man/man4/pci.4

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.Dd August 31, 2025
.Dt PCI 4
.Os
.Sh NAME
.Nm pci
.Nd generic PCI/PCIe bus driver
.Sh SYNOPSIS
To compile the PCI bus driver into the kernel,
place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd device pci
.Ed
.Pp
To compile in support for Single Root I/O Virtualization
.Pq SR-IOV :
.Bd -ragged -offset indent
.Cd options PCI_IOV
.Ed
.Pp
To compile in support for native PCI-express HotPlug:
.Bd -ragged -offset indent
.Cd options PCI_HP
.Ed
.Sh DESCRIPTION
The
.Nm
driver provides support for
.Tn PCI
and
.Tn PCIe
devices in the kernel and limited access to
.Tn PCI
devices for userland.
.Pp
The
.Nm
driver provides a
.Pa /dev/pci
character device that can be used by userland programs to read and write
.Tn PCI
configuration registers.
Programs can also use this device to get a list of all
.Tn PCI
devices, or all
.Tn PCI
devices that match various patterns.
.Pp
Since the
.Nm
driver provides a write interface for
.Tn PCI
configuration registers, system administrators should exercise caution when
granting access to the
.Nm
device.
If used improperly, this driver can allow userland applications to
crash a machine or cause data loss.
In particular, driver only allows operations on the opened
.Pa /dev/pci
to modify system state if the file descriptor was opened for writing.
For instance, the
.Dv PCIOCREAD
and
.Dv PCIOCBARMMAP
operations require a writeable descriptor, because reading a config register
or a BAR read access could have function-specific side-effects.
.Pp
The
.Nm
driver implements the
.Tn PCI
bus in the kernel.
It enumerates any devices on the
.Tn PCI
bus and gives
.Tn 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
.Tn PCI
children when
.Tn PCI
client drivers are dynamically
loaded at runtime.
The
.Nm
driver also includes support for PCI-PCI bridges,
various platform-specific Host-PCI bridges,
and basic support for
.Tn PCI
VGA adapters.
.Sh IOCTLS
The following
.Xr ioctl 2
calls are supported by the
.Nm
driver.
They are defined in the header file
.In sys/pciio.h .
.Bl -tag -width 012345678901234
.It PCIOCGETCONF
This
.Xr ioctl 2
takes a
.Va pci_conf_io
structure.
It allows the user to retrieve information on all
.Tn PCI
devices in the system, or on
.Tn PCI
devices matching patterns supplied by the user.
The call may set
.Va errno
to any value specified in either
.Xr copyin 9
or
.Xr copyout 9 .
The
.Va pci_conf_io
structure consists of a number of fields:
.Bl -tag -width match_buf_len
.It pat_buf_len
The length, in bytes, of the buffer filled with user-supplied patterns.
.It num_patterns
The number of user-supplied patterns.
.It patterns
Pointer to a buffer filled with user-supplied patterns.
.Va patterns
is a pointer to
.Va num_patterns
.Va pci_match_conf
structures.
The
.Va pci_match_conf
structure consists of the following elements:
.Bl -tag -width pd_vendor
.It pc_sel
.Tn PCI
domain, bus, slot and function.
.It pd_name
.Tn PCI
device driver name.
.It pd_unit
.Tn PCI
device driver unit number.
.It pc_vendor
.Tn PCI
vendor ID.
.It pc_device
.Tn PCI
device ID.
.It pc_class
.Tn PCI
device class.
.It flags
The flags describe which of the fields the kernel should match against.
A device must match all specified fields in order to be returned.
The match flags are enumerated in the
.Va pci_getconf_flags
structure.
Hopefully the flag values are obvious enough that they do not need to
described in detail.
.El
.It match_buf_len
Length of the
.Va matches
buffer allocated by the user to hold the results of the
.Dv PCIOCGETCONF
query.
.It num_matches
Number of matches returned by the kernel.
.It matches
Buffer containing matching devices returned by the kernel.
The items in this buffer are of type
.Va pci_conf ,
which consists of the following items:
.Bl -tag -width pc_subvendor
.It pc_sel
.Tn PCI
domain, bus, slot and function.
.It pc_hdr
.Tn PCI
header type.
.It pc_subvendor
.Tn PCI
subvendor ID.
.It pc_subdevice
.Tn PCI
subdevice ID.
.It pc_vendor
.Tn PCI
vendor ID.
.It pc_device
.Tn PCI
device ID.
.It pc_class
.Tn PCI
device class.
.It pc_subclass
.Tn PCI
device subclass.
.It pc_progif
.Tn PCI
device programming interface.
.It pc_revid
.Tn PCI
revision ID.
