vt_simplefb: support armv7 and U-Boot-placed framebuffer nodes

[seuros]

Summary

Search for simple-framebuffer nodes under both /chosen and the root,
as U-Boot may place them there, and skip disabled nodes.

Honor memory-region, fix address handling on 32-bit arm, and use the
pmap_preboot API for framebuffer mappings before pmap_bootstrap().

[seuros]

sys/arm/arm/machdep.c

118

@imp , i saw that in some place we started to use booleans.

[ray]

sys/dev/vt/hw/simplefb/simplefb.c

153

I'm afraid not "/chosen" means it is not ready, in most cases. Or at least unwanted.

[mmel]

I'm not thrilled with this solution.
The standard pre-pmap_bootstrap() KVA is 128 MB, including:

• kernel text/data/bss (section-mapped from KERNBASE);
• early page tables (kern_pt1, kern_pt2tab and initial L2 pages);
• whatever initarm() takes (dpcpu, stacks, msgbuf, etc.).

So there are only tens of megabytes of free KVA for the framebuffer.

[mmel]

Another thing: Wouldn't it be much cleaner to extend the implementation of pmap_mapdev_attr() for early boot phase of armv7? subr_devmap.c looks ready for this job.

[adrian]

In D59264#1359759, @mmel wrote:

Another thing: Wouldn't it be much cleaner to extend the implementation of pmap_mapdev_attr() for early boot phase of armv7? subr_devmap.c looks ready for this job.

should we even pull in kib@ or someone else to talk about the kvm layout and pmap stuff?

-a