DDI_DEVICE_ACC_ATTR(9S) Data Structures for Drivers DDI_DEVICE_ACC_ATTR(9S)

ddi_device_acc_attr - data access attributes structure

#include <sys/ddi.h>
#include <sys/sunddi.h>

illumos DDI specific (illumos DDI)

The ddi_device_acc_attr structure describes the data access characteristics and requirements of the device.

ushort_t     devacc_attr_version;
uchar_t      devacc_attr_endian_flags;
uchar_t      devacc_attr_dataorder;
uchar_t      devacc_attr_access;

The devacc_attr_version member identifies the version number of this structure. The current version number is DDI_DEVICE_ATTR_V1.

The devacc_attr_endian_flags member describes the endian characteristics of the device. Specify one of the following values:


Data access with no byte swapping


Structural data access in big-endian format


Structural data access in little endian format

DDI_STRUCTURE_BE_ACC and DDI_STRUCTURE_LE_ACC describe the endian characteristics of the device as big-endian or little-endian, respectively. Although most of the devices have the same endian characteristics as their buses, examples of devices that have opposite endian characteristics of the buses do exist. When DDI_STRUCTURE_BE_ACC or DDI_STRUCTURE_LE_ACC is set, byte swapping is automatically performed by the system if the host machine and the device data formats have opposite endian characteristics. The implementation can take advantage of hardware platform byte swapping capabilities.

When you specify DDI_NEVERSWAP_ACC, byte swapping is not invoked in the data access functions.

The devacc_attr_dataorder member describes the order in which the CPU references data. Specify one of the following values.


Data references must be issued by a CPU in program order. Strict ordering is the default behavior.


The CPU can reorder the data references. This includes all kinds of reordering. For example, a load followed by a store might be replaced by a store followed by a load.


The CPU can merge individual stores to consecutive locations. For example, the CPU can turn two consecutive byte stores into one half-word store. It can also batch individual loads. For example, the CPU might turn two consecutive byte loads into one half-word load. DDI_MERGING_OK_ACC also implies reordering.


The CPU can cache the data it fetches and reuse it until another store occurs. The default behavior is to fetch new data on every load. DDI_LOADCACHING_OK_ACC also implies merging and reordering.


The CPU can keep the data in the cache and push it to the device, perhaps with other data, at a later time. The default behavior is to push the data right away. DDI_STORECACHING_OK_ACC also implies load caching, merging, and reordering.

These values are advisory, not mandatory. For example, data can be ordered without being merged, or cached, even though a driver requests unordered, merged, and cached together.

The values defined for devacc_attr_access are:


If an I/O fault occurs, the system will take the default action, which might be to panic.


Using this value indicates that the driver is hardened: able to cope with the incorrect results of I/O operations that might result from an I/O fault. The value also indicates that the driver will use ddi_fm_acc_err_get(9F) to check access handles for faults on a regular basis.

If possible, the system should not panic on such an I/O fault, but should instead mark the I/O handle through which the access was made as having faulted.

This value is advisory: it tells the system that the driver can continue in the face of I/O faults. The value does not guarantee that the system will not panic, as that depends on the nature of the fault and the capabilities of the system. It is quite legitimate for an implementation to ignore this flag and panic anyway.


This value indicates that an I/O fault is anticipated and should be handled as gracefully as possible. For example, the framework should not print a console message.

This value should be used when it is not certain that a device is physically present: for example, when probing. As such, it provides an alternative within the DDI access framework to the existing peek/poke functions, which don't use access handles and cannot be integrated easily into a more general I/O fault handling framework.

In order to guarantee safe recovery from an I/O fault, it might be necessary to acquire exclusive access to the parent bus, for example, or to synchronize across processors on an MP machine. "Cautious" access can be quite expensive and is only recommended for initial probing and possibly for additional fault-recovery code.

The following examples illustrate the use of device register address mapping setup functions and different data access functions.