.It pd_name
Driver name.
.It pd_unit
Driver unit number.
.It pd_numa_domain
Driver NUMA domain.
.It pc_reported_len
Length of the valid portion of the encompassing
.Vt pci_conf
structure.
This should always be equivalent to the offset of the
.Va pc_spare
member.
.It pc_spare
Reserved for future use.
.El
.It offset
The offset is passed in by the user to tell the kernel where it should
start traversing the device list.
The value passed out by the kernel
points to the record immediately after the last one returned.
The user may
pass the value returned by the kernel in subsequent calls to the
.Dv PCIOCGETCONF
ioctl.
If the user does not intend to use the offset, it must be set to zero.
.It generation
.Tn PCI
configuration generation.
This value only needs to be set if the offset is set.
The kernel will compare the current generation number of its internal
device list to the generation passed in by the user to determine whether
its device list has changed since the user last called the
.Dv PCIOCGETCONF
ioctl.
If the device list has changed, a status of
.Va PCI_GETCONF_LIST_CHANGED
will be passed back.
.It status
The status tells the user the disposition of his request for a device list.
The possible status values are:
.Bl -ohang
.It PCI_GETCONF_LAST_DEVICE
This means that there are no more devices in the PCI device list matching
the specified criteria after the
ones returned in the
.Va matches
buffer.
.It PCI_GETCONF_LIST_CHANGED
This status tells the user that the
.Tn PCI
device list has changed since his last call to the
.Dv PCIOCGETCONF
ioctl and he must reset the
.Va offset
and
.Va generation
to zero to start over at the beginning of the list.
.It PCI_GETCONF_MORE_DEVS
This tells the user that his buffer was not large enough to hold all of the
remaining devices in the device list that match his criteria.
.It PCI_GETCONF_ERROR
This indicates a general error while servicing the user's request.
If the
.Va pat_buf_len
is not equal to
.Va num_patterns
times
.Fn sizeof "struct pci_match_conf" ,
.Va errno
will be set to
.Er EINVAL .
.El
.El
.It PCIOCREAD
This
.Xr ioctl 2
reads the
.Tn PCI
configuration registers specified by the passed-in
.Va pci_io
structure.
The
.Va pci_io
structure consists of the following fields:
.Bl -tag -width pi_width
.It pi_sel
A
.Va pcisel
structure which specifies the domain, bus, slot and function the user would
like to query.
If the specific bus is not found, errno will be set to ENODEV and -1 returned
from the ioctl.
.It pi_reg
The
.Tn PCI
configuration registers the user would like to access.
.It pi_width
The width, in bytes, of the data the user would like to read.
This value
may be either 1, 2, or 4.
3-byte reads and reads larger than 4 bytes are
not supported.
If an invalid width is passed, errno will be set to EINVAL.
.It pi_data
The data returned by the kernel.
.El
.It PCIOCWRITE
This
.Xr ioctl 2
allows users to write to the
.Tn PCI
configuration registers specified in the passed-in
.Va pci_io
structure.
The
.Va pci_io
structure is described above.
The limitations on data width described for
reading registers, above, also apply to writing
.Tn PCI
configuration registers.
.It PCIOCATTACHED
This
.Xr ioctl 2
allows users to query if a driver is attached to the
.Tn PCI
device specified in the passed-in
.Va pci_io
structure.
The
.Va pci_io
structure is described above, however, the
.Va pi_reg
and
.Va pi_width
fields are not used.
The status of the device is stored in the
.Va pi_data
field.
A value of 0 indicates no driver is attached, while a value larger than 0
indicates that a driver is attached.
.It PCIOCBARMMAP
This
.Xr ioctl 2
command allows userspace processes to
.Xr mmap 2
the memory-mapped PCI BAR into its address space.
The input parameters and results are passed in the
.Va pci_bar_mmap
structure, which has the following fields:
.Bl -tag -width Vt struct pcise pbm_sel
.It Vt void *pbm_map_base
Reports the established mapping base to the caller.
If
.Va PCIIO_BAR_MMAP_FIXED
flag was specified, then this field must be filled before the call
with the desired address for the mapping.
.It Vt size_t pbm_map_length
Reports the mapped length of the BAR, in bytes.
Its
.Vt size_t
value is always multiple of machine pages.
.It Vt uint64_t pbm_bar_length
Reports length of the bar as exposed by the device.
.It Vt int pbm_bar_off
Reports offset from the mapped base to the start of the
first register in the bar.
.It Vt struct pcisel pbm_sel
Should be filled before the call.
Describes the device to operate on.
.It Vt int pbm_reg
The BAR index to mmap.
.It Vt int pbm_flags
Flags which augments the operation.
See below.
