Message-ID: <1AE640813FDE7649BE1B193DEA596E88B9B1F6@SHSMSX101.ccr.corp.intel.com>
Date:	Fri, 28 Sep 2012 05:24:14 +0000
From:	"Zheng, Lv" <lv.zheng@...el.com>
To:	"Brown, Len" <len.brown@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: RE: [PATCH v4 0/2] ACPI: DBGP/DBG2 early console support for LPIA.

Forgot to Cc x86 maintainers, will send again. Sorry for the noise.

> -----Original Message-----
> From: Zheng, Lv
> Sent: Friday, September 28, 2012 10:40 AM
> To: Brown, Len
> Cc: linux-kernel@...r.kernel.org; linux-acpi@...r.kernel.org; Zheng, Lv
> Subject: [PATCH v4 0/2] ACPI: DBGP/DBG2 early console support for LPIA.
> 
> Microsoft Debug Port Table (DBGP or DBG2) is used by the Windows SoC
> platforms to describe their debugging facilities.
> Recent Low Power Intel Architecture (LPIA) platforms have utilized
> this for the SPI UART debug ports that are resident on their debug
> boards.
> 
> This patch set enables the DBGP/DBG2 debug ports as an Linux early
> console launcher.
> The SPI UART debug ports support is also refined to co-exist with this
> new usage model.
> 
> To use this facility on LPIA platforms, you need to enable the following
> kernel configurations:
>   CONFIG_EARLY_PRINTK_ACPI=y
>   CONFIG_EARLY_PRINTK_INTEL_MID_SPI=y
> Then you need to append the following kernel parameter to the kernel
> command line in your the boot loader configuration file:
>   earlyprintk=acpi
> 
> There is a dilemma in designing this patch set.  There should be three
> steps to enable an early console for an operating system:
> 1. Probe: In this stage, the Linux kernel can detect the early consoles
>           and the base address of their register block can be determined.
>           This can be done by parsing the descriptors in the ACPI
> DBGP/DBG2
>           tables.  Note that acpi_table_init() must be called before
>           parsing.
> 2. Setup: In this stage, the Linux kernel can apply user specified
>           configuration options (ex. baudrate of serial ports) for the
>           early consoles.  This is done by parsing the early parameters
>           passed to the kernel from the boot loaders.  Note that
>           parse_early_params() is called very early to allow parameters to
>           be passed to other kernel subsystems.
> 3. Start: In this stage, the Linux kernel can make the console available
>           to output messages.  Since early consoles are always used for
>           kernel boot up debugging, this must be done as early as possible
>           to arm the kernel with more testability the kernel subsystems.
>           Note that, this stage happens when the register_console() is
>           called.
> The preferred sequence for the above steps is:
>    +-----------------+    +-------------------+    +--------------------+
>    | ACPI DBGP PROBE | -> | EARLY_PARAM SETUP | -> | EARLY_RPINTK
> START |
>    +-----------------+    +-------------------+    +--------------------+
> But unfortunately, in the current x86 implementation, early parameters and
> early printk initialization are called before acpi_table_init() which
> requires early memory mapping facility.
> There are some choices for me to design this patch set:
> 1. Invoking acpi_table_init() before parse_early_param() to maintain the
>    sequence:
>    +-----------------+    +-------------------+    +--------------------+
>    | ACPI DBGP PROBE | -> | EARLY_PARAM SETUP | -> | EARLY_RPINTK
> START |
>    +-----------------+    +-------------------+    +--------------------+
>    This requires other subsystem maintainers' review to ensure no
>    regressions will be introduced.  As far as I know, one kind of issue
>    might be found in EFI subsystsm:
>    The EFI boot services and runtime services are mixed up in the x86
>    specific initialization process before the ACPI table initialization.
>    Things are much worse that you even cannot disable the runtime services
>    while still allow the boot services codes to be executed in the kernel
>    compilation stage.  Enabling the early consoles after the ACPI table
>    initialization will make it difficult to debug the runtime BIOS bugs.
>    If any progress is made to the kernel boot sequences, please let me
>    know.  I'll be willing to redesign the ACPI DBGP/DBG2 console probing
>    facility.  You can reach me at <zetalog@...il.com>.
> 2. Modifying above sequece to make it look like:
>    +-------------------+    +-----------------+    +--------------------+
