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

[Kryštof Černý]

> On Mon, 9 Nov 2026, Kryštof Černý via B4 Relay wrote:
>
> > From: Kryštof Černý <cleverline1mc@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > Signed-off-by: Miroslav Ondra <ondra@xxxxxxxxx>
> > Signed-off-by: Kryštof Černý <cleverline1mc@xxxxxxxxx>
> > ---
> >   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.

Thanks for the information, I started the work about a month ago
and not sure if the patch existed before. I will test the other series 
on my hardware. I like that approach more.