Re: [Qemu-devel] [RFC PATCH 3/3] mc146818rtc: export the timezone information

[Paolo Bonzini]

Il 28/02/2013 03:12, H. Peter Anvin ha scritto:
> From: "H. Peter Anvin" <address@hidden>
> 
> There is no standard method for storing timezone information
> associated with the classic PC/AT RTC, however, there are standard
> methods in ACPI (Time and Alarm Device) and EFI (GetTime/SetTime) for
> getting this information.
> 
> Since these are abstract methods, it is qreally firmware-specific how
> it is stored, however, since Qemu initializes the RTC in the virtual
> environment that information needs to come from Qemu in the first
> place.
> 
> Non-PC platforms that use the MC146181 RTC may have their own
> firmware-specific methods as well.
> 
> The most logical place to stash this information is in the RTC CMOS;
> not only is it logically co-located with the relevant information, but
> it is also very easy to access from ACPI bytecode.  Thus, save the
> timezone information in two bytes in CMOS that have no known standard
> definition, but are yet within the 64 bytes that even the most basic
> RTC CMOS implementations including the original MC146181 support.
> 
> Note: all timezones currently in use in the world are on 15-minutes
> boundaries, which would allow this information to be stored in a
> single signed byte.  However, both EFI and ACPI use a minute-granular
> interface (specified as -1440 to +1440 with 2047 used to mean
> "unknown", this requires a minimum of 12 bits to represent); this
> follows that model.

Interesting, do you have SeaBIOS and/or OVMF patches for this?

Paolo

> Signed-off-by: H. Peter Anvin <address@hidden>
> Cc: "Kevin O'Connor" <address@hidden>
> Cc: David Woodhouse <address@hidden>
> ---
>  hw/mc146818rtc.c      | 6 ++++++
>  hw/mc146818rtc_regs.h | 2 ++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/hw/mc146818rtc.c b/hw/mc146818rtc.c
> index 2fb11f6..72541dd 100644
> --- a/hw/mc146818rtc.c
> +++ b/hw/mc146818rtc.c
> @@ -681,6 +681,7 @@ static void rtc_set_date_from_host(ISADevice *dev)
>  {
>      RTCState *s = DO_UPCAST(RTCState, dev, dev);
>      struct tm tm;
> +    int minuteseast;
>  
>      qemu_get_timedate(&tm, 0);
>  
> @@ -690,6 +691,11 @@ static void rtc_set_date_from_host(ISADevice *dev)
>  
>      /* set the CMOS date */
>      rtc_set_cmos(s, &tm);
> +
> +    /* Set the timezone information as a signed 16-bit number of minutes */
> +    minuteseast = ((int64_t)s->base_rtc - (int64_t)mktime(&tm)) / 60;
> +    s->cmos_data[RTC_TIMEZONE_L] = (uint8_t)(minuteseast);
> +    s->cmos_data[RTC_TIMEZONE_H] = (uint8_t)(minuteseast >> 8);
>  }
>  
>  static int rtc_post_load(void *opaque, int version_id)
> diff --git a/hw/mc146818rtc_regs.h b/hw/mc146818rtc_regs.h
> index ccdee42..7dd5e0d 100644
> --- a/hw/mc146818rtc_regs.h
> +++ b/hw/mc146818rtc_regs.h
> @@ -47,6 +47,8 @@
>  /* PC cmos mappings */
>  #define RTC_CENTURY              0x32
>  #define RTC_IBM_PS2_CENTURY_BYTE 0x37
> +#define RTC_TIMEZONE_L           0x3e
> +#define RTC_TIMEZONE_H           0x3f
>  
>  #define REG_A_UIP 0x80
>  
>