Re: [PATCH 08/11] ACPICA: Tables: Back port acpi_get_table_with_size() and early_acpi_os_unmap_memory() from Linux kernel

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

On Fri, Dec 9, 2016 at 2:59 AM, Zheng, Lv <lv.zheng@intel.com> wrote:
> Hi, Rafael and Dan
>
>> From: Dan Williams [mailto:dan.j.williams@intel.com]
>> Subject: Re: [PATCH 08/11] ACPICA: Tables: Back port acpi_get_table_with_size() and
>> early_acpi_os_unmap_memory() from Linux kernel
>>
>> On Thu, Dec 8, 2016 at 5:18 AM, Rafael J. Wysocki <rafael@kernel.org> wrote:
>> > On Thu, Dec 8, 2016 at 2:11 AM, Dan Williams <dan.j.williams@intel.com> wrote:
>> >> On Tue, Nov 29, 2016 at 11:21 PM, Lv Zheng <lv.zheng@intel.com> wrote:
>> >>> ACPICA commit cac6790954d4d752a083e6122220b8a22febcd07
>> >>>
>> >>> This patch back ports Linux acpi_get_table_with_size() and
>> >>> early_acpi_os_unmap_memory() into ACPICA upstream to reduce divergences.
>> >>>
>> >>> The 2 APIs are used by Linux as table management APIs for long time, it
>> >>> contains a hidden logic that during the early stage, the mapped tables
>> >>> should be unmapped before the early stage ends.
>> >>>
>> >>> During the early stage, tables are handled by the following sequence:
>> >>>  acpi_get_table_with_size();
>> >>>  parse the table
>> >>>  early_acpi_os_unmap_memory();
>> >>> During the late stage, tables are handled by the following sequence:
>> >>>  acpi_get_table();
>> >>>  parse the table
>> >>> Linux uses acpi_gbl_permanent_mmap to distinguish the early stage and the
>> >>> late stage.
>> >>>
>> >>> The reasoning of introducing acpi_get_table_with_size() is: ACPICA will
>> >>> remember the early mapped pointer in acpi_get_table() and Linux isn't able to
>> >>> prevent ACPICA from using the wrong early mapped pointer during the late
>> >>> stage as there is no API provided from ACPICA to be an inverse of
>> >>> acpi_get_table() to forget the early mapped pointer.
>> >>>
>> >>> But how ACPICA can work with the early/late stage requirement? Inside of
>> >>> ACPICA, tables are ensured to be remained in "INSTALLED" state during the
>> >>> early stage, and they are carefully not transitioned to "VALIDATED" state
>> >>> until the late stage. So the same logic is in fact implemented inside of
>> >>> ACPICA in a different way. The gap is only that the feature is not provided
>> >>> to the OSPMs in an accessible external API style.
>> >>>
>> >>> It then is possible to fix the gap by providing an inverse of
>> >>> acpi_get_table() from ACPICA, so that the two Linux sequences can be
>> >>> combined:
>> >>>  acpi_get_table();
>> >>>  parse the table
>> >>>  acpi_put_table();
>> >>> In order to work easier with the current Linux code, acpi_get_table() and
>> >>> acpi_put_table() is implemented in a usage counting based style:
>> >>>  1. When the usage count of the table is increased from 0 to 1, table is
>> >>>     mapped and .Pointer is set with the mapping address (VALIDATED);
>> >>>  2. When the usage count of the table is decreased from 1 to 0, .Pointer
>> >>>     is unset and the mapping address is unmapped (INVALIDATED).
>> >>> So that we can deploy the new APIs to Linux with minimal effort by just
>> >>> invoking acpi_get_table() in acpi_get_table_with_size() and invoking
>> >>> acpi_put_table() in early_acpi_os_unmap_memory(). Lv Zheng.
>> >>>
>> >>> Link: https://github.com/acpica/acpica/commit/cac67909
>> >>> Signed-off-by: Lv Zheng <lv.zheng@intel.com>
>> >>> Signed-off-by: Bob Moore <robert.moore@intel.com>
>> >>
>> >> This commit in -next (071b39575679 ACPICA: Tables: Back port
>> >> acpi_get_table_with_size() and early_acpi_os_unmap_memory() from Linux
>> >> kernel) causes a regression in my nfit/nvdimm test environment. The
>> >> nfit produced by QEMU no longer results in a nvdimm bus being created.
>> >>
>> >> I have not root caused it, but I'm using the following command line
>> >> options to create an nfit in qemu-2.6.  Reverting the commit leads
>> >> compile failures.
>> >
>> > Would the build problems go away if you reverted "ACPICA: Tables:
>> > Allow FADT to be customized with virtual address" (linux-next commit
>> > cf334d3174f9) in addition to it?
>>
>> Yes, reverting those two commits gets me back to a functional environment:
>>
>> Revert "ACPICA: Tables: Allow FADT to be customized with virtual address"
>> Revert "ACPICA: Tables: Back port acpi_get_table_with_size() and
>> early_acpi_os_un
>
> To Dan:
> It seems in drivers/acpi/nfit/core.c.
> The returned table size is used by the NFIT code.
> I think it should be changed to use table_header->length.
>
> To Rafael:
> I can offer a quick fix for this by returning table_header->length from acpi_get_table_with_size().

OK

I've dropped the problematic ACPICA commit for now (along with the one
that depended on it) to prevent the issue from going in.

The change that you are suggesting would defeat the purpose of what
the NFIT code does.

Thanks,
Rafael