Tammy Noergaard , in Embedded Systems Architecture (2d Edition), 2013
8.5 Summary
This chapter discussed device drivers, the blazon of software needed to manage the hardware in an embedded system. The chapter besides introduced a general set of device driver routines, which make up almost device drivers. Interrupt handling (on the PowerPC platform), retentiveness management (on the PowerPC platform), I2C bus (on a PowerPC-based platform), and I/O (Ethernet and RS-232 on PowerPC and ARM-based platforms) were real-world examples provided, along with pseudocode to demonstrate how device driver functionality tin can be implemented.
The next chapter, Chapter ix, Embedded Operating Systems, is an introduction to the technical fundamentals of embedded operating systems and their part within a design.
Peng Zhang , in Avant-garde Industrial Control Engineering, 2010
(1) Device driver content
Device drivers brand up the major function of all operating systems kernels. Like other parts of the operating system, they operate in a highly privileged surroundings and can crusade disaster if they get things wrong. Device drivers control the interaction between the operating system and the device that they are controlling. For instance, the file system makes employ of a general block device interface when writing blocks to an IDE disk. The driver takes care of the details and makes device-specific things happen. Device drivers are specific to the controller chip that they are driving, which is why, for example, you demand the NCR810 SCSI driver if your system has an NCR810 SCSI controller.
Every device commuter has two important data structures; the device information structure and the static structure. These are used to install the device commuter and to share information amid the entry point routines. The device information structure is a static file that is passed to the install entry betoken. The purpose of the information structure is to laissez passer the data required to install a major device into the install entry point where information technology is used to initialize the static construction. The static structure is used to pass data between the different entry points, and is initialized with the information stored in the information structure. The operating system communicates with the driver through its entry point routines.
Colin Walls , in Embedded Software (Second Edition), 2012
7.6.1 The Two Sides of a Device Driver
Most device drivers are written to back up interrupts. Truthful, device drivers can be more efficiently written without interrupts; even so, the reality is that most are written to back up them. Since information technology is easier to write not-interrupt-driven device drivers, if you learn how to write one that uses them, all other implementations are petty.
Let's quickly review the basics of a device commuter: initialize, send, receive, control, and close. There, that was rather painless! When you get right down to it, at that place is not much to a device driver in most cases. Sure, some extra error processing might need to be done, or you may want to provide an ioctl() function, but what information technology all boils downwardly to is that yous are sending and receiving data. Not also complicated! So why do we get all tied up in knots when nosotros have to write a device driver? Well, it's generally because, unless y'all have some sophisticated tools, y'all cannot see what the device is doing until information technology works! Nevertheless, with the right framework, making a device work apace is quite straightforward.
So what are the ii sides? I side interfaces with the tasking surround, and the other side is an interrupt service routine (ISR) and its related processing. What are the potential problems with coordinating the two sides? One is information abuse, and the other is task scheduling. If the ISR is manipulating pointers into the receive buffer, and those pointers are not protected from dual access, then your buffer data or control structures can be corrupted. Likewise, if y'all do not properly handle scheduling issues, a task waiting for a device driver service can bring a system to its knees.
Peng Zhang , in Industrial Control Technology, 2008
5.2.three.2 Driver Content
Every device driver has ii important data structures; the device information structure and the static structure. These are used to install the device commuter and to share information among the entry point routines. The device information structure is a static file that is passed to the install entry point. The purpose of the information construction is to pass the information required to install a major device into the install entry point where it is used to initialize the static construction. The static construction is used to pass information between the unlike entry points and is initialized with the information stored in the information structure. The operating system communicates with the commuter through its entry point routines.
Joseph Yiu , in The Definitive Guide to Arm® Cortex®-M0 and Cortex-M0+ Processors (Second Edition), 2015
3.v.3 Organization of the CMSIS-Core
A CMSIS compliant device commuter contains the following:
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Cadre Peripheral Access Layer—Name definitions, address definitions, and helper functions to admission core registers and core internal peripherals similar the NVIC and SysTick timer.
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Device Peripheral Access Layer (MCU specific)—Annals name definitions, accost definitions, and device driver code to access peripherals.
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Access Functions for Peripherals (MCU specific)—Optional helper functions for peripherals. Note that another CMSIS project chosen CMSIS-Driver is ongoing to create a common peripheral API to enable awarding code and middleware to be adult for multiple microcontroller platforms.
The office of these layers is illustrated in Figure 3.xix.