.It Vt int pbm_memattr
The caching attribute for the mapping.
Typical values are
.Dv VM_MEMATTR_UNCACHEABLE
for control registers BARs, and
.Dv VM_MEMATTR_WRITE_COMBINING
for frame buffers.
Regular memory-like BAR should be mapped with
.Dv VM_MEMATTR_DEFAULT
attribute.
.El
.Pp
Currently defined flags are:
.Bl -tag -width PCIIO_BAR_MMAP_ACTIVATE
.It PCIIO_BAR_MMAP_FIXED
The resulted mappings should be established at the address
specified by the
.Va pbm_map_base
member, otherwise fail.
.It PCIIO_BAR_MMAP_EXCL
Must be used together with
.Dv PCIIO_BAR_MMAP_FIXED
If the specified base contains already established mappings, the
operation fails instead of implicitly unmapping them.
.It PCIIO_BAR_MMAP_RW
The requested mapping allows both reading and writing.
Without the flag, read-only mapping is established.
Note that it is common for the device registers to have side-effects
even on reads.
.It PCIIO_BAR_MMAP_ACTIVATE
(Unimplemented) If the BAR is not activated, activate it in the course
of mapping.
Currently attempt to mmap an inactive BAR results in error.
.El
.It PCIOCBARIO
This
.Xr ioctl 2
command allows users to read from and write to BARs.
The I/O request parameters are passed in a
.Va struct pci_bar_ioreq
structure, which has the following fields:
.Bl -tag
.It Vt struct pcisel pbi_sel
Describes the device to operate on.
.It Vt int pbi_op
The operation to perform.
Currently supported values are
.Dv PCIBARIO_READ
and
.Dv PCIBARIO_WRITE .
.It Vt uint32_t pbi_bar
The index of the BAR on which to operate.
.It Vt uint32_t pbi_offset
The offset into the BAR at which to operate.
.It Vt uint32_t pbi_width
The size, in bytes, of the I/O operation.
1-byte, 2-byte, 4-byte and 8-byte perations are supported.
.It Vt uint32_t pbi_value
For reads, the value is returned in this field.
For writes, the caller specifies the value to be written in this field.
.Pp
Note that this operation maps and unmaps the corresponding resource and
so is relatively expensive for memory BARs.
The
.Va PCIOCBARMMAP
.Xr ioctl 2
can be used to create a persistent userspace mapping for such BARs instead.
.El
.El
.Sh LOADER TUNABLES
Tunables can be set at the
.Xr loader 8
prompt before booting the kernel, or stored in
.Xr loader.conf 5 .
The current value of these tunables can be examined at runtime via
.Xr sysctl 8
nodes of the same name.
Unless otherwise specified,
each of these tunables is a boolean that can be enabled by setting the
tunable to a non-zero value.
.Bl -tag -width indent
.It Va hw.pci.clear_bars Pq Defaults to 0
Ignore any firmware-assigned memory and I/O port resources.
This forces the
.Tn PCI
bus driver to allocate resource ranges for memory and I/O port resources
from scratch.
.It Va hw.pci.clear_buses Pq Defaults to 0
Ignore any firmware-assigned bus number registers in PCI-PCI bridges.
This forces the
.Tn PCI
bus driver and PCI-PCI bridge driver to allocate bus numbers for secondary
buses behind PCI-PCI bridges.
.It Va hw.pci.clear_pcib Pq 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 allocate memory and I/O port resources
for resource windows from scratch.
.Pp
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 existing window
regions when possible to satisfy a resource request.
As a result,
both
.Va hw.pci.clear_bars
and
.Va hw.pci.clear_pcib
must be enabled to fully ignore firmware-supplied resource assignments.
.It Va hw.pci.default_vgapci_unit Pq Defaults to -1
By default,
the first
.Tn 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
.Va vgapci Ns Ar X
device.
.It Va hw.pci.do_power_nodriver Pq Defaults to 0
Place devices into a low power state
.Pq D3
when a suitable device driver is not found.
Can be set to one of the following values:
.Bl -tag -width indent
.It 3
Powers down all
.Tn PCI
devices without a device driver.
.It 2
Powers down most devices without a device driver.
PCI devices with the display, memory, and base peripheral device classes
are not powered down.
.It 1
Similar to a setting of 2 except that storage controllers are also not
powered down.
.It 0
All devices are left fully powered.
.El
.Pp
A
.Tn PCI
device must support power management to be powered down.
Placing a device into a low power state may not reduce power consumption.
.It Va hw.pci.do_power_resume Pq Defaults to 1
Place
.Tn PCI
devices into the fully powered state when resuming either the system or an
individual device.
