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PCI(4) Kernel Interfaces Manual PCI(4) NAME pci -- generic PCI/PCIe bus driver SYNOPSIS To compile the PCI bus driver into the kernel, place the following line in your kernel configuration file: device pci To compile in support for Single Root I/O Virtualization (SR-IOV): options PCI_IOV To compile in support for native PCI-express HotPlug: options PCI_HP DESCRIPTION The pci driver provides support for PCI and PCIe devices in the kernel and limited access to PCI devices for userland. The pci driver provides a /dev/pci character device that can be used by userland programs to read and write PCI configuration registers. Programs can also use this device to get a list of all PCI devices, or all PCI de- vices that match various patterns. Since the pci driver provides a write interface for PCI configuration reg- isters, system administrators should exercise caution when granting access to the pci device. If used improperly, this driver can allow userland ap- plications to crash a machine or cause data loss. In particular, driver only allows operations on the opened /dev/pci to modify system state if the file descriptor was opened for writing. For instance, the PCIOCREAD and PCIOCBARMMAP operations require a writeable descriptor, because reading a config register or a BAR read access could have function-specific side-ef- fects. The pci driver implements the PCI bus in the kernel. It enumerates any de- vices on the PCI bus and gives PCI client drivers the chance to attach to them. It assigns resources to children, when the BIOS does not. It takes care of routing interrupts when necessary. It reprobes the unattached PCI children when PCI client drivers are dynamically loaded at runtime. The pci driver also includes support for PCI-PCI bridges, various platform-spe- cific Host-PCI bridges, and basic support for PCI VGA adapters. IOCTLS The following ioctl(2) calls are supported by the pci driver. They are de- fined in the header file <sys/pciio.h>. PCIOCGETCONF PCIOCREAD PCIOCWRITE PCIOCATTACHED PCIOCBARMMAP PCIOCBARIO LOADER TUNABLES Tunables can be set at the loader(8) prompt before booting the kernel, or stored in loader.conf(5). The current value of these tunables can be exam- ined at runtime via sysctl(8) nodes of the same name. Unless otherwise specified, each of these tunables is a boolean that can be enabled by set- ting the tunable to a non-zero value. hw.pci.clear_bars (Defaults to 0) Ignore any firmware-assigned memory and I/O port resources. This forces the PCI bus driver to allocate resource ranges for memory and I/O port resources from scratch. hw.pci.clear_buses (Defaults to 0) Ignore any firmware-assigned bus number registers in PCI-PCI bridges. This forces the PCI bus driver and PCI-PCI bridge driver to allocate bus numbers for secondary buses behind PCI-PCI bridges. hw.pci.clear_pcib (Defaults to 0) Ignore any firmware-assigned memory and I/O port resource windows in PCI-PCI bridges. This forces the PCI-PCI bridge driver to allo- cate memory and I/O port resources for resource windows from scratch. By default the PCI-PCI bridge driver will allocate windows that contain the firmware-assigned resources devices behind the bridge. In addition, the PCI-PCI bridge driver will suballocate from exist- ing window regions when possible to satisfy a resource request. As a result, both hw.pci.clear_bars and hw.pci.clear_pcib must be en- abled to fully ignore firmware-supplied resource assignments. hw.pci.default_vgapci_unit (Defaults to -1) By default, the first PCI VGA adapter encountered by the system is assumed to be the boot display device. This tunable can be set to choose a specific VGA adapter by specifying the unit number of the associated vgapciX device. hw.pci.do_power_nodriver (Defaults to 0) Place devices into a low power state (D3) when a suitable device driver is not found. Can be set to one of the following values: 3 Powers down all PCI devices without a device driver. 2 Powers down most devices without a device driver. PCI de- vices with the display, memory, and base peripheral device classes are not powered down. 1 Similar to a setting of 2 except that storage controllers are also not powered down. 0 All devices are left fully powered. A PCI device must support power management to be powered down. Placing a device into a low power state may not reduce power con- sumption. hw.pci.do_power_resume (Defaults to 1) Place PCI devices into the fully powered state when resuming either the system or an individual device. Setting this to zero is dis- couraged as the system will not attempt to power up non-powered PCI devices after a suspend. hw.pci.do_power_suspend (Defaults to 1) Place PCI devices into a low power state when suspending either the system or individual devices. Normally the D3 state is used as the low power state, but firmware may override the desired power state during a system suspend. hw.pci.enable_ari (Defaults to 1) Enable support for PCI-express Alternative RID Interpretation. This is often used in conjunction with SR-IOV. hw.pci.enable_io_modes (Defaults to 1) Enable memory or I/O port decoding in a PCI device's command regis- ter if it has firmware-assigned memory or I/O port resources. The firmware (BIOS) in some systems does not enable memory or I/O port decoding for some devices even when it has assigned resources to the device. This enables decoding for such resources during bus probe. hw.pci.enable_msi (Defaults to 1) Enable support for Message Signalled Interrupts (MSI). MSI inter- rupts can be disabled by setting this tunable to 0. hw.pci.enable_msix (Defaults to 1) Enable support for extended Message Signalled Interrupts (MSI-X). MSI-X interrupts can be disabled by setting this tunable to 0. hw.pci.enable_pcie_ei (Defaults to 0) Enable support for PCI-express Electromechanical Interlock. hw.pci.enable_pcie_hp (Defaults to 1) Enable support for native PCI-express HotPlug. hw.pci.honor_msi_blacklist (Defaults to 1) MSI and MSI-X interrupts are disabled for certain chipsets known to have broken MSI and MSI-X implementations when this tunable is set. It can be set to zero to permit use of MSI and MSI-X interrupts if the chipset match is a false positive. hw.pci.iov_max_config (Defaults to 1MB) The maximum amount of memory permitted for the configuration para- meters used when creating Virtual Functions via SR-IOV. This tun- able can also be changed at runtime via sysctl(8). hw.pci.realloc_bars (Defaults to 0) Attempt to allocate a new resource range during the initial device scan for any memory or I/O port resources with firmware-assigned ranges that conflict with another active resource. hw.pci.usb_early_takeover (Defaults to 1 on amd64 and i386) Disable legacy device emulation of USB devices during the initial device scan. Set this tunable to zero to use USB devices via legacy emulation when using a custom kernel without USB controller drivers. hw.pci<D>.<B>.<S>.INT<P>.irq These tunables can be used to override the interrupt routing for legacy PCI INTx interrupts. Unlike other tunables in this list, these do not have corresponding sysctl nodes. The tunable name in- cludes the address of the PCI device as well as the pin of the de- sired INTx IRQ to override: <D> The domain (or segment) of the PCI device in decimal. <B> The bus address of the PCI device in decimal. <S> The slot of the PCI device in decimal. <P> The interrupt pin of the PCI slot to override. One of `A', `B', `C', or `D'. The value of the tunable is the raw IRQ value to use for the INTx interrupt pin identified by the tunable name. Mapping of IRQ val- ues to platform interrupt sources is machine dependent. DEVICE WIRING You can wire the device unit at a given location with device.hints(5). BSF Based Wiring Devices may be wired to a Bus / Slot / Function (BSF) address. This is the form reported by pciconf(8) Entries of the form hints.<name>.<unit>.at="pci<B>:<S>:<F>" or hints.<name>.<unit>.at="pci<D>:<B>:<S>:<F>" will force the driver name to probe and attach at unit unit for any PCI device found to match the speci- fication, where: <D> The domain (or segment) of the PCI device in decimal. Defaults to 0 if unspecified. <B> The bus address of the PCI device in decimal. <S> The slot of the PCI device in decimal. <F> The function of the PCI device in decimal. The code to do the matching requires an exact string match. Do not specify the angle brackets (< >) in the hints file. Wiring multiple devices to the same name and unit produces undefined results. Examples Given the following lines in /boot/device.hints: hint.nvme.3.at="pci6:0:0" hint.igb.8.at="pci14:0:0" If there is a device that supports igb(4) at PCI bus 14 slot 0 function 0, then it will be assigned igb8 for probe and attach. Likewise, if there is an nvme(4) device at PCI bus 6 slot 0 function 0, then it will be assigned nvme3 for probe and attach. If another type of card is in either of these locations, the name and unit of that card will be the default names and will be unaffected by these hints. If other igb or nvme cards are located elsewhere, they will be assigned their unit numbers sequentially, skipping the unit numbers that have 'at' hints. Location Based Wiring While simple to locate where to place a device for BSF wiring, the bus num- ber of that is not invariant. Any number of changes to the devices within the system can cause this value to vary from boot to boot. The UEFI Stan- dard defines a device path that's based only on the invariant parts of the address: The root complex (domain), the slot number and the function. These paths are hard to construct by hand, please see devctl(8) `getpath' command with a `UEFI' locator. The above example could also be expressed as hint.nvme.3.at="PciRoot(0x2)/Pci(0x1,0x3)/Pci(0x0,0x0)/Pci(0x0,0x0)/Pci(0x0,0x0)" hint.nvme.8.at="PciRoot(0x1)/Pci(0x2,0x2)/Pci(0x0,0x0)/Pci(0x0,0x0)" The advantage of this notation is that you can specify the exact location a device will be at. For deployments of multiple systems with the same con- figuration, this can be helpful in managing the devices. However, even slight variation in motherboards can cause the path to change substan- tially. It is also less natural to think of the UEFI Device Paths since little else will report it. FILES /dev/pci Character device for the pci driver. SEE ALSO device.hints(5) pciconf(8) HISTORY The pci driver (not the kernel's PCI support code) first appeared in FreeBSD 2.2, and was written by Stefan Esser and Garrett Wollman. Support for device listing and matching was re-implemented by Kenneth Merry, and first appeared in FreeBSD 3.0. AUTHORS Kenneth Merry <ken@FreeBSD.org> BUGS It is not possible for users to specify an accurate offset into the device list without calling the PCIOCGETCONF at least once, since they have no way of knowing the current generation number otherwise. This probably is not a serious problem, though, since users can easily narrow their search by specifying a pattern or patterns for the kernel to match against. FreeBSD ports 15.quarterly March 10, 2026 PCI(4)
NAME | SYNOPSIS | DESCRIPTION | IOCTLS | LOADER TUNABLES | DEVICE WIRING | FILES | SEE ALSO | HISTORY | AUTHORS | BUGS
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