15:12

0x0

RW/P

Compliance Preset/De-emphasis (CD)

For 8.0 GT/s and higher Data Rate: This field sets the Transmitter Preset in Polling.Compliance state if the entry occurred due to the Enter Compliance bit being 1b. Results are undefined if a reserved preset encoding is used when entering Polling.Compliance in this way.
For 5.0 GT/s Data Rate: This bit sets the de-emphasis level in Polling.Compliance state if the entry occurred due to the Enter Compliance bit being 1b.
Encodings:
0001b -3.5 dB
0000b -6 dB
When the Link is operating at 2.5 GT/s, the setting of this field has no effect.
The default value of this field is 0000b.
This bit is intended for debug, compliance testing purposes. System firmware and software is allowed to modify this bit only during debug or compliance testing. In all other cases, the system must ensure that this field is set to the default value.

11

0x0

RW/P

Compliance SOS (CSOS)

When set to 1b, the LTSSM is required to send SKP Ordered Sets periodically in between the (modified) compliance patterns.
The default value of this bit is 0b.
This bit is applicable when the Link is operating at 2.5 GT/s or 5.0 GT/s data rates only.

10

0x0

RW/P

Enter Modified Compliance (EMC)

When this bit is set to 1b, the device transmits Modified Compliance Pattern if the LTSSM enters Polling.Compliance substate.
Default value of this bit is 0b.
This register is intended for debug, compliance testing purposes only. System firmware and software is allowed to modify this register only during debug or compliance testing. In all other cases, the system must ensure that this register is set to the default value.

9:7

0x0

RW/P

Transmit Margin (TM)

This field controls the value of the non-deemphasized voltage level at the Transmitter pins. This field is reset to 000b on entry to the LTSSM Polling.Configuration substate (see PCI Express Chapter 4 for details of how the Transmitter voltage level is determined in various states).
Encodings:
000b Normal operating range
001b 800-1200 mV for full swing and 400-700 mV for half-swing
010b - (n-1) Values must be monotonic with a non-zero slope. The value of n must be greater than 3 and less than 7. At least two of these must be below the normal operating range of n: 200-400 mV for full-swing and 100-200 mV for half-swing
n - 111b reserved
For a Multi-Function device associated with an Upstream Port, the field in Function 0 is of type RWS, and only Function 0 controls the component's Link behavior. In all other Functions of that device, this field is of type RsvdP.
Default value of this field is 000b.
Components that support only the 2.5 GT/s speed are permitted to hardwire this bit to 000b.
This register is intended for debug, compliance testing purposes only. System firmware and software is allowed to modify this register only during debug or compliance testing. In all other cases, the system must ensure that this register is set to the default value.

6

0x0

RW/P

Selectable De-emphasis (SD)

When the Link is operating at 5.0 GT/s speed, this bit selects the level of de-emphasis for an Upstream component.
Encodings:
1b -3.5 dB
0b -6 dB
When the Link is not operating at 5.0 GT/s speed, the setting of this bit has no effect.

5

0x0

RO

Hardware Autonomous Speed Disable (HASD)

Reserved
This port cannot autonomously change speeds.

4

0x0

RW/P

Enter Compliance (EC)

Software is permitted to force a Link to enter Compliance mode at the speed indicated in the Target Link Speed field by setting this bit to 1b in both components on a Link and then initiating a hot reset on the Link.
Default value of this bit following Fundamental Reset is 0b.
This bit is intended for debug, compliance testing purposes only. System firmware and software is allowed to modify this bit only during debug or compliance testing. In all other cases, the system must ensure that this bit is set to the default value.

3:0

0x1

RW/V/P

Target Link Speed (TLS)

This field sets an upper limit on Link operational speed by restricting the values advertised by the upstream component in its training sequences.
The encoded value specifies a bit location in the Supported Link Speeds Vector (in the Link Capabilities 2 register) that corresponds to the current Link speed.
Defined encodings are:
0001b: Supported Link Speeds Vector field bit 0.
0010b: Supported Link Speeds Vector field bit 1.
0011b: Supported Link Speeds Vector field bit 2.
0100b: Supported Link Speeds Vector field bit 3.
0101b: Supported Link Speeds Vector field bit 4.
0110b: Supported Link Speeds Vector field bit 5.
0111b: Supported Link Speeds Vector field bit 6.
All other encodings are reserved.
If a value is written to this field that does not correspond to a supported speed, as indicated by the Supported Link Speeds Vector, the result is undefined.
The default value of this field is GEN1.
Note: This register field could be used by a driver to limit the link speed to 2.5 GT/s or 5 GT/s data rate.