Commit 35627cfa authored by David Johnson's avatar David Johnson

Explain why we hardwire the serial port to 0x3f8 in /boot/grub/grub.conf

on images that use grub as bootloader.  No way to fix GRUB itself to
handle this little problem without spending a bunch more time.
parent 6353beca
This note applies to images using GRUB as the bootloader (currently, FC6-STD).
In /boot/grub/grub.conf, we have hardwired the serial port address for COM1 to
0x3f8. This is valid for most machines (probably all modern ones!). The
reason it's set this way, instead of GRUB's "--unit=0" way is because some
BIOSes whack the BIOS data area containing the COM port addresses if console
redirection is enabled. For instance, the Emulab pc850 BIOS writes 0x2f8 into
the the BDA for the COM1 address, and 0x0 for COM2). This is an issue since
GRUB uses the "unit" number (i.e., /dev/ttySX) to index into the COM address
section of the BDA, and thus obtain the correct port address. Unfortunately,
there doesn't seem to be a way to help GRUB get the correct address out of the
BIOS (reinitializing the COM unit specified in grub.conf|menu.lst via BIOS int
does not work, and it seems unlikely that anything else would -- the only way
out is to do something more along the lines of serial device drivers; i.e.,
the Linux ones.
If you don't get output on serial with the FC6 image on your hardware, try
removing the "--port=0x3f8" argument from the "serial" option line in
/boot/grub/grub.conf.
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