[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 <ramesh_bios at yahoo.com> 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.

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Best regards

Peter K