illumos: manual page: srv.9e

SRV(9E) Driver Entry Points SRV(9E)

NAME

srv - service queued messages

SYNOPSIS

#include <sys/types.h>
#include <sys/stream.h>
#include <sys/stropts.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>

int prefixrsrv(queue_t *q); /* read side */

int prefixwsrv(queue_t *q); /* write side */

INTERFACE LEVEL

Architecture independent level 1 (DDI/DKI). This entry point is required
for STREAMS.

ARGUMENTS

q
Pointer to the queue(9S) structure.

DESCRIPTION

The optional service srv() routine may be included in a STREAMS module or
driver for many possible reasons, including:

o to provide greater control over the flow of messages in a
stream;

o to make it possible to defer the processing of some messages
to avoid depleting system resources;

o to combine small messages into larger ones, or break large
messages into smaller ones;

o to recover from resource allocation failure. A module's or
driver's put(9E) routine can test for the availability of a
resource, and if it is not available, enqueue the message for
later processing by the srv() routine.

A message is first passed to a module's or driver's put(9E) routine,
which may or may not do some processing. It must then either:

o Pass the message to the next stream component with
putnext(9F).

o If a srv() routine has been included, it may call putq(9F) to
place the message on the queue.

Once a message has been enqueued, the STREAMS scheduler controls the
service routine's invocation. The scheduler calls the service routines in
FIFO order. The scheduler cannot guarantee a maximum delay srv() routine
to be called except that it will happen before any user level process are
run.

Every stream component (stream head, module or driver) has limit values
it uses to implement flow control. Each component should check the
tunable high and low water marks to stop and restart the flow of message
processing. Flow control limits apply only between two adjacent
components with srv() routines.

STREAMS messages can be defined to have up to 256 different priorities
to support requirements for multiple bands of data flow. At a minimum, a
stream must distinguish between normal (priority zero) messages and high
priority messages (such as M_IOCACK). High priority messages are always
placed at the head of the srv() routine's queue, after any other enqueued
high priority messages. Next are messages from all included priority
bands, which are enqueued in decreasing order of priority. Each priority
band has its own flow control limits. If a flow controlled band is
stopped, all lower priority bands are also stopped.

Once the STREAMS scheduler calls a srv() routine, it must process all
messages on its queue. The following steps are general guidelines for
processing messages. Keep in mind that many of the details of how a srv()
routine should be written depend of the implementation, the direction of
flow (upstream or downstream), and whether it is for a module or a
driver.

1. Use getq(9F) to get the next enqueued message.

2. If the message is high priority, process (if appropriate) and
pass to the next stream component with putnext(9F).

3. If it is not a high priority message (and therefore subject to
flow control), attempt to send it to the next stream component
with a srv() routine. Use bcanputnext(9F) to determine if this
can be done.

4. If the message cannot be passed, put it back on the queue with
putbq(9F). If it can be passed, process (if appropriate) and
pass with putnext().

RETURN VALUES

Ignored.

WARNINGS

Each stream module must specify a read and a write service srv() routine.
If a service routine is not needed (because the put() routine processes
all messages), a NULL pointer should be placed in module's qinit(9S)
structure. Do not use nulldev(9F) instead of the NULL pointer. Use of
nulldev(9F) for a srv() routine can result in flow control errors.

March 15, 2019 SRV(9E)