Too late.
Translation: LinuxBIOS cannot access the serial port on a PCI card until late in the boot process, after initializing and scanning the PCI bus. Only a serial port integrated into the mainboard chipset can be initialized early enough to be useful for debugging.
That's not exactly true. A serial port that has its address space subtractively decoded (at least at boot time) certainly is easier to set up; but it certainly isn't impossible to set up just enough of the PCI fan-out to reach a plugin card; esp. if you do this for debug purposes only (i.e., not for the end-user), you can hardcode all of this.
But yes, it's very unfortunate "modern" systems are so unfriendly to low-level bringup. At the very least it makes it really really hard to have some uniform ("generic") working early setup code.
Segher