Example 1 Using ddi_device_acc_attr() in ddi_regs_map_setup(9F)

This example demonstrates the use of the ddi_device_acc_attr() structure in ddi_regs_map_setup(9F). It also shows the use of ddi_get16(9F) and ddi_put16(9F) functions in accessing the register contents.

dev_info_t *dip;
uint_t     rnumber;
ushort_t  *dev_addr;
offset_t   offset;
offset_t   len;
ushort_t   dev_command;
ddi_device_acc_attr_t dev_attr;
ddi_acc_handle_t handle;

* setup the device attribute structure for little endian,
* strict ordering and 16-bit word access.
*/ dev_attr.devacc_attr_version = DDI_DEVICE_ATTR_V1; dev_attr.devacc_attr_endian_flags = DDI_STRUCTURE_LE_ACC; dev_attr.devacc_attr_dataorder = DDI_STRICTORDER_ACC; dev_attr.devacc_attr_access = DDI_DEFAULT_ACC; /*
* set up the device registers address mapping
*/ ddi_regs_map_setup(dip, rnumber, (caddr_t *)&dev_addr, offset, len,
&dev_attr, &handle); /* read a 16-bit word command register from the device */ dev_command = ddi_get16(handle, dev_addr); dev_command |= DEV_INTR_ENABLE; /* store a new value back to the device command register */ ddi_put16(handle, dev_addr, dev_command);

Example 2 Accessing a Device with Different Apertures

The following example illustrates the steps used to access a device with different apertures. Several apertures are assumed to be grouped under one single "reg" entry. For example, the sample device has four different apertures, each 32 Kbyte in size. The apertures represent YUV little-endian, YUV big-endian, RGB little-endian, and RGB big-endian. This sample device uses entry 1 of the "reg" property list for this purpose. The size of the address space is 128 Kbyte with each 32 Kbyte range as a separate aperture. In the register mapping setup function, the sample driver uses the offset and len parameters to specify one of the apertures.

ulong_t	*dev_addr;
ddi_device_acc_attr_t dev_attr;
ddi_acc_handle_t handle;
uchar_t buf[256];

* setup the device attribute structure for never swap,
* unordered and 32-bit word access.
*/ dev_attr.devacc_attr_version = DDI_DEVICE_ATTR_V1; dev_attr.devacc_attr_endian_flags = DDI_NEVERSWAP_ACC; dev_attr.devacc_attr_dataorder = DDI_UNORDERED_OK_ACC; dev_attr.devacc_attr_access = DDI_DEFAULT_ACC; /*
* map in the RGB big-endian aperture
* while running in a big endian machine
* - offset 96K and len 32K
*/ ddi_regs_map_setup(dip, 1, (caddr_t *)&dev_addr, 96*1024, 32*1024,
&dev_attr, &handle); /*
* Write to the screen buffer
* first 1K bytes words, each size 4 bytes
*/ ddi_rep_put32(handle, buf, dev_addr, 256, DDI_DEV_AUTOINCR);

Example 3 Functions That Call Out the Data Word Size

The following example illustrates the use of the functions that explicitly call out the data word size to override the data size in the device attribute structure.

struct device_blk {
	ushort_t	d_command;	/* command register */
	ushort_t	d_status;	/* status register */
	ulong	    d_data;		/* data register */
} *dev_blkp;
dev_info_t *dip;
caddr_t	dev_addr;
ddi_device_acc_attr_t dev_attr;
ddi_acc_handle_t handle;
uchar_t buf[256];

* setup the device attribute structure for never swap,
* strict ordering and 32-bit word access.
*/ dev_attr.devacc_attr_version = DDI_DEVICE_ATTR_V1; dev_attr.devacc_attr_endian_flags = DDI_NEVERSWAP_ACC; dev_attr.devacc_attr_dataorder= DDI_STRICTORDER_ACC; dev_attr.devacc_attr_access = DDI_DEFAULT_ACC; ddi_regs_map_setup(dip, 1, (caddr_t *)&dev_blkp, 0, 0,
&dev_attr, &handle); /* write command to the 16-bit command register */ ddi_put16(handle, &dev_blkp->d_command, START_XFER); /* Read the 16-bit status register */ status = ddi_get16(handle, &dev_blkp->d_status); if (status & DATA_READY)
/* Read 1K bytes off the 32-bit data register */
ddi_rep_get32(handle, buf, &dev_blkp->d_data,

See attributes(7) for descriptions of the following attributes:

Interface Stability Committed

attributes(7), ddi_fm_acc_err_get(9F), ddi_get16(9F), ddi_put16(9F), ddi_regs_map_setup(9F)

Writing Device Drivers

August 18, 2019 OmniOS