Why does Mac mini forget primary display on reboot

[thebart]

I have an M1 mini with a main monitor on USB C and a TV connected to HDMI as extended display. But every time I reboot it switches to the TV and sends no signal to the monitor. I always think uh oh why isn't it booting up then remember about this stupid bug. I have to pull the HDMI plug and then it decides I have a monitor after all

It's been this way through 3 OS versions. I filed a bug report which evidently went straight to /dev/null

 

[Bigwaff]

But every time I reboot it switches to the TV and sends no signal to the monitor.

What brand and model TV? Is your TV powered off or in stand-by mode? What brand and model display? Is there an adapter in the USB-C chain?

 

[thebart]

What brand and model TV? Is your TV powered off or in stand-by mode? What brand and model display? Is there an adapter in the USB-C chain?

An LG OLED. It's turned off, but I suppose with modern electronics, it's on stand by of some sort to avoid full boot of the OS. CEC is disabled because that stuff is wonky.

The monitor is Dell U2723QE. Connection is not chained, just a straight USB C to USB C cable

 

[Honza1]

It does not forget the main monitor, it does not start/see the USB-C monitor and therefore switches to the only monitor it sees. So the question is why the USB-C monitor does not start. I have something similar with usb-c hard drive which is not started/revived on boot and needs to be power cycled to be found. Annoying!
Did you try different USB-C cable? There are so many different variations on USB-C cable that it may be simply the cable. Keep in mind that when you plug in USB-C cable, it needs to handshake between device and its (cable) one end chip and the other device and the other cable end chip what each can do and then eventually all need to agree what all can do. No surprise that there would be situations which may not work perfectly, considering how many different manufacturers make the devices/cables today.

 

[PaulD-UK]

"...it does not start/see the USB-C monitor and therefore switches to the only monitor it sees. So the question is why the USB-C monitor does not start."

I've had the same problem with a Dell monitor (and an M1 mini).
I think the problem is that when the M1 mini boots up it takes a while before it sends a signal to interrogate the external display(s), and the Dell has gone into some complex self-check/input interrogation mode and isn't ready for the wake up signal and then has decides no signal is incoming, and does something else...

So the Dell doesn't see the Mac's signal, but hasn't yet returned to standby mode, so it isn't ready to wake up.
By the time the Dell is ready the Mac has already stopped sending the wake-up command...

With simple monitors that don't have a complex self-check software start-up routine, everything works fine.

I sorted it by using exactly such a simple monitor - a DIY iMac 5K conversion, which works faultlessly.

 

[Bigwaff]

The monitor is Dell U2723QE. Connection is not chained, just a straight USB C to USB C cable

Might be worth the try... a USB-C > DisplayPort cable

UGREEN USB C to DisplayPort 1.4 Cable 8K@60Hz 4K@240Hz Unidirectional Thunderbolt 4/3 to DisplayPort Cord USB C to DP 32.4Gbps Compatible iPhone 16, MacBook Pro/Air, iPad, Mac Mini, Galaxy, XPS, 6.6FT

Brief Introduction UGREEN premium USB C to DisplayPort 1.4 cable connects a phone or computer with a USB Type-C or Thunderbolt 4/3 port to a monitor with DisplayPort input and there is no need for a driver, or software. High 8K Resolution : This UGREEN Type C to DP cord supports up to 8K@60H...

www.amazon.com

 

[thebart]

Might be worth the try... a USB-C > DisplayPort cable

UGREEN USB C to DisplayPort 1.4 Cable 8K@60Hz 4K@240Hz Unidirectional Thunderbolt 4/3 to DisplayPort Cord USB C to DP 32.4Gbps Compatible iPhone 16, MacBook Pro/Air, iPad, Mac Mini, Galaxy, XPS, 6.6FT

Brief Introduction UGREEN premium USB C to DisplayPort 1.4 cable connects a phone or computer with a USB Type-C or Thunderbolt 4/3 port to a monitor with DisplayPort input and there is no need for a driver, or software. High 8K Resolution : This UGREEN Type C to DP cord supports up to 8K@60H...

www.amazon.com

Thanks. But won't I lose functionality of using the monitor as a hub?