Re: Using serial port on RPi

[Michael van Elst <mlelstv%serpens.de@localhost>]

On Sun, Jul 03, 2022 at 02:28:02PM +0000, John Klos wrote:
> > > I'd like to use the primary serial port on a Raspberry Pi 3. However, I
> > > haven't quite figured out how to tell the kernel to not use the serial
> > > port to mirror the graphics console.
> > 
> > Use a kernel without option WSDISPLAY_MULTICONS.
> 
> That's great!
> 
> Interestingly, I still get a segment of the kernel's boot messages on the
> serial port:

Yes. MULTICONS stops the mirroring, but the initial console is still
the serial port as defined in the DTB.

I should try what happens when the stdout-path isn't pointing to
the uart.



> [   1.0000030] com0 at simplebus1: BCM AUX UART, 1-byte FIFO
> [   1.0000030] com0: console
...
> [   1.0000030] genfb0 at simplebus1: switching to framebuffer console

Here it changes when interpreting the cmdline.txt option. Before it
couldn't use the framebuffer, so it's either serial port or nothing
(or a driver you write).

N.B. It should be possible to keep the bits as GPIO and only configure
them as UART when the kernel already switched the console.


> It points out another issue: removing WSDISPLAY_MULTICONS means the system
> can be halted without the inevitable USB panic about half the time. With
> WSDISPLAY_MULTICONS, every halt leads to a panic if any USB device is
> plugged in:

Hmm... 


> [  46.6207747] fp ffffc0007f053dc0 smscphy_service() at ffffc0000038b884 netbsd:smscphy_service+0xe4
> [  46.6407795] fp ffffc0007f053e00 mii_tick() at ffffc0000037cb14 netbsd:mii_tick+0x54

Not sure how that is related to the console. But apparently the MII
ticker hasn't been stopped correctly. Do you use a NFS root ?



Greetings,
-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."