PCIe Bus Allocation in Dell Precision 5820 | 7820 | 7920 Tower

PCIe Bus Allocation in Dell Precision 5820 | 7820 | 7920 Tower


Related Systems


Precision 7920 Tower

Precision 7820 Tower

Precision 5820 Tower

Summary

This post provides information about a new feature provided as a BIOS setup option on the above generation of Precision Workstations.

Background

All PCIe devices (including root ports, switches, and endpoints) must be assigned bus numbers during system POST (before the operating system starts). The purpose is to provide system software (operating system, device drivers, applications, etc.) an enumerated way to address PCIe devices. The current system architecture provides a maximum of 256 bus numbers and requires them to be allocated among PCIe SLOTs.

Typical PCIe add-in cards with a single endpoint device will consume just one bus number. However, some cards can consume many more. These include cards with multiple functions or bridge cards that attach to external PCIe expansion chassis. Also, cards that support hot-plugging devices (such as Thunderbolt) require bus numbers to be reserved for devices that may be attached to the system later, after POST is complete and the operating system is running.

Implementation

To provide flexibility to accommodate many different end customer configurations, a BIOS setup option is provided to allow customers to select a preferred PCIe bus number allocation scheme. The option is in the "Advanced Configurations" menu and is entitled "PCIe Bus Allocation". There are five options, detailed below.

  • Default
    • Bus numbers are evenly distributed among PCIe SLOTs for balanced resources.
    • This setting is used for all shipping configurations except those containing Thunderbolt add-in cards. It is expected to be the best setting for customers using typical PCIe add-in cards.
  • Optimize for Thunderbolt

    • Additional bus numbers are allocated to SLOT1, SLOT4, and SLOT5 for Thunderbolt support.
    • Thunderbolt configurations require at least 65 buses for each SLOT that supports Thunderbolt.
    • This setting is automatically selected when Thunderbolt support is enabled.
  • Option 1
    • Provides increased bus numbers to SLOT2 and SLOT4.
    • Useful to support two external expansion cards.
    • Maintains a moderate number of buses for other SLOTs.
  • Option 2
    • Provides even more bus numbers for SLOT4
    • Maintains a moderate number of buses for other SLOTs.
  • Option 3
    • Maximizes the number of buses available for SLOT4.
    • Maintains a minimum number of buses for other SLOTs.
    • Useful to support maximum daisy-chained external expansion from one system card.

Note that bus number allocation is for software enumeration and does not relate to the physical number of PCIe lanes provided to each SLOT. Bus number allocation does not affect performance.

Additional Details

The below tables show the PCIe bus allocation for each implementation.

SLOT1 44 66 18 18 10
SLOT2 46 12 42 20 12
SLOT3 10 12 10 10 10
SLOT4 46 66 75 129 177
SLOT5 11 65 11 11 11
SLOT6_CPU1 44 12 46 36 12
SLOT7_CPU1 44 12 42 20 12
Default TBT Option1 Option2 Option3
Table 1: Precision 7920 Tower with dual CPU’s

SLOT1 65 66 18 18 10
SLOT2 67 12 90 20 12
SLOT3 11 12 11 11 11
SLOT4 67 66 91 161 177
SLOT5 11 65 11 11 11
Default TBT Option1 Option2 Option3
Table 2: Precision 7820 Tower with dual CPU’s

SLOT1 76 66 18 18 10
SLOT2 78 44 86 20 12
SLOT3 10 12 10 10 10
SLOT4 78 66 128 194 210
SLOT5 11 65 10 10 10
Default TBT Option1 Option2 Option3
Table 3: Precision 7920, 7820, and 5820 Tower with single CPU



Article ID: SLN312134

Last Date Modified: 08/13/2018 09:50 AM


Rate this article

Accurate
Useful
Easy to understand
Was this article helpful?
Yes No
Send us feedback
Comments cannot contain these special characters: <>()\
Sorry, our feedback system is currently down. Please try again later.

Thank you for your feedback.