it now works better: with pci=nocrs the radeon card initialize and kdm+Xorg works somehow.... I've now the screen flickering problem at high resolutions(starting at 1024x768, 800x600 is fine).
if the problem can't be fixed, would buying an nvidia nv50 workarround the issue and permit me to have a resolution up to 1440x900 (that would also give me 3d acceleration(trisquel,the distribution that I use, don't include the non-free 3d microcode for ATI cards) with nouveau,if I understood well they have no non-free microcode requirement and I was told theses cards were stable enough now).
Altough if I understand well the PCI express reset gpio has to work to be able to enable graphics on the PCI express slot.
Denis.