Re: [PATCH 1/2] serial: 8250: Add RX trigger levels for U6_16550A

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Mon, 9 Nov 2026, Kryštof Černý via B4 Relay wrote:

> From: Kryštof Černý <cleverline1mc@gmail.com>
> 
> PORT_U6_16550A has a 64-byte FIFO but no rxtrig_bytes table, so the
> rx_trig_bytes sysfs attribute is not registered for ports of this type
> and the RX trigger level cannot be read or changed from userspace.
> 
> Add a table with 1, 16, 32 and 62 bytes, i.e. 1 character, FIFO 1/4
> full, FIFO 1/2 full and FIFO 2 less than full. These are the levels the
> Synopsys DesignWare UART uses for a 64-byte FIFO, and they match the
> existing PORT_ALTR_16550_F64 table.
> 
> The default FCR value of the type is unchanged, so the trigger level a
> port starts with stays at 1/2 full. The only visible change for ports
> already detected as U6_16550A by autoconfig is the new sysfs attribute.
> 
> This prepares for 8250_dw selecting PORT_U6_16550A for DesignWare UARTs
> with a 64-byte FIFO.
> 
> Co-developed-by: Miroslav Ondra <ondra@faster.cz>
> Signed-off-by: Miroslav Ondra <ondra@faster.cz>
> Signed-off-by: Kryštof Černý <cleverline1mc@gmail.com>
> ---
>  drivers/tty/serial/8250/8250_port.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 36b2b0a2c05c..a7acede62f40 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -163,6 +163,7 @@ static const struct serial8250_config uart_config[] = {
>  		.fifo_size	= 64,
>  		.tx_loadsz	= 64,
>  		.fcr		= UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10,
> +		.rxtrig_bytes	= {1, 16, 32, 62},

Reusing U6 entry for DW UARTs seems misleading.

We've better mechanism coming up for defining rxtrig in a one pending 
series so this constant friction with the uart config types can be put 
into rest.

Please build on top of that other series and help in testing and reviewing 
it.


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 i.