Setting this to zero is discouraged as the system will not attempt to power
up non-powered PCI devices after a suspend.
.It Va hw.pci.do_power_suspend Pq Defaults to 1
Place
.Tn 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.
.It Va hw.pci.enable_ari Pq Defaults to 1
Enable support for PCI-express Alternative RID Interpretation.
This is often used in conjunction with SR-IOV.
.It Va hw.pci.enable_io_modes Pq Defaults to 1
Enable memory or I/O port decoding in a PCI device's command register if it has
firmware-assigned memory or I/O port resources.
The firmware
.Pq 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.
.It Va hw.pci.enable_msi Pq Defaults to 1
Enable support for Message Signalled Interrupts
.Pq MSI .
MSI interrupts can be disabled by setting this tunable to 0.
.It Va hw.pci.enable_msix Pq Defaults to 1
Enable support for extended Message Signalled Interrupts
.Pq MSI-X .
MSI-X interrupts can be disabled by setting this tunable to 0.
.It Va hw.pci.enable_pcie_ei Pq Defaults to 0
Enable support for PCI-express Electromechanical Interlock.
.It Va hw.pci.enable_pcie_hp Pq Defaults to 1
Enable support for native PCI-express HotPlug.
.It Va hw.pci.honor_msi_blacklist Pq 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.
.It Va hw.pci.iov_max_config Pq Defaults to 1MB
The maximum amount of memory permitted for the configuration parameters
used when creating Virtual Functions via SR-IOV.
This tunable can also be changed at runtime via
.Xr sysctl 8 .
.It Va hw.pci.realloc_bars Pq 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.
.It Va hw.pci.usb_early_takeover Pq Defaults to 1 on Tn amd64 and Tn 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.
.It Va 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 includes the address of the PCI device as well as the
pin of the desired INTx IRQ to override:
.Bl -tag -width indent
.It <D>
The domain
.Pq or segment
of the PCI device in decimal.
.It <B>
The bus address of the PCI device in decimal.
.It <S>
The slot of the PCI device in decimal.
.It <P>
The interrupt pin of the PCI slot to override.
One of
.Ql A ,
.Ql B ,
.Ql C ,
or
.Ql D .
.El
.Pp
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 values to platform interrupt sources is machine dependent.
.El
.Sh DEVICE WIRING
You can wire the device unit at a given location with
.Xr device.hints 5 .
.Ss BSF Based Wiring
Devices may be wired to a Bus / Slot / Function (BSF) address.
This is the form reported by
.Xr pciconf 8
Entries of the form
.Va hints.<name>.<unit>.at="pci<B>:<S>:<F>"
or
.Va hints.<name>.<unit>.at="pci<D>:<B>:<S>:<F>"
will force the driver
.Va name
to probe and attach at unit
.Va unit
for any PCI device found to match the specification, where:
.Bl -tag -width -indent
.It <D>
The domain
.Pq or segment
of the PCI device in decimal.
Defaults to 0 if unspecified.
.It <B>
The bus address of the PCI device in decimal.
.It <S>
The slot of the PCI device in decimal.
.It <F>
The function of the PCI device in decimal.
.El
.Pp
The code to do the matching requires an exact string match.
Do not specify the angle brackets
.Pq < >
in the hints file.
Wiring multiple devices to the same
.Va name
and
.Va unit
produces undefined results.
.Ss Examples
Given the following lines in
.Pa /boot/device.hints :
.Bd -literal
hint.nvme.3.at="pci6:0:0"
hint.igb.8.at="pci14:0:0"
.Ed
.Pp
If there is a device that supports
.Xr 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
.Xr 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.
.Ss Location Based Wiring
While simple to locate where to place a device for BSF wiring, the
bus number 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 Standard 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
.Xr devctl 8
.Sq Cm getpath
command with a
.Sq Ar UEFI
locator.
The above example could also be expressed as
.Bd -literal
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)"
.Ed
.Pp
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 configuration, this can be
helpful in managing the devices.
However, even slight variation in motherboards can cause the path to change
substantially.
It is also less natural to think of the UEFI Device Paths since little else
will report it.
.Sh FILES
.Bl -tag -width /dev/pci -compact
.It Pa /dev/pci
Character device for the
.Nm
driver.
.El
.Sh SEE ALSO
.Xr device.hints 5
.Xr pciconf 8
.Sh HISTORY
The
.Nm
driver (not the kernel's
.Tn PCI
support code) first appeared in
.Fx 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
.Fx 3.0 .
.Sh AUTHORS
.An Kenneth Merry Aq Mt ken@FreeBSD.org
.Sh BUGS
It is not possible for users to specify an accurate offset into the device
list without calling the
.Dv 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.