>    | EARLY_PARAM SETUP | -> | ACPI DBGP PROBE | -> | EARLY_RPINTK
> START |
>    +-------------------+    +-----------------+    +--------------------+
>    Early consoles started in this style will lose some debuggabilities in
>    the kernel boot up.  If the system does not crash very early,
>    developers still can see the bufferred kernel outputs when the
>    register_console() is called.
>    Current early console implementation need to be modified to split their
>    initialization codes into tow part:
>    1. Detecting hardware.  This can be called in the PROBE stage.
>    2. Applying user parameters.  This can be called in the SETUP stage.
>    Individual early console drver maintainers need to be involved to avoid
>    regressions that might occur on this modification as the maintainers
>    might offer the real tests rather than I can do.
> 3. Introducing a barely new debugging facility that does not relate to the
>    current early console implementation to allow automatic detection for
>    the early consoles.
>    +-------------------+    +--------------------+
>    | EARLY_PARAM SETUP | -> | EARLY_RPINTK START |
>    +-------------------+    +--------------------+
>    +-----------------+    +--------------------+
>    | ACPI DBGP PROBE | -> | EARLY_RPINTK START |
>    +-----------------+    +--------------------+
>    This is what the patch set has done to enable this new usage model.
>    It is known as "ACPI early console launcher mode".
>    Early consoles started in this style will lose some debuggabilities in
>    the kernel boot up.  If the system does not crash very early,
>    developers still can see the bufferred kernel outputs when the
>    register_console() is called.
>    Note that the user configuration can not be applied to the registered
>    early consoles in this way as the acpi_table_init() is still called
>    after the parse_early_param().  Instead, the early consoles should
>    derive the hardware settings used in the BIOS/bootloaders.
>    As a launcher, ACPI DBGP will not actually output kernel messages
>    without the real early console drivers, that's why the
>    CONFIG_EARLY_PRINTK_INTEL_MID_SPI is still need to be enabled along
>    along with the CONFIG_EARLY_PRINTK_ACPI.
>    In order to disable this facility by default and enable it at runtime,
>    an kernel parameter "earlyprintk=acpi" is introduced.  This makes the
>    actual sequence looks like:
>    +-------------------+    +--------------------+
>    | EARLY_PARAM SETUP | -> | EARLY_RPINTK START |
>    +-------------------+    +....................+    +-----------------+
>                             | ACPI DBGP LAUNCH   | -> | ACPI DBGP
> PROBE | ->
>                             +--------------------+    +-----------------+
>       +--------------------+
>    -> | EARLY_RPINTK START |
>       +--------------------+
> 
> Version 3 of this patch set is the first version that will be published
> in the Linux community.  Earlier versions are reviewed internally in
> Intel.  This patch set keeps the version number to track the history
> that is known to Intel internal developers.
> Version 4 of this patch set fixed bunch of CodingStyle issues.
> 
> Lv Zheng (2):
>   ACPI: Add early console framework for DBGP/DBG2.
>   ACPI: Add Intel MID SPI early console support.
> 
>  Documentation/kernel-parameters.txt        |    2 +
>  arch/x86/Kconfig.debug                     |   38 +++++
>  arch/x86/include/asm/mrst.h                |    2 +-
>  arch/x86/kernel/acpi/boot.c                |    1 +
>  arch/x86/kernel/early_printk.c             |   25 +++-
>  arch/x86/platform/mrst/early_printk_mrst.c |  186 +----------------------
>  drivers/acpi/Makefile                      |    2 +
>  drivers/acpi/early_printk.c                |  201
> +++++++++++++++++++++++++
>  drivers/platform/x86/Makefile              |    2 +
>  drivers/platform/x86/early/Makefile        |    5 +
>  drivers/platform/x86/early/intel_mid_spi.c |  220
> ++++++++++++++++++++++++++++
>  include/acpi/actbl2.h                      |    1 +
>  include/linux/acpi.h                       |   24 +++
>  include/linux/intel_mid_early.h            |   12 ++
>  14 files changed, 540 insertions(+), 181 deletions(-)
>  create mode 100644 drivers/acpi/early_printk.c
>  create mode 100644 drivers/platform/x86/early/Makefile
>  create mode 100644 drivers/platform/x86/early/intel_mid_spi.c
>  create mode 100644 include/linux/intel_mid_early.h
> 
> --
> 1.7.10

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