Paolo, The RTC driver runs and produces the runtime services before the ACPI tables are available so what you suggest is not always possible.
Mike > -----Original Message----- > From: edk2-devel [mailto:edk2-devel-boun...@lists.01.org] On Behalf Of Paolo > Bonzini > Sent: Wednesday, December 9, 2015 3:50 AM > To: Ni, Ruiyu <ruiyu...@intel.com>; Scott Duplichan <sc...@notabs.org> > Cc: 'edk2-devel@lists.01.org' <edk2-de...@ml01.01.org>; Zeng, Star > <star.z...@intel.com> > Subject: Re: [edk2] [Patch] PcAtChipsetPkg/Rtc: Fix a UEFI Win7 boot hang > issue > > > > On 09/12/2015 12:16, Ni, Ruiyu wrote: > > Scott, I debugged the issue further and had the below findings: > > According to the ACPI spec 6.0 5.2.9 Fixed ACPI Description Table > > (FADT), the FADT.Century can be set to 0 indicating the RTC doesn't > > support to store century value. But the Win7 boot loader > > hal!HalpInitializeCmos() will firstly read and save the FADT.Century > > to a 4-byte location pointed by hal!_HalpCmosCenturyOffset. When the > > FADT.Century is 0, it will save 0x32 (default value) to that > > location. Later hal!HalpReadCmosTime() skips to read the century > > value from CMOS when BIT 7 of the value poited by > > hal!_HalpCmosCenturyOffset is set. So in order to tell Windows that > > RTC doesn't support to store century, we need to set the FADT.Century > > to 0x80 other than 0. In summary, if FADT.Century is 0, Win7 boot > > loader reads century from 0x32; if FADT.Century & BIT7 != 0, it > > doesn't read century from CMOS; otherwise, it reads century from CMOS > > index FADT.Century. > > > > But that caused another issue. Linux code strictly follows the ACPI > > spec which reads the century value from FADT.Century if it doesn't > > equal to zero. If we leave 0x80 in FADT.Century, Linux kernel will > > reads century from 0x80 location (actually from location 0 because > > CMOS address is 7-bit). Location 0 stores the seconds of the time > > which means Linux will read random century value. > > > > So do you agree if a platform needs to support booting both Windows > > and Linux, it had better to set FADT.Century to 0x32 and save correct > > century value (0x20) to CMOS address 0x32? > > That's fair enough, but you should not use RTC_ADDRESS_CENTURY > unconditionally in PcRtcSetTime. Instead you should read the FADT > yourself and use the FADT.Century value if it is non-zero. If it is > zero, I suppose writing the century to 0x32 is the only thing you can do. > > Paolo > _______________________________________________ > edk2-devel mailing list > edk2-devel@lists.01.org > https://lists.01.org/mailman/listinfo/edk2-devel _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel
