Re: [PATCH] serial: 8250: don't register ports that have no hardware

[Lucas Tanure <tanure@linux.com>]

On 19/08/2026 05:42, Jiri Slaby wrote:
> On 18. 08. 26, 20:52, Lucas Tanure wrote:
>> The 8250 driver creates a fixed set of port slots at boot and
>> registers a ttyS device for each one, even when the slot describes no
>> hardware. On device tree platforms those slots are empty, but they
>> still take the ttyS0..ttyS3 names.
>>
>> New Amlogic SoCs (S4, T7) name their real UARTs ttyS as well. With
>> both drivers built in, as in arm64 defconfig, the real port fails to 
>> bind:
>>
>>    sysfs: cannot create duplicate filename '/class/tty/ttyS0'
>>    meson_uart fe078000.serial: Cannot register tty device on line 0
>>
>> Skip slots that have no I/O and no memory address. Legacy x86 slots
>> come with resources and are not affected.
>>
>> Signed-off-by: Lucas Tanure <tanure@linux.com>
>> Assisted-by: Claude:claude-fable-5
>> ---
>>   drivers/tty/serial/8250/8250_core.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/ 
>> serial/8250/8250_core.c
>> index f49862d90eeb..e66e0cdbb210 100644
>> --- a/drivers/tty/serial/8250/8250_core.c
>> +++ b/drivers/tty/serial/8250/8250_core.c
>> @@ -378,6 +378,14 @@ void __init serial8250_register_ports(struct 
>> uart_driver *drv, struct device *de
>>           if (up->port.dev)
>>               continue;
>> +        /*
>> +         * Skip slots that describe no hardware: they would only
>> +         * take up ttyS<n> names. Real ports get their node when
>> +         * they claim a slot in serial8250_register_8250_port().
>> +         */
>> +        if (!up->port.iobase && !up->port.mapbase && !up->port.membase)
>> +            continue;
> 
Hi,
I am working on upstreaming the Khadas VIM4 (Amlogic T7). On that board
the serial console does not come up at all: meson_uart fails to register
because the 8250 stub ports already own the ttyS names.
> So you can no longer set up those ports via TIOCSSERIAL?
> 
> And what about already registered consoles?
> 
> Why does meson register ttyS* in the first place?
Because of commit e71aab9d6132 ("tty: serial: meson: apply ttyS devname
instead of ttyAML for new SoCs"). meson_uart carries two uart_drivers,
ttyAML and ttyS, and A1, S4 and T7 are wired to the ttyS one.
> 
> F*ck AI proposed changes.
> 
> thanks,

I will talk to Amlogic list to see if there is other solution.

thanks,
lucas