STREAMS drivers have five different points of contact with the kernel:
These are the routines that allow the kernel to find the driver binary in the file system and load it into, or unload it from, the running kernel. The entry points include _init(9E), _info(9E), and _fini(9E).
The entry points allow the driver to determine a device's presence and initialize its state. These routines are accessed through the dev_ops(9S) data structure during system initialization. They include getinfo(9E), identify(9E), probe(9E), attach(9E), and detach(9E).
The table-driven entry points are accessed through cb_ops(9S), the character and block access table, when an application calls the appropriate interface. The members of the cb_ops(9S) structure include pointers to entry points that perform the device's functions, such as read(9E), write(9E), ioctl(9E). The cb_ops(9S) table contains a pointer to the streamtab(9S)structure.
These entry points are contained in the streamtab; they read and process the STREAMS messages that travel through the queue structures. Examples of STREAMS queue processing entry points are put(9E) and srv(9E).
These points of contact are discussed in the following sections.
As with other SunOS 5 drivers, STREAMS drivers are dynamically linked and loaded when referred to for the first time. For example, when the system is initially booted, the STREAMS pseudo-tty slave, pseudo-driver (pts(7D)) is loaded automatically into the kernel when it is first opened.
In STREAMS, the header declarations differ between drivers and modules. (The word module is used in two different ways when talking about drivers. There are STREAMS modules, which are pushable nondriver entities, and there are kernel-loadable modules, which are components of the kernel.). See the appropriate chapters in Writing Device Drivers.
The kernel configuration mechanism must distinguish between STREAMS devices and traditional character devices because system calls to STREAMS drivers are processed by STREAMS routines, not by the system driver routines. The streamtab pointer in the cb_ops(9S) structure provides this distinction. If it is NULL, there are no STREAMS routines to execute; otherwise, STREAMS drivers initialize streamtab with a pointer to a streamtab(9S) structure containing the driver's STREAMS queue processing entry points.
STREAMS drivers' initialization entry points must perform the same tasks as those of non-STREAMS drivers. See Writing Device Drivers for more information.
In non-STREAMS drivers, most of the driver's work is accomplished through the entry points in the cb_ops(9S) structure. For STREAMS drivers, most of the work is accomplished through the message-based STREAMS queue processing entry points.
Figure 9-1 shows multiple streams (corresponding to minor devices) connecting to a common driver. There are two distinct streams opened from the same major device. Consequently, they have the same streamtab and the same driver procedures.
Multiple instances (minor devices) of the same driver are handled during the initial open for each device. Typically, a driver stores the queue address in a driver-private structure "uniquely identified" by the minor device number. (The DDI/DKI provides a mechanism for uniform handling of driver-private structures; see ddi_soft_state(9F)). The q_ptr of the queue points to the private data structure entry. When the messages are received by the queue, the calls to the driver put and service procedures pass the address of the queue, allowing the procedures to determine the associated device through the q_ptr field.
STREAMS guarantees that only one open or close can be active at a time per major/minor device pair.
STREAMS device drivers have processing routines that are registered with the framework through the streamtab structure. The put
procedure is a driver's entry point, but it is a message (not system) interface. STREAMS drivers and STREAMS modules implement these entry points similarly, as described in "Entry Points".
The stream head translates write(2) and ioctl(2) calls into messages and sends them downstream to be processed by the driver's write queue put(9E) procedure. read is seen directly only by the stream head, which contains the functions required to process system calls. A STREAMS driver does not check system interfaces other than open and close, but it can detect the absence of a read indirectly if flow control propagates from the stream head to the driver and affects the driver's ability to send messages upstream.
For read-side processing, when the driver is ready to send data or other information to a user process, it prepares a message and sends it upstream to the read queue of the appropriate (minor device) stream. The driver's open routine generally stores the queue address corresponding to this stream.
For write-side (or output) processing, the driver receives messages in place of a write call. If the message cannot be sent immediately to the hardware, it may be stored on the driver's write message queue. Subsequent output interrupts can remove messages from this queue.
A driver is at the end of a stream. As a result, drivers must include standard processing for certain message types that a module might be able to pass to the next component. For example, a driver must process all M_IOCTL messages; otherwise, the stream head blocks for an M_IOCNAK, M_IOCACK, or until the timeout (potentially infinite) expires. If a driver does not understand an ioctl(2), an M_IOCNAK message is sent upstream.
Messages that are not understood by the drivers should be freed.
The stream head locks up the stream when it receives an M_ERROR message, so driver developers should be careful when using the M_ERROR message.
Most hardware drivers have an interrupt handler routine. You must supply an interrupt routine for the device's driver. The interrupt handling for STREAMS drivers is not fundamentally different from that for other device drivers. Drivers usually register interrupt handlers in their attach(9E)entry point, using ddi_add_intr(9F). Drivers unregister the interrupt handler at detach time using ddi_remove_intr(9F).
The system also supports software interrupts. The routines ddi_add_softintr(9F) and ddi_remove_softintr(9F) register and unregister (respectively) soft-interrupt handlers. A software interrupt is generated by calling ddi_trigger_softintr(9F).
See Writing Device Drivers for more information.
STREAMS drivers can prevent unloading through the standard driver detach(9E) entry point.