Please attach the dmesg output when booting in the problematic case.  The dmesg output attached to the ubunutu bug looks fine, but it's not clear whether this is the problematic case or the working case.  I'm inclined to say this is an efi stub issue.

Created attachment 116979 [details]
dmesg with system booted thru kernel's efi stub

Hi Alex.
Right now, I've attached the dmesg output you're requesting.
Regarding the bug related the the efi stub, I'd rather say no. I don't think so because one of the first thing I've tried is to boot the the system with the old kernel, the one shipped with the older version of Ubuntu (i.e. the one involved when system used to work perfectly), and the bug was still there.
In other words, I've tested the following combinations:
Old kernel (3.19.0-16) + Ubuntu 14 --> Working
Old kernel (3.19.0-16) + Ubuntu 15 --> Not working
New kernel (3.19.0-22) + Ubuntu 15 --> Not working

(In reply to Raffaele Candeliere from comment #2)
> Created attachment 116979 [details]
> dmesg with system booted thru kernel's efi stub

The kernel driver is loading fine.

(In reply to Raffaele Candeliere from comment #3)
> In other words, I've tested the following combinations:
> Old kernel (3.19.0-16) + Ubuntu 14 --> Working
> Old kernel (3.19.0-16) + Ubuntu 15 --> Not working
> New kernel (3.19.0-22) + Ubuntu 15 --> Not working

It's likely a packaging or configuration issue with the newer version of Ubuntu then.  X has no dependencies on what was used to boot the system.  Are you using any special xorg config file?  Can you start X manually or as root?

(In reply to Alex Deucher from comment #5)
> It's likely a packaging or configuration issue with the newer version of
> Ubuntu then.  X has no dependencies on what was used to boot the system. 
> Are you using any special xorg config file?  Can you start X manually or as
> root?
Mmhhh. Not sure about that, but, again I don't think so. As you can see in the Ubuntu bug report, there's no xorg.conf file. I'm simply using the default shipped with Ubuntu, that is, no config file. I simply install Ubuntu with the usual iso image and that's it. It used to work out of the box. The tweak I'm using is only about the way Ubuntu boots (either via GRUB or directly via the EFI stub). No other system wide tweak is in place.
Can I start X manually? No, I can't if I issue a startx command I get the same error.
What I haven't tried is to configure X manually, that is, writing my own xorg.conf specifying the radeon driver, but it would be a gross workaround, not a fix. But if you need it as a test, I can try that.

(In reply to Raffaele Candeliere from comment #6)
> What I haven't tried is to configure X manually, that is, writing my own
> xorg.conf specifying the radeon driver, but it would be a gross workaround,
> not a fix. But if you need it as a test, I can try that.

Can you try that?  It would help narrow down what the problem is.  Additionally does using the same ddx (xf86-video-ati) and xserver version from the previous version of ubuntu work?  Can you try that similar to how you tested the older kernel with a newer ubunutu?  Also does starting X as root work?  It could be that the configuration with the efi stub loader sets device permissions differently or something like that.

(In reply to Alex Deucher from comment #7)
> Also does starting X as root work?  It could be that the configuration with
> the efi stub loader sets device permissions differently or something like that.

Indeed, I'd try that first.

Created attachment 116990 [details]
XOrg.2.log when booting thru EFI Stub

(In reply to Michel Dänzer from comment #8)
> (In reply to Alex Deucher from comment #7)
> > Also does starting X as root work?  It could be that the configuration with
> > the efi stub loader sets device permissions differently or something like that.
> 
> Indeed, I'd try that first.

Hi Michel.
I've tried it. Same result. Doesn't work.
My sensation is that X *is* detecting the card. As you can see in the Xorg.log (right now I've attached one) X is detecting an ATI video card because it tries first to load the fglrx module. Upon failure (I have no fglrx driver installed indeed) it then tries to load the ati module and it *loads* it. But when it comes to setting all the roundabout up it fails: "no device detected"

The line

 (II) [KMS] Kernel modesetting enabled.

in the log means the radeon driver Probe hook returns TRUE, so this looks like a Xorg issue.

I've tried the latest Ubuntu update (Ubuntu 15.10, Xorg server 1.7.2, kernel 4.2.0-21).
Same problem.

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