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

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

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

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

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

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),

Writing Device Drivers

illumos August 18, 2019 DDI_DEVICE_ACC_ATTR(9S)