Fedora 16 black screen with Radeon HD 6470M: partially solved?

[M. Fioretti]

Greetings,

SHORT VERSION: I think I've found a temporary solution to this Fedora
install problem (found on other distros too), but I am not sure about
_why_ it works, and it's the best solution. Any feedback is very
appreciated.

I've got a HP G6 1273SL laptop with this chipset and a live cd of
Fedora 16 x86_64, KDE spin. 

When running live from the CD there were NO graphic problems.  So I
went and installed it on the laptop hard disk. The install was
successful, or so anaconda said, so I rebooted.

I got the GRUB 2 menu just fine. However, when I select or let start
the default entry, the screen goes completely black. If I remove the
"quiet" option in the grub entry I see that the last thing before it
goes black is "radeon something..." (too fast to read it...)

So it seems to me that is the same issue mentioned here:

http://ask.fedoraproject.org/question/179/black-screen-while-startingwhen-first-half-of

I have done a bit of research and found this:

http://askubuntu.com/questions/28848/what-does-the-kernel-boot-parameter-set-acpi-osi-linux-do

linked from

https://answers.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+question/172376

which also mentions the same problem on Fedora, and indeed, if I
enter "e" in grub and add to its initrd line the two parameters

acpi_osi=Linux acpi_backlight=vendor

Fedora boots without problems, but the laptop is quite warm.

This other page says even the first parameter alone is OK:

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/765438/comments/2

Now the questions:

1) first of all, I admit I don't really understand how and why this
   works, that is if this is a real/ complete / best solution. Should
   I add those two parameters to GRUB_CMDLINE_LINUX_DEFAULT, rerun

     	 grub2-mkconfig -o /boot/grub2/grub.cfg

   and go on? I believe i should also try some combination of
   vgaswitcheroo and/or radeon.modeset=0 as mentioned in other "
   Radeon HD 6470M and black linux screen" threads like

   http://ubuntuforums.org/showthread.php?p=11271523
   http://ubuntuforums.org/showthread.php?t=1781662

   that, as far as I undertsand, should be better since they should
   completely turn off that chip and make the laptop cooler but... how
   to implement them in Fedora? This not clear to me.

2) why there was no problem with the live cd???  What are the options
   used there that make the distro work from the cd but not from the
   hard disk?

Please note that, for me, at least for the short/medium term, it is
perfectly OK for me to use only the on-board graphich chipset instead
of the radeon one, I don't need to do heavy 3D stuff with this
laptop...

	Thanks, hope this helps.

	Marco