Introduction to Device Context
This section introduces device context and the context management model.
What Is a Device Context?
The context of a device is the current state of the device hardware.
The device driver manages the device context for a process on behalf of
the process. The driver must maintain a separate device context for each process
that accesses the device. The device driver has the responsibility to restore the
correct device context when a process accesses the device.
Context Management Model
Frame buffers provide a good example of device context management. An accelerated frame
buffer enables user processes to directly manipulate the control registers of the
device through memory-mapped access. Because these processes do not use traditional system calls,
a process that accesses the device need not call the device driver.
However, the device driver must be notified when a process is about to
access a device. The driver needs to restore the correct device context and
needs to provide any necessary synchronization.
To resolve this problem, the device context management interfaces enable a device driver
to be notified when a user process accesses memory-mapped regions of the device,
and to control accesses to the device's hardware. Synchronization and management of the
various device contexts are the responsibility of the device driver. When a user
process accesses a mapping, the device driver must restore the correct device context
for that process.
A device driver is notified whenever a user process performs any of the
Accesses a mapping
Duplicates a mapping
Frees a mapping
Creates a mapping
The following figure shows multiple user processes that have memory-mapped a device. The
driver has granted process B access to the device, and process B no
longer notifies the driver of accesses. However, the driver is still notified if
either process A or process C accesses the device.
Figure 11-1 Device Context Management
At some point in the future, process A accesses the device. The
device driver is notified and blocks future access to the device by
process B. The driver then saves the device context for process B. The
driver restores the device context of process A. The driver then grants access
to process A, as illustrated in the following figure. At this point, the
device driver is notified if either process B or process C accesses the
Figure 11-2 Device Context Switched to User Process A
On a multiprocessor machine, multiple processes could attempt to access the device at
the same time. This situation can cause thrashing. Some devices require a longer
time to restore a device context. To prevent more CPU time from being
used to restore a device context than to actually use that device context,
the minimum time that a process needs to have access to the
device can be set using devmap_set_ctx_timeout(9F).
The kernel guarantees that once a device driver has granted access to a
process, no other process is allowed to request access to the same device
for the time interval specified by devmap_set_ctx_timeout(9F).