AHCI(4) FreeBSD Kernel Interfaces Manual AHCI(4)


ahciSerial ATA Advanced Host Controller Interface driver


To compile this driver into the kernel, place the following lines in your kernel configuration file:

device pci
device scbus
device ahci

Alternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):


The following tunables are settable from the loader(8):

hint.ahci. X .msi
controls Message Signaled Interrupts (MSI) usage by the specified controller.

MSI disabled;
single MSI vector used, if supported (default);
multiple MSI vectors used, if supported;
hint.ahci. X .ccc
controls Command Completion Coalescing (CCC) usage by the specified controller. Non-zero value enables CCC and defines maximum time (in ms), request can wait for interrupt, if there are some more requests present on controller queue. CCC reduces number of context switches on systems with many parallel requests, but it can decrease disk performance on some workloads due to additional command latency.
hint.ahcich. X .pm_level
controls SATA interface Power Management for the specified channel, allowing some power to be saved at the cost of additional command latency. Possible values:

interface Power Management is disabled (default);
device is allowed to initiate PM state change, host is passive;
host initiates PARTIAL PM state transition every time port becomes idle;
host initiates SLUMBER PM state transition every time port becomes idle.
driver initiates PARTIAL PM state transition 1ms after port becomes idle;
driver initiates SLUMBER PM state transition 125ms after port becomes idle.

Some controllers, such as ICH8, do not implement modes 2 and 3 with NCQ used. Because of artificial entering latency, performance degradation in modes 4 and 5 is much smaller then in modes 2 and 3.

Note that interface Power Management complicates device presence detection. A manual bus reset/rescan may be needed after device hot-plug, unless hardware implements Cold Presence Detection.

hint.ahcich. X .sata_rev
setting to nonzero value limits maximum SATA revision (speed). Values 1, 2 and 3 are respectively 1.5, 3 and 6Gbps.
setting to nonzero value forces driver attach to some known AHCI-capable chips even if they are configured for legacy IDE emulation. Default is 1.


This driver provides the CAM(4) subsystem with native access to the SATA ports of AHCI-compatible controllers. Each SATA port found is represented to CAM as a separate bus with one target, or, if HBA supports Port Multipliers, 16 targets. Most of the bus-management details are handled by the SATA-specific transport of CAM. Connected ATA disks are handled by the ATA protocol disk peripheral driver ada(4). ATAPI devices are handled by the SCSI protocol peripheral drivers cd(4), da(4), sa(4), etc.

Driver features include support for Serial ATA and ATAPI devices, Port Multipliers (including FIS-based switching, when supported), hardware command queues (up to 32 commands per port), Native Command Queuing, SATA interface Power Management, device hot-plug and Message Signaled Interrupts.

Driver supports "LED" enclosure management messages, defined by the AHCI. When supported by hardware, it allows to control per-port activity, locate and fault LEDs via the led(4) API or emulated ses(4) device for localization and status reporting purposes. Supporting AHCI controllers may transmit that information to the backplane controllers via SGPIO interface. Backplane controllers interpret received statuses in some way (IBPI standard) to report them using present indicators.

AHCI hardware is also supported by ataahci driver from ata(4) subsystem. If both drivers are loaded at the same time, this one will be given precedence as the more functional of the two.


The ahci driver supports AHCI compatible controllers having PCI class 1 (mass storage), subclass 6 (SATA) and programming interface 1 (AHCI).

Also, in cooperation with atamarvell and atajmicron drivers of ata(4), it supports AHCI part of legacy-PATA + AHCI-SATA combined controllers, such as JMicron JMB36x and Marvell 88SE61xx.


activity LED device nodes
fault LED device nodes
locate LED device nodes


The ahci driver first appeared in FreeBSD 8.0.


Alexander Motin <mav@FreeBSD.org>.
July 25, 2012 FreeBSD