10.1 kernel device config for drm2 and kms drivers

[Ben Morrow]

Quoth =?windows-1252?Q?Jean-S=E9bastien_P=E9dron?= <dumbbell at FreeBSD.org>:
> On 11.09.2014 23:14, Ben Morrow wrote:
> > I didn't realise it was possible to load radeonkms after boot.
> > Presumably if you do that you lose even more of the boot messages when
> > the new console loads? Currently it appears to lose everything that was
> > above the top of the screen when the new driver loaded; it would be
> > better (if possible) if it were to keep the scrollback as well.
> 
> When a KMS driver is loaded, the history buffer of vt(4) grows and the
> current data is preserved. Therefore you should not lose any data.
> 
> One caveat is that the cursor is put at the top of the screen, so the
> previous content is off-screen. Another issue is that the history size
> (hard-coded) is admittedly very small. But data should be there.
> 
> Could you please describe your problem in more details?

Previously, with syscons, I could usually scroll back after boot had
finished and the login: prompt appeared and get all the way up to the
start of the kernel messages (the Copyright line). 

Now, with vt, and loading radeonkms from rc.conf rather than
loader.conf, if I switch to the console, I can: scroll up through 8
screens of kernel and userland console messages until I come to a screen
which has

    info: [drm] Initialised drm 1.1.0 20060810

in approximately the middle; then I can scroll up one more screen only,
which takes me to somewhere in the middle of probing the USB ports.
Looking at dmesg suggests there should be 3 or 4 more screens of
messages above that.

Obviously I can't be certain without trying more things that this isn't
just because the rest has gone off the top of the buffer, but ISTR when
I was first changing the machine over to vt that when I didn't load
radeonkms and stayed with vt_vga I could still scroll all the way up. I
may be misremembering, and/or making assumptions based on the fact the
end of the scrollback is about a screenful above drm loading; I can do
more experiments next time I reboot the machine if that would be
helpful.

Ben