ACPI Control Method Batteries have a _BIF and/or _BIX object which
provide static properties of the battery. FreeBSD acpi_cmbat module
supported _BIF object only, which was deprecated as of ACPI 4.0.
_BIX is an extended version of _BIF defined in ACPI 4.0 or later.
As of writing, _BIX has two revisions. One is in ACPI 4.0 (rev.0) and
another is in ACPI 6.0 (rev.1). It seems that hardware vendors still
stick to _BIF only or _BIX rev.0 + _BIF for the maximum compatibility.
Microsoft requires _BIX rev.0 for Windows machines, so there are some
laptop machines with _BIX rev.0 only. In this case, FreeBSD does not
recognize the battery information.
After this change, the acpi_cmbat module gets battery information from
_BIX or _BIF object and internally uses _BIX rev.1 data structure as
the primary information store in the kernel. ACPIIO_BATT_GET_BI[FX]
returns an acpi_bi[fx] structure built by using information obtained
from a _BIF or a _BIX object found on the system. The revision number
field can be used to check which field is available. The acpiconf(8)
utility will show additional information if _BIX is available.
Although ABIs of ACPIIO_BATT_* were changed, the existing APIs for
userland utilities are not changed and the backward-compatible ABIs
are provided. This means that older versions of acpiconf(8) can also
work with the new kernel. The (union acpi_battery_ioctl_arg) was
padded to 256 byte long to avoid another ABI change in the future.
A _BIX object with its revision number >1 will be treated as
compatible with the rev.1 _BIX format.
Reviewed by: takawata
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23728
294de6bbd6