Add serial to wl_output::geometry?

Pekka Paalanen ppaalanen at
Fri Aug 18 07:47:24 UTC 2017

On Thu, 03 Aug 2017 08:27:06 -0400
Simon Ser <contact at> wrote:

> Hello,
> I'm writing a Wayland client that needs to keep track of screens
> across reboots. I registered a wl_output::geometry event handler.
> This event already has the fields "make" and "model", but is missing
> "serial". Is there a way to get an output serial (something like
> "HS3P505873")? Would it be possible to add it to the protocol or is
> it too late? Is there another way to identify screens?

Hi Simon,

these are very good questions. I have a vague recollection this has
been discussed before on wayland-devel@, but personally I may have been
skeptical about the use cases at the time. Identifying an output and
monitor serial are not exactly equivalent, so what we should have
depends on the use case.

If you actually want to identify a physical monitor, then serial (if
such is available in hardware to begin with!) would be good. If you
just wanted to identify an output e.g. based on the physical connector
or desktop layout, then serial is obviously not the solution.

Anyway, nowadays I think adding the physical monitor serial to
wl_output as a new event should be fine. There are some details to
discuss like if the monitor hardware does not provide a reliable
serial, should the compositor maybe manufacture some GUID or just leave
the serial unset. What about "monitors" that are not physical devices,
like a window in another window system - should one relay the serial
from the other window system's monitor, manufacture an artificial GUID
based on e.g. window system connection, or just not send a serial at

Maybe it would be best to just say the serial is from a locally
connected monitor hardware (or equivalent), and leave the cases where
the hardware serial is not available or the output is not a local
physical monitor to not send a serial.

What do you want to identify the physical monitors for, exactly?

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