[LinuxBIOS] Re: RTC and linuxbios checksum versus pc checksum

[Peter Karlsson]

On Thu, 31 Mar 2005, ramesh bios wrote:
> Well, I got things to work in the end by copying the
> standard bios's CR bit settings for the W83977AF.
> Regretably though, I have gained no understanding of
> why or how it's working. Amazingly, the standard bios
> sets CR30 on the W83977AF, ie: the RTC enable/disable
> bit to disabled. So the RTC appears to increment
> properly when that bit is disabled (which makes no
> sense to me at all, ie: disable the RTC and it ticks
> properly?). When that bit is enabled, and no other
> changes, the RTC has weird incrementation behavior.
> Almost seemed like random bits in mm:ss were changing.
> Also, I never understood why bytes in 0E-7F, the user
> ram changes during normal ticking of the clock. Oh
> well, it's working now so I guess I'll just push this
> mystery to the back of my mind. If anyone happens to
> understand this stuff, I'll sleep better once I've
> understood this.
>
> Thanks.
>
> --- ramesh bios  wrote:
> > I was looking through linuxbios' freebios (v1) code
> > and see the following:
> >
> > rtc_checksum_valid(PC_CKS_RANGE_START,
> >
> > PC_CKS_RANGE_END,PC_CKS_LOC);
> > then
> > rtc_checksum_valid(LB_CKS_RANGE_START,
> >
> > LB_CKS_RANGE_END,LB_CKS_LOC);
> > then finally
> > rtc_set_checksum(PC_CKS_RANGE_START,
> >
> > PC_CKS_RANGE_END,PC_CKS_LOC);
> >
> > I looked in 2.6.11's mc146818 code and I don't see
> > it
> > writing a checksum so I'm not certain it's valid to
> > check for a checksum on boot since stuff like
> > hwclock
> > may/would have written to the cmos during normal
> > operation. Out of curiosity, how come there is a LB
> > checksum and then a PC checksum and then we write a
> > PC
> > checksum at the end. Is it for legacy compatibility,
> > I
> > didn't find any mention of it in the mailing list
> > and
> > google.
> >
> > Thanks.

The @clustermatic.org mail address is not to be used anymore.

If anyone knows anything about the above question please put this in the 
wiki-faq...

Best regards

Peter K

[Ronald G. Minnich]

On Fri, 1 Apr 2005, Peter Karlsson wrote:
> If anyone knows anything about the above question please put this in the
> wiki-faq...

I think it's another hardware bug, and I think the description as ramesh 
wrote it would be nice to have in the FAQ. One of these "set this bit, 
don't ask why" type situations.

ron

[Richard Smith]

> mystery to the back of my mind. If anyone happens to
> understand this stuff, I'll sleep better once I've
> understood this.
> I think it's another hardware bug, and I think the description as ramesh
> wrote it would be nice to have in the FAQ. One of these "set this bit,
> don't ask why" type situations.

Check for errata notes on the part.  Perhaps the polarity of the
enable/disable bit is flipped and incorrect in datasheet.