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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 14 Dec 2017 17:10:17 -0800
Message-ID: <CAPcyv4gnZ3NJsEUugjBrsBHcs9-yqxkvs_G4V1HHGWF2JDi13g@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] acpi: HMAT support in acpi_parse_entries_array()
To: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Anaczkowski, Lukasz" <lukasz.anaczkowski@intel.com>,
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On Wed, Dec 13, 2017 at 6:10 PM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> The current implementation of acpi_parse_entries_array() assumes that each
> subtable has a standard ACPI subtable entry of type struct
> acpi_subtable_header.  This standard subtable header has a one byte length
> followed by a one byte type.
>
> The HMAT subtables have to allow for a longer length so they have subtable
> headers of type struct acpi_hmat_structure which has a 2 byte type and a 4
> byte length.

Hmm, NFIT has a 2 byte type and a 2 byte length, so its one more
permutation. I happened to reinvent sub-table parsing in the NFIT
driver, but it might be nice in the future to refactor that to use the
common parsing.

>
> Enhance the subtable parsing in acpi_parse_entries_array() so that it can
> handle these new HMAT subtables.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> ---
>  drivers/acpi/tables.c | 52 ++++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 41 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/acpi/tables.c b/drivers/acpi/tables.c
> index 80ce2a7d224b..f777b94c234a 100644
> --- a/drivers/acpi/tables.c
> +++ b/drivers/acpi/tables.c
> @@ -218,6 +218,33 @@ void acpi_table_print_madt_entry(struct acpi_subtable_header *header)
>         }
>  }
>
> +static unsigned long __init
> +acpi_get_entry_type(char *id, void *entry)
> +{
> +       if (strncmp(id, ACPI_SIG_HMAT, 4) == 0)
> +               return ((struct acpi_hmat_structure *)entry)->type;
> +       else
> +               return ((struct acpi_subtable_header *)entry)->type;
> +}

It seems inefficient to make all checks keep asking "is HMAT?".
Especially if we want to extend this to other table types should we
instead setup and pass a pair of function pointers to parse the
sub-table format?