linux-kernel - Re: [PATCH v3 1/3] acpi: HMAT support in acpi_parse_entries

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Date:   Thu, 14 Dec 2017 17:10:17 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Anaczkowski, Lukasz" <lukasz.anaczkowski@...el.com>,
        "Box, David E" <david.e.box@...el.com>,
        "Kogut, Jaroslaw" <Jaroslaw.Kogut@...el.com>,
        "Koss, Marcin" <marcin.koss@...el.com>,
        "Koziej, Artur" <artur.koziej@...el.com>,
        "Lahtinen, Joonas" <joonas.lahtinen@...el.com>,
        "Moore, Robert" <robert.moore@...el.com>,
        "Nachimuthu, Murugasamy" <murugasamy.nachimuthu@...el.com>,
        "Odzioba, Lukasz" <lukasz.odzioba@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Schmauss, Erik" <erik.schmauss@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>,
        "Zheng, Lv" <lv.zheng@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Balbir Singh <bsingharora@...il.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Len Brown <lenb@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, devel@...ica.org,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH v3 1/3] acpi: HMAT support in acpi_parse_entries_array()

On Wed, Dec 13, 2017 at 6:10 PM, Ross Zwisler
<ross.zwisler@...ux.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@...ux.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?