Hi!

Introduction of the Platform Environment Control Interface (PECI) bus
device driver. PECI is a one-wire bus interface that provides a
communication channel between Intel processor and chipset components to
external monitoring or control devices. PECI is designed to support the
following sideband functions:

* Processor and DRAM thermal management
  - Processor fan speed control is managed by comparing Digital Thermal
    Sensor (DTS) thermal readings acquired via PECI against the
    processor-specific fan speed control reference point, or TCONTROL.
    Both TCONTROL and DTS thermal readings are accessible via the processor
    PECI client. These variables are referenced to a common temperature,
    the TCC activation point, and are both defined as negative offsets from
    that reference.
  - PECI based access to the processor package configuration space provides
    a means for Baseboard Management Controllers (BMC) or other platform
    management devices to actively manage the processor and memory power
    and thermal features.

* Platform Manageability
  - Platform manageability functions including thermal, power, and error
    monitoring. Note that platform 'power' management includes monitoring
    and control for both the processor and DRAM subsystem to assist with
    data center power limiting.
  - PECI allows read access to certain error registers in the processor MSR
    space and status monitoring registers in the PCI configuration space
    within the processor and downstream devices.
  - PECI permits writes to certain registers in the processor PCI
    configuration space.

* Processor Interface Tuning and Diagnostics
  - Processor interface tuning and diagnostics capabilities
    (Intel(c) Interconnect BIST). The processors Intel(c) Interconnect
    Built In Self Test (Intel(c) IBIST) allows for infield diagnostic
    capabilities in the Intel UPI and memory controller interfaces. PECI
    provides a port to execute these diagnostics via its PCI Configuration
    read and write capabilities.

* Failure Analysis
  - Output the state of the processor after a failure for analysis via
    Crashdump.

PECI uses a single wire for self-clocking and data transfer. The bus
requires no additional control lines. The physical layer is a self-clocked
one-wire bus that begins each bit with a driven, rising edge from an idle
level near zero volts. The duration of the signal driven high depends on
whether the bit value is a logic '0' or logic '1'. PECI also includes
variable data transfer rate established with every message. In this way,
it is highly flexible even though underlying logic is simple.

The interface design was optimized for interfacing to Intel processor and
chipset components in both single processor and multiple processor
environments. The single wire interface provides low board routing
overhead for the multiple load connections in the congested routing area
near the processor and chipset components. Bus speed, error checking, and
low protocol overhead provides adequate link bandwidth and reliability to
transfer critical device operating conditions and configuration
information.

This implementation provides the basic framework to add PECI extensions
to the Linux bus and device models. A hardware specific 'Adapter' driver
can be attached to the PECI bus to provide sideband functions described
above. It is also possible to access all devices on an adapter from
userspace through the /dev interface. A device specific 'Client' driver
also can be attached to the PECI bus so each processor client's features
can be supported by the 'Client' driver through an adapter connection in
the bus. This patch set includes Aspeed 24xx/25xx PECI driver and a generic
PECI hwmon driver as the first implementation for both adapter and client
drivers on the PECI bus framework.

Ok, how does this interact with ACPI/SMM BIOS/Secure mode code? Does
Linux _need_ to control the fan? Or is SMM BIOS capable of doing all
the work itself and Linux has just read-only access for monitoring
purposes?

Pavel

-- (english) http://www.livejournal.com/~pavelmachek
(cesky, pictures)
http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html