Ubuntu
linux package

Bug #1377878
Comment #39

Comment 39 for bug 1377878

Revision history for this message

Adam Smith (adamsmith) wrote on 2014-11-25:

#39

This is caused by the radeonfb kernel module being now compiled as a module, rather than being 'built-in' to the kernel. It's hard to see where/when/why this change happened in Ubuntu, but it probably came from an upstream kernel change https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748398 .

A bit of history: The radeonfb module was purposefully builtin on Ubuntu PPC to break KMS (as it would cause freezing like above) and also, radeonfb is necessary for suspend. UMS was removed entirely from the radeon xorg driver in 12.10, and people have in fact been using the fbdev xorg driver (which uses radeonfb) by default. For most users, KMS works if forcing pci mode (agpmode=-1), however Lubuntu-qa had an eMac user at the time of 12.10 who couldn't use radeon KMS (black screen), and so if the switch was made to CONFIG_FB_RADEON=m then he would have had to fail the ISOs. The kernel configuration of framebuffers/KMS has hence remained the same from 12.04-14.04

It is not very helpful to mix up 14.04 and 14.10 as has been done in the confusing discussion above. The kernel configs are different and the default xorg driver will be different. Necessary boot parameters will be different.

The described freezing bug with radeon KMS under PowerPC has been present for years and has been reported many times. However, the actual cause is not known, and those who may have the knowledge to fix it don't have the hardware or time to devote to a solution. Forcing pci mode remains the only workaround to get the radeon xorg driver working on most machines.

Testing the vivid desktop/live ISO today, I couldn't use radeonfb even following previoulsy working methods for framebuffer modules (http://ubuntuforums.org/showthread.php?t=2079873&p=12336938#post12336938 ). It seems video=offb:off doesn't work anymore, although I don't know why.

Also, I note 'nomodeset' doesn't work. You have to use radeon.modeset=0 to turn off KMS.

Using fbdev with the openfirmware framebuffer (offb) is not an option because of the very limited number of colours.

Despite all this, building radeonfb as a module is probably the best thing to do nowadays, but it will make it harder for some users.

So how can we improve things? Well, changing the boot.msg on ISOs would be good! Currently it says:

"If the system fails to boot at all (the typical symptom is a white screen that doesn't go away), use 'live video=ofonly'"

Where this came from has been lost by the sands of time, but it doesn't relate to any PowerPC mac these days and hasn't been good advice since, well 10.04, and possibly before that. Now that framebuffers are modules (that are blacklisted), the boot parameter video=ofonly in fact does nothing. IT HAS TO BE CHANGED as it is just wasting people's time.

It would be far better to point users to the troubleshooting section of the PowerPC wiki pages:

wiki.ubuntu.com/PowerPCFAQ#Troubleshooting

This is caused by the radeonfb kernel module being now compiled as a module, rather than being 'built-in' to the kernel.  It's hard to see where/when/why this change happened in Ubuntu, but it probably came from an upstream kernel change https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748398 .

A bit of history: The radeonfb module was purposefully builtin on Ubuntu PPC to break KMS (as it would cause freezing like above) and also, radeonfb is necessary for suspend.  UMS was removed entirely from the radeon xorg driver in 12.10, and people have in fact been using the fbdev xorg driver (which uses radeonfb) by default.  For most users, KMS works if forcing pci mode (agpmode=-1), however Lubuntu-qa had an eMac user at the time of 12.10 who couldn't use radeon KMS (black screen), and so if the switch was made to CONFIG_FB_RADEON=m then he would have had to fail the ISOs.  The kernel configuration of framebuffers/KMS has hence remained the same from 12.04-14.04

It is not very helpful to mix up 14.04 and 14.10 as has been done in the confusing discussion above.  The kernel configs are different and the default xorg driver will be different.  Necessary boot parameters will be different.

The described freezing bug with radeon KMS under PowerPC has been present for years and has been reported many times.  However, the actual cause is not known, and those who may have the knowledge to fix it don't have the hardware or time to devote to a solution.  Forcing pci mode remains the only workaround to get the radeon xorg driver working on most machines.

Testing the vivid desktop/live ISO today, I couldn't use radeonfb even following previoulsy working methods for framebuffer modules (http://ubuntuforums.org/showthread.php?t=2079873&p=12336938#post12336938 ).  It seems video=offb:off doesn't work anymore, although I don't know why.

Also, I note 'nomodeset' doesn't work.  You have to use radeon.modeset=0 to turn off KMS.