ATI Radeon Mobility 3470 fails with Kernel 5

[Craig M]

Hi,

I'm trying to track down a problem with an ancient graphics device ATI
Radeon Mobility 3470.

Ubuntu/Kubuntu 18 is shipped kernel 4.18 and the radeon drivers work well.
With 18.02 and later it ships with kernel 5.0 and the radeon drivers don't.

What I'm seeing is hard to describe. It looks a lot like the scan
lines are interleaved and there's a lot of noise. It's not the typical
screen tearing, nor is it just snow (black and white noise). I can
*just* make out what it being displayed onscreen, but hurts the eyes a
lot to try and do so!

If I hold the kernel back at 4.18 things are just fine.

Some detailed info (from a working 4.18 kernel). Note that the
returned information from a 5.x kernel isn't all that much different:

$ lshw -c video
*-display
      description: VGA compatible controller
      product: RV620/M82 [Mobility Radeon HD 3450/3470]
      vendor: Advanced Micro Devices, Inc. [AMD/ATI]
      physical id: 0
      bus info: pci at 0000:01:00.0
      version: 00
      width: 32 bits
      clock: 33MHz
      capabilities: pm pciexpress msi vga_controller bus_master
cap_list rom       configuration: driver=radeon latency=0
resources: irq:26 memory:c0000000-c7ffffff ioport:9000(size=256)
memory:c8020000-c802ffff memory:c0000-dffff


$ modinfo radeon

filename:
/lib/modules/4.18.0-17-generic/kernel/drivers/gpu/drm/radeon/radeon.ko
license:        GPL and additional rights description:    ATI Radeon
author:         Gareth Hughes, Keith Whitwell, others.

...
name:           radeon
vermagic:       4.18.0-17-generic SMP mod_unload
signat:         PKCS#7
signer:
sig_key:
sig_hashalgo:   md4
....



I'm just trying to track down the driver changes between 4.18 and 5.0
to see what I can do to change settings. Any help  would be greatly
appreciated. I've had a quick look through
https://gitlab.freedesktop.org/xorg/driver/xf86-video-ati but I don't
know where to start (as there's no correlation between releases and
kernels, from what I can see).

Regards,
Craig.