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

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Date:   Wed, 23 May 2018 13:34:52 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     "Prakash, Prashanth" <pprakash@...eaurora.org>,
        Al Stone <ahs3@...hat.com>
Cc:     ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH v4 3/3] mailbox: ACPI: erroneous error message when parsing the ACPI, PCCT

On Thursday, May 17, 2018 9:48:39 PM CEST Prakash, Prashanth wrote:
> 
> On 5/17/2018 4:24 AM, Rafael J. Wysocki wrote:
> > On Thu, May 17, 2018 at 12:01 AM, Al Stone <ahs3@...hat.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.
> 
> Looks good.
> 
> Reviewed-by: Prashanth Prakash <pprakash@...eaurora.org>

Applied, thanks!