DRMKMS problem on i386 i915 chipset

[Mike Pumford <mpumford%black-star.demon.co.uk@localhost>]

Having a problem getting a 7-BETA KMS kernel to work on my now quite old 
Sony VAIO laptop.

At the point where the console should switch to graphics mode I get a 
load of random colour bars on the screen. The system is still running at 
that point so by typing blind I've been able to gather some debug 
information.

agp0 at pchb0: i915-family chipset
agp0: detected 7932k stolen memory
agp0: aperture at 0xc0000000, size 0x10000000
i915drmkms0 at pci0 dev 2 function 0: vendor 0x8086 product 0x27a2 (rev. 
0x03)
drm: Memory usable by graphics device = 256M
drm: Supports vblank timestamp caching Rev 2 (21.10.2013).
drm: Driver supports precise vblank timestamp query.
drm: failed to find VBIOS tables
i915drmkms0: interrupting at ioapic0 pin 16 (i915)
drm: GMBUS [i915 gmbus panel] timed out, falling back to bit banging on 
pin 3
drm: initialized overlay support
intelfb0 at i915drmkms0
i915drmkms0: info: registered panic notifier
intelfb0: framebuffer at 0xdb070000, size 640x480, depth 32, stride 2560
DRM error in intel_pipe_config_compare: mismatch in 
adjusted_mode.flags(DRM_MODE_FLAG_PHSYNC) (expected 0, found 1)
warning: 
/work/netbsd/7-stable/src/sys/external/bsd/drm2/dist/drm/i915/intel_display.c:9891: 
pipe state doesn't match!

Making an educated guess I thought that the lack of VBIOS data might be 
triggering the failure to detect the panel so I stuck some debug prints 
in pci_map_rom_md. The code gets as far the call to bus_space_map but 
that call returns an error. I have verified that there appears to be a 
valid VBIOS at 0xc0000 using dd on /dev/mem to dump the first 1MB of 
memory. I can see the string that the VBIOS parser looks for so if the 
bus_space_map() call succeeded I'd probably have a working system.

I guess this code needs to map the memory the same way that /dev/mem 
does to have the BIOS accesses in the DRM code succeed?

I'll keep on digging but any help anyone can offer would be much 
appreciated.

Mike