# libATA Developer's Guide

## Introduction

libATA is a library used inside the Linux kernel to support ATA host controllers and devices. libATA provides an ATA driver API, class transports for ATA and ATAPI devices, and SCSI<->ATA translation for ATA devices according to the T10 SAT specification.

This Guide documents the libATA driver API, library functions, library internals, and a couple sample ATA low-level drivers.

## libata Driver API

struct ata_port_operations is defined for every low-level libata hardware driver, and it controls how the low-level driver interfaces with the ATA and SCSI layers.

FIS-based drivers will hook into the system with ->qc_prep() and ->qc_issue() high-level hooks. Hardware which behaves in a manner similar to PCI IDE hardware may utilize several generic helpers, defining at a bare minimum the bus I/O addresses of the ATA shadow register blocks.

### struct ata_port_operations

#### Disable ATA port

    void (*port_disable) (struct ata_port *);

Called from ata_bus_probe() and ata_bus_reset() error paths, as well as when unregistering from the SCSI module (rmmod, hot unplug).

#### Post-IDENTIFY device configuration

    void (*dev_config) (struct ata_port *, struct ata_device *);

Called after IDENTIFY [PACKET] DEVICE is issued to each device found. Typically used to apply device-specific fixups prior to issue of SET FEATURES - XFER MODE, and prior to operation.

#### Set PIO/DMA mode

    void (*set_piomode) (struct ata_port *, struct ata_device *);
    void (*set_dmamode) (struct ata_port *, struct ata_device *);
    void (*post_set_mode) (struct ata_port *ap);

Hooks called prior to the issue of SET FEATURES - XFER MODE command. dev->pio_mode is guaranteed to be valid when ->set_piomode() is called, and dev->dma_mode is guaranteed to be valid when ->set_dmamode() is called. ->post_set_mode() is called unconditionally, after the SET FEATURES - XFER MODE command completes successfully.

->set_piomode() is always called (if present), but ->set_dma_mode() is only called if DMA is possible.

#### Taskfile read/write

    void (*tf_load) (struct ata_port *ap, struct ata_taskfile *tf);
    void (*tf_read) (struct ata_port *ap, struct ata_taskfile *tf);

->tf_load() is called to load the given taskfile into hardware registers / DMA buffers. ->tf_read() is called to read the hardware registers / DMA buffers, to obtain the current set of taskfile register values.

#### ATA command execute

    void (*exec_command)(struct ata_port *ap, struct ata_taskfile *tf);

causes an ATA command, previously loaded with ->tf_load(), to be initiated in hardware.

#### Per-cmd ATAPI DMA capabilities filter

    int (*check_atapi_dma) (struct ata_queued_cmd *qc);

Allow low-level driver to filter ATA PACKET commands, returning a status indicating whether or not it is OK to use DMA for the supplied PACKET command.

#### Read specific ATA shadow registers

    u8   (*check_status)(struct ata_port *ap);
    u8   (*check_altstatus)(struct ata_port *ap);
    u8   (*check_err)(struct ata_port *ap);

Reads the Status/AltStatus/Error ATA shadow register from hardware. On some hardware, reading the Status register has the side effect of clearing the interrupt condition.

#### Select ATA device on bus

    void (*dev_select)(struct ata_port *ap, unsigned int device);

Issues the low-level hardware command(s) that causes one of N hardware devices to be considered 'selected' (active and available for use) on the ATA bus. This generally has no meaning on FIS-based devices.

#### Reset ATA bus

    void (*phy_reset) (struct ata_port *ap);

The very first step in the probe phase. Actions vary depending on the bus type, typically. After waking up the device and probing for device presence (PATA and SATA), typically a soft reset (SRST) will be performed. Drivers typically use the helper functions ata_bus_reset() or sata_phy_reset() for this hook.

#### Control PCI IDE BMDMA engine

    void (*bmdma_setup) (struct ata_queued_cmd *qc);
    void (*bmdma_start) (struct ata_queued_cmd *qc);
    void (*bmdma_stop) (struct ata_port *ap);
    u8   (*bmdma_status) (struct ata_port *ap);

When setting up an IDE BMDMA transaction, these hooks arm (->bmdma_setup), fire (->bmdma_start), and halt (->bmdma_stop) the hardware's DMA engine. ->bmdma_status is used to read the standard PCI IDE DMA Status register.

These hooks are typically either no-ops, or simply not implemented, in FIS-based drivers.

#### High-level taskfile hooks

    void (*qc_prep) (struct ata_queued_cmd *qc);
    int (*qc_issue) (struct ata_queued_cmd *qc);

Higher-level hooks, these two hooks can potentially supercede several of the above taskfile/DMA engine hooks. ->qc_prep is called after the buffers have been DMA-mapped, and is typically used to populate the hardware's DMA scatter-gather table. Most drivers use the standard ata_qc_prep() helper function, but more advanced drivers roll their own.

->qc_issue is used to make a command active, once the hardware and S/G tables have been prepared. IDE BMDMA drivers use the helper function ata_qc_issue_prot() for taskfile protocol-based dispatch. More advanced drivers implement their own ->qc_issue.

#### Timeout (error) handling

    void (*eng_timeout) (struct ata_port *ap);

This is a high level error handling function, called from the error handling thread, when a command times out. Most newer hardware will implement its own error handling code here. IDE BMDMA drivers may use the helper function ata_eng_timeout().

#### Hardware interrupt handling

    irqreturn_t (*irq_handler)(int, void *, struct pt_regs *);
    void (*irq_clear) (struct ata_port *);

->irq_handler is the interrupt handling routine registered with the system, by libata. ->irq_clear is called during probe just before the interrupt handler is registered, to be sure hardware is quiet.

#### SATA phy read/write

    u32 (*scr_read) (struct ata_port *ap, unsigned int sc_reg);
    void (*scr_write) (struct ata_port *ap, unsigned int sc_reg,
                       u32 val);

Read and write standard SATA phy registers. Currently only used if ->phy_reset hook called the sata_phy_reset() helper function.

#### Init and shutdown

    int (*port_start) (struct ata_port *ap);
    void (*port_stop) (struct ata_port *ap);
    void (*host_stop) (struct ata_host_set *host_set);

->port_start() is called just after the data structures for each port are initialized. Typically this is used to alloc per-port DMA buffers / tables / rings, enable DMA engines, and similar tasks.

->port_stop() is called after ->host_stop(). It's sole function is to release DMA/memory resources, now that they are no longer actively being used.

->host_stop() is called after all ->port_stop() calls have completed. The hook must finalize hardware shutdown, release DMA and other resources, etc.

## libata Library

!Edrivers/scsi/libata-core.c

## libata Core Internals

!Idrivers/scsi/libata-core.c

## libata SCSI translation/emulation

!Edrivers/scsi/libata-scsi.c !Idrivers/scsi/libata-scsi.c

## ata_piix Internals

!Idrivers/scsi/ata_piix.c

## sata_sil Internals

!Idrivers/scsi/sata_sil.c

## Thanks

The bulk of the ATA knowledge comes thanks to long conversations with Andre Hedrick (www.linux-ide.org), and long hours pondering the ATA and SCSI specifications.

Thanks to Alan Cox for pointing out similarities between SATA and SCSI, and in general for motivation to hack on libata.

libata's device detection method, ata_pio_devchk, and in general all the early probing was based on extensive study of Hale Landis's probe/reset code in his ATADRVR driver (www.ata-atapi.com).