154] Run /sbin/init as init process
NT [   12.429776] platform cpufreq-dt: deferred probe pending: (reason 
unknown)
[   12.434259] platform regulator-vddcpu: deferred probe pending: 
pwm-regulator: Failed to get PWM
[[6if;9]Uigmkfl-tl ocretbo nrnee .[[6if;9]Uigmkfl-tl ocretbo nrnee .
[[6if;9]Uigmkfl-tl ocretbo nrnee .
[[6if;9]Uigmkfl-tl ocretbo nrnee .
[[6if;9]Uigmkfl-tl ocretbo nrnee .
[[6if;9]Uigmkfl-tl ocretbo nrnee .
[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[2l[1[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
[.. yteiigteiiilhtlgeet dvcs..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
[.. yteiigteiiilhtlgeet dvcs..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
[.. yteiigteiiilhtlgeet dvcs..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
[.. yteiigteiiilhtlgeet dvcs..[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
[.. yteiigteiiilhtlgeet dvcs..[  105.613420] debugfs: Directory 
'ff800280.cec' with parent 'regmap' already present!
[[6if;9]Uigmkfl-tl ocretbo nrnee .
[.. trightlgeet ipthr ytm-dv5c7[Gmo [94m8[2h[0.
..]Snhszn h nta opu vns(usses..[2l[11[[2 k;9?5[  105.638809] mc: Linux 
media interface: v0.10
[.. atn o dvt eflyppltd.[  105.707390] meson-vrtc ff8000a8.rtc: 
registered as rtc0


I found this patch by bisecting today's linux-next. I've checked the 
changes in the related UART drivers and I don't see any obvious issues 
though. Let me know if I can help debugging this issue somehow.

I've trimmed recipient list due to my smtp server limitation.


Best regards

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