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

From: Lucas Tanure

Date: Wed Aug 19 2026 - 03:31:16 EST

On 19/08/2026 07:49, Greg Kroah-Hartman wrote:
On Wed, Aug 19, 2026 at 07:40:17AM +0100, Lucas Tanure wrote:
On 19/08/2026 06:57, Greg Kroah-Hartman wrote:
On Tue, Aug 18, 2026 at 07:52:00PM +0100, 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:

Then fix that bug, don't break everyone else's systems because of it.

Hi,

Understood. I thought preventing 8250 from probing on devices that don't use
it would be a good idea, and fix my bug. Thanks for the feedback.

I will tak to Amlogic list the ttyS name collision introduced for the new
Amlogic SoCs in e71aab9d6132.

That was an explicit decision by Amlogic to do that, so there should not
be any collisions as their systems should NOT be having the older 8250
device tree entries.

So perhaps just fix your device tree to not be broken?

thanks,

greg k-h
Hi,

There are no 8250 nodes in the Vim4 device tree.
At __serial8250_isa_init_ports() in 8250_platform.c it creates nr_uarts ports unconditionally:

for (i = 0; i < nr_uarts; i++)
serial8250_setup_port(i);

And serial8250_init calls serial8250_isa_init_ports unconditionally. So by just probing the module the serial ports are created.

Am I missing something? I can also boot with 8250.nr_uarts=0 to make the issue go away, but I would prefer a code fix for the colision.

I will take the naming question to the Amlogic list, but I wanted to double check my understand of the code.

Thanks,
Lucas