Hello!
i`m sysadmin for quite some time and while being that, i have come across the
one or another system, which refused to boot a linux kernel. typical symptom i
have seen is a blinking cursor in the upper left just after kernel/initrd were
loaded.
i never spent much time on that and either choose another system for linux or
choose "failsafe" option, if the installer of my favourite distro gave that
option.
since a colleague of mine was hit by that problem some weeks ago and i also
came across that again recently, i started to investigate deeper and found,
that EDD Bios probe is the problem here.
i found more than a handful of references on the net where people report
similar problems. many discussion threads contained that typical smattering
babble and seldom somebody gave that essential hint "try edd=off" for which i`m
sure it would have helped many times.
that`s why i started to spend some thoughts on this how to make this
easier/better for the user.
so
- it seems there are buggy Bios implementations out there which have problems
with EDD
- your favourite distro may have set CONFIG_EDD=y|m , so EDD probe is on by
default quite often nowadays.
- setting "edd=off" when you get that hang on boot is _not_ obvious.
- adressing this issue may be a little bit late, since i have mostly seen that
problem on older machines, but not on recently bought
- i have at least two different systems with different types of chipsets to
demonstrate this
on one of those, i added some printf`s to edd.c and this routine seems to be
problematic and never returns
/* Extended Get Device Parameters */
ei->params.length = sizeof(ei->params);
ax = 0x4800;
dx = devno;
asm("pushfl; int $0x13; popfl"
: "+a" (ax), "+d" (dx), "=m" (ei->params)
: "S" (&ei->params)
: "ebx", "ecx", "edi");
i had a short conversation with matt domsch and hpa, who both think that
additional printf`s would be an easy solution and not too bad to be added.
here is a quick and dirty initial patch from me:
--- linux-2.6.23/arch/x86/boot/main.c.orig 2007-12-09 11:40:24.315346712
+0100
+++ linux-2.6.23/arch/x86/boot/main.c 2007-12-09 16:11:43.644512504 +0100
@@ -152,7 +152,10 @@
/* Query EDD information */
#if defined(CONFIG_EDD) || defined(CONFIG_EDD_MODULE)
- query_edd();
+ printf("Probing EDD (query Bios for boot-device information)\n");
+ printf("If boot hangs here, you may have a buggy Bios. Try edd=skipmbr
or edd=off");
+ query_edd();
+ printf("\rOK
\n");
#endif
/* Do the last things and invoke protected mode */
go_to_protected_mode();
ok, fore sure it`s better to do that stuff in query_edd() itself, but before
making a better version, i`d like to discuss if such patch would get accepted
at all and if it`s a valid approach to let the kernel print some line which
get`s overwritten ("\rOK + lot`s of whitespaces") milliseconds later on
successful function return.
regards
roland
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