PCIe interface card hot plug mechanism analysis

Inserting the interface card directly into the slot without shutting down requires some mechanisms and procedures, otherwise the motherboard or interface card may be easily damaged. Here are some countermeasures for PCIe hot plugging.

Strictly speaking, hot plugging of PCIe (PCI Express) results from the standard specification for hot-swap controllers (SHPC 1.0) for the PCI bus. The PCIe hot plug design concept is part of a "No Surprises" strategy where users must notify the system before inserting or removing the interface card; the system software must be prepared to notify the user through the monitor. Hot plugged state.

Recalling the past development of the PCI bus, it was not specifically defined for hot-plugging at first. Later, due to the needs of industrial applications, hot-swappable mechanisms were introduced on the market. CompactPCI is an example.

From a hot-plug system point of view, PCI and PCIe are conceptually identical and both require a "hot-plug controller" for control.

The PCI bus is a shared bus interface (Share Bus), so the logic of isolation measures is necessary. The PCIe interface architecture is a point-to-point connection type, which eliminates the existence of isolated logic circuits and allows distributed hot-swap controllers to process the relevant ports. PCIe constitutes a standard software interface for each port. Because of differences in program interfaces, PCI and PCIe require different driver software.

From the perspective of the physical interface, the difference between PCIe and PCI is that the pin of PCIe defines the signals (PRSNT1 and PRSNT2) for detecting the presence of the interface card. Even if it is accidentally or maliciously removed, system software still has enough time to process it.

Hot swap in PCIe environment

To fully support the hot-swap function in the PCIe environment, you need a combination of hardware and software. In terms of software, the operating system OS covers user interfaces, hot-swappable system software services, and device drivers. On the hardware side, the port device must have a hot swap controller, interface card power switch logic, interface card reset line, attention button, power indicator/attention indicator, and interface card presence detection pin. (ie PRSNT1# and PRSNT2# signals) and so on.

In the following, we will first explain the insertion and removal steps of the hot-pluggable interface card. First, we must understand what the internal meaning of the "On" and "Off" states of the slot is.

The slot "On" has the following key points:
â—† Power is applied to the interface card â—† The REFCLK reference clock signal is turned on â—† The link is in dynamic state or enters power saving mode through the ASPM (L0s or L1 state)
â—† Close the PERST# signal

The opposite of the "Off" slot:
â—† Turn off the power supply on the interface card â—† The REFCLK reference clock signal turns off â—† The transmission link is inactive (Inactive)
â—† Start PERST# signal

Under normal operating conditions, the Power Indicator (green) turns on, and the Attention Indicator (amber or yellow) turns off.

The software uses the transfer mode of the configuration write transaction to send a request to the hot swap controller and control the power and indicator status added to the interface.

When the user wants to remove the interface card, the trigger can be activated in two ways, through the "button" or the "user interface" of the software program.

The detailed steps for using key removal are as follows:
◆ Step 1: The user presses the “alarm indicator” button, the hot swap controller obtains this information, and sends an interrupt signal to the “Root Complex” side. The system reads the slot information and confirms the button. ◆ Step 2: Hot The plug-in controller flashes the power indicator light. ◆ Step 3: The hot-swap service routine stops the driver program and does not issue any request action to the interface card. ◆ Step 4: The software moves to the link control register of the interface card slot port, making the link Both ports are inoperative ◆ Step 5: The hot swap controller closes the slot ◆ Step 6: The software sends a “Power Off” request to instruct the user to remove the interface card ◆ Step 7: Release the manual interface card retention latch (MRL) allows the hot-swap controller to remove SMBus and JTAG switch signals from the slot, and then remove the interface card. ◆ Step 8: The system uses the resources occupied by the interface card, such as storage space, interrupt signal lines, etc. Transfer to other components

The insertion of the interface card is basically the reverse of the above process.

