Re: [PATCH v4 3/3] mailbox: ACPI: erroneous error message when parsing the ACPI, PCCT

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, May 17, 2018 at 12:01 AM, Al Stone <ahs3@redhat.com> wrote:
> There have been multiple reports of the following error message:
>
> [    0.068293] Error parsing PCC subspaces from PCCT
>
> This error message is not correct.  In multiple cases examined, the PCCT
> (Platform Communications Channel Table) concerned is actually properly
> constructed; the problem is that acpi_pcc_probe() which reads the PCCT
> is making the assumption that the only valid PCCT is one that contains
> subtables of one of two types: ACPI_PCCT_TYPE_HW_REDUCED_SUBSPACE or
> ACPI_PCCT_TYPE_HW_REDUCED_TYPE2.  The number of subtables of these
> types are counted and as long as there is at least one of the desired
> types, the acpi_pcc_probe() succeeds.  When no subtables of these types
> are found, regardless of whether or not any other subtable types are
> present, the error mentioned above is reported.
>
> In the cases reported to me personally, the PCCT contains exactly one
> subtable of type ACPI_PCCT_TYPE_GENERIC_SUBSPACE.  The function
> acpi_pcc_probe() does not count it as a valid subtable, so believes
> there to be no valid subtables, and hence outputs the error message.
>
> An example of the PCCT being reported as erroneous yet perfectly fine
> is the following:
>
>                     Signature : "PCCT"
>                  Table Length : 0000006E
>                      Revision : 05
>                      Checksum : A9
>                        Oem ID : "XXXXXX"
>                  Oem Table ID : "XXXXX   "
>                  Oem Revision : 00002280
>               Asl Compiler ID : "XXXX"
>         Asl Compiler Revision : 00000002
>
>         Flags (decoded below) : 00000001
>                      Platform : 1
>                      Reserved : 0000000000000000
>
>                 Subtable Type : 00 [Generic Communications Subspace]
>                        Length : 3E
>
>                      Reserved : 000000000000
>                  Base Address : 00000000DCE43018
>                Address Length : 0000000000001000
>
>             Doorbell Register : [Generic Address Structure]
>                      Space ID : 01 [SystemIO]
>                     Bit Width : 08
>                    Bit Offset : 00
>          Encoded Access Width : 01 [Byte Access:8]
>                       Address : 0000000000001842
>
>                 Preserve Mask : 00000000000000FD
>                    Write Mask : 0000000000000002
>               Command Latency : 00001388
>           Maximum Access Rate : 00000000
>       Minimum Turnaround Time : 0000
>
> To fix this, we count up all of the possible subtable types for the
> PCCT, and only report an error when there are none (which could mean
> either no subtables, or no valid subtables), or there are too many.
> We also change the logic so that if there is a valid subtable, we
> do try to initialize it per the PCCT subtable contents.  This is a
> change in functionality; previously, the probe would have returned
> right after the error message and would not have tried to use any
> other subtable definition.
>
> Tested on my personal laptop which showed the error previously; the
> error message no longer appears and the laptop appears to operate
> normally.

I'd like to know the Prashanth's opinion here.