[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 at 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 at faster.cz>
>> Signed-off-by: Miroslav Ondra <ondra at faster.cz>
>> Signed-off-by: Kryštof Černý <cleverline1mc at 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.

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.