PCIe Hot Swap Controller

The "PCI Hot Plug" version 1.0 specification does not define a standard register and user interface, so the software/hardware used by each vendor varies. When consumers purchase products from different vendors, they must learn and become familiar with their use. For this reason, the PCI Hot Swap Controller (HPC) Version 1.0 specification specifically makes the following two important provisions:
â—† Defined the standard user interface (User Interface), making it easier for consumers to use â—† For hot swap controllers, a standardized programming interface (Programming Interface) is provided to enable the incorporation of a standardized hot plug driver software Operating system becomes feasible

Unfortunately, the PCIe scratchpad is not defined in HPC. The standard hot-swap controller driver software is slightly different when it comes to PCI and PCIe.

Let's talk about the standard user interface first. The standard user interface must have an alarm reminder, power indicator, manual interface card retention latch (MRL), slot number identification (Slot Numbering Identification), software interface, and prompt buttons.

The alarm reminder should be as close to the interface slot as possible, with yellow or amber LED indicators. The main function of this indicator is to attract the user's high attention and inform the hot swap operation failed. Therefore, the indicator is normally off.

The power indicator mainly reflects that the main power has been allocated to the slot and is controlled by the hot plug-related software. The display color is green. The light is on to indicate that the interface card cannot be inserted or removed.

The main role of MRL is to fix the interface card, and you can choose to add additional sensors. The switching signals controlled by the MRL are nothing more than Vaux auxiliary power and SMbus signals (SMBCLK and SMBDAT signals).

A careful study of PCIe's core specifications and the CEM (Card Electro Mechanical) specification reveals that the specification defines the PCIe hotswap required slot signals. among them:
◆ PWRLED#: Power indicator output signal ◆ ATNLED#: Alarm indicator output signal ◆ PWREN: Main power supply to slot control signal ◆ REFCLKEN#: Reference clock to slot control signal ◆ PERST#: Basic reset signal ◆ PRSNT1 #: Connector end grounded ◆ PRSNT2#: Interface card presence signal, pull-high resistor on the system board termination ◆ PWRFLT#: External logic is used to inform the hot-swap controller that the power supply has an incorrect input signal ◆ AUXEN#: AUX signal and Auxiliary power control signal ◆ MRL#: MRL sensor input signal ◆ BUTTON#: “Attention Button”, used to notify the user of hot-swap controller to perform hot plugging action

PCIe's standard program interface for hot-swap controllers is implemented through the PCI Express Capability register.

Slot Capabilities must read the Device Capability Register to know whether buttons and indicators are supported.

System-controlled hot-swap events and actions are accomplished through the slot control registers (Slot Control Registers). The relevant bit definitions are shown in Figure 6.

Hot-swap controllers track various events and notify hot-swappable system driver software, which is then confirmed by status bits.

Usually there will be many slots on the system mainboard. The identification between each slot requires a Physical Slot ID, but these identification values ​​are specified by the system designer and must also allow Root or Switch such as the chip value. The port of the component is known. After all, the hardware must initially initialize the Slot Status Registers for each slot.

Hot swap power budget

The PCIe specification mentions the issue of Power Budgeting, which is the Power Budgeting Capability mentioned in the specification, and is part of a design option. Although it is not 100% for hot swappable interface cards, at least the system designer has enough power budget for the interface card design to minimize the conflict between each other and also can precisely control the thermal processing.

The specification defines the configuration registers that support the power budget, but does not describe the detailed implementation.

The system firmware must know the overall system available power, the power configuration of the system equipment, and the slot on the interface.

A hot-swappable port has the Slot Power Limit and Slot Power Scale fields in the Slot Capacity register. When the power software or firmware initializes, this field must be loaded with a value indicating that the port is enabled. The maximum power supply available. At the same time, the port will use Set_Slot_Power_Limit to inform the interface card.

For interface cards that support power budgets, the Device Capabilities register stores only the number of messages sent from the port. At the same time, the values ​​in the power budget capacity register are used to report power-related information. The role of the device driver is to verify that the power supply is sufficient before starting.


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