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

Dell Precision 7920 Tower

Dell Precision 7820 Tower

Dell Precision 5820 Tower

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

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.

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.

Table 1: Precision 7920 Tower with dual CPU’s

Table 2: Precision 7820 Tower with dual CPU’s

Table 3: Precision 7920, 7820, and 5820 Tower with single CPU