Motorola MCP750 Specifications download pdf (Page 176)

evice Driver Programming

11-2

• Linked lists, state ﬁelds, and other data structures used by the driver

• Global variables

• Hardware registers

As indicated in the previous section, the mechanisms that can be used are spin locks, sleep

locks, and synchronization variables.

When multithreading a driver, the ﬁrst step is to check the areas of the program level code

that raise IPL. These areas are protecting device driver structures from being corrupted by

program level/interrupt level interactions. A driver's program-level code can, for example,

be adding a message to a queue while its interrupt-level code is attempting to remove a

message from the same queue. The program-level code might be similar to the following:

message_qp = message_Q;

message_qp->next = new_message_p;

The interrupt-level code might be similar to the following:

message_qp = message_Q;

message_Q = message_qp->next;

If an interrupt occurs between the two lines of code at program level and then the two lines

of code are executed at interrupt level, the new message added at program level is lost. To

prevent this problem from occurring, the queue structures can be protected by using the

LOCK and UNLOCK facilities (see the “Spin Locks” section, p. 11-5). The program-level

code can be changed as follows:

old_ipl = LOCK(Q_lock, plhi);

message_qp = message_Q;

message_qp->next = new_message_p;

UNLOCK(Q_lock, old_ipl);

The interrupt-level code can be changed as follows:

old_ipl = LOCK(Q_lock, plhi);

message_qp = message_Q;

message_Q = message_qp->next;

UNLOCK(Q_lock, old_ipl);

By using the spin locks, you protect program level from interrupt level and interrupt level

from program level.

Any areas of code that raise the IPL to protect code must be protected via a spin lock. The

spin lock must be locked at both program and interrupt level. Simply raising the IPL is not

good enough protection because raising the IPL does not prevent an interrupt from occur-

ring on another CPU.

The next step in multithreading a driver is to look at the data structures that the driver uses.

The most common data structures that need coordination are linked lists and state fields.

Any time items are added or removed from linked lists the operation must be protected.

State ﬁelds are often bit ﬁelds that are set to indicate a current condition in the driver. The

setting and resetting of these bits is often done because of asynchronous events. Changes

to these state ﬁelds must be protected as well as checks on the state ﬁeld. If the execution

1 2 ... 171 172 173 174 175 176 177 178 179 180 181 ... 343 344

Comments to this Manuals

No comments

Motorola MCP750 Specifications Page 176

Comments to this Manuals

Related products and manuals for PC/workstation barebones Motorola MCP750