ANDREW N. SLOSS , ... CHRIS WRIGHT , in ARM System Developer's Guide, 2004
eleven.2.4.2 SWI Exception
Whenever a device driver is chosen from an application, the phone call goes through the SWI handler mechanism. The SWI educational activity forces the processor to change from user to SVC mode.
The core SWI handler is shown here. The commencement activity of the handler is to store registers r0 to r12 to the SVC stack.
The adjacent action calculates the address of the SWI instruction and loads that education into annals r10. The SWI number is obtained by masking the top 8 $.25. The address of the SVC stack is and so copied into register r1 and is used equally the second parameter when calling the SWI C handler.
The spsr is and then copied to register r2 and stored on the stack. This is only required when a nested SWI telephone call occurs. The handler then jumps to the code that calls the C handler routine.
The code that follows the BL teaching returns back to the callee plan as shown here. This is achieved by restoring the spsr from the stack and loading all the user banked registers back, including the pc.
The link register has been set in the BL didactics. This lawmaking is executed when the SWI C handler is complete.
The C handler, eventsSWIHandler, shown in Effigy 11.3, is chosen with register r0 containing the SWI number and register r1 pointing to the registers stored on the SVC stack.
Jagdish Gediya , ... Zening Wang , in Software Applied science for Embedded Systems (Second Edition), 2019
1.7.3.iv User Infinite Drivers
Traditionally, device drivers were adult in the Linux kernel. Some of the drivers were complex and when these buggy drivers misbehaved, past accessing memory which a driver was non supposed to access, the outcome was catastrophic—the unabridged Linux kernel could hang or crash. This was a trouble—no 1 wanted a crashed/hung arrangement, not even during the software development phase.
There was one further challenge—users didn't want the source code of their driver or software to be publicly available as brash by Linux'southward GPL license. Hence, such users started looking for alternative ways of using their drivers on Linux-based systems.
User space drivers came to the rescue in these scenarios. In the case of user space drivers, the driver software runs in user space as an application plan. User space software doesn't fall under GPL so the user can retain the privacy of their source lawmaking in their modules. Likewise, if the user space driver tries to misbehave by accessing unauthorized regions, Linux detects this and prevents the driver from doing so.
Linux makes this possible by exposing a specific device'southward configuration space (mostly, memory-mapped device configuration registers) and DMA-capable RAM to user space. Since a user space commuter can access its device's memory space and DMA-capable regions without involving the Linux kernel, these drivers are very useful in example the device processes a lot of traffic. For case, Ethernet interfaces demand huge amounts of network-level processing to cater to network traffic at line rate. In such cases user infinite network drivers and stacks are condign popular in the open-source world—DPDK, an open up-source project, uses user space drivers to provide maximum throughput. Interrupt handling is a claiming as interrupts force the CPU to enter supervisory mode—hence interrupt routines cannot be implemented completely in user space. The solution to this outcome is to have a small Linux kernel driver for the device with an extremely small interrupt subroutine to just notify the user space driver of the interrupt event. Another approach could be to disable the interrupt and allow the user space commuter practise the polling for events. Some solutions use a mix of the ii approaches: (ane) during loftier-traffic conditions, utilise polling mode in user infinite and keep the interrupt disabled; and (two) during scarce traffic, enable the interrupt and expect for notification from the kernel driver of the device.
The Linux kernel provides "UIO" and "vfio" frameworks to help develop user space drivers.
Tammy Noergaard , in Demystifying Embedded Systems Middleware, 2010
2.5 Operating Systems and Device Driver Access for Middleware
While middleware can access device drivers directly, as introduced in the previous department, an embedded OS can as well include an abstraction layer API that allows for device driver access. When providing device access, or any blazon of I/O access to middleware, virtually Os APIs categorize their associated device drivers as some combination of:
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Character, a driver that allows hardware access via a (graphic symbol) byte stream
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Block, a driver that allows hardware admission via some smallest addressable set up of bytes at whatever given time
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Network, a commuter that allows hardware admission via data in the form of networking packets
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Virtual, a commuter that allows I/O access to virtual (software) devices
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Miscellaneous Monitor and Control, a driver that allows I/O access to hardware that is non accessible via the other categories higher up.
For an example of an Os block device interface, vxWorks provides an I/O interface, called CBIO (cache blocked input output), that allows different file systems, such as JFS, dosFS, etc., to exist ported to one standard vxWorks interface regardless of the underlying hardware storage medium (see Figures 2.11d and 2.11e). Every bit stated in the previous department, to port JFS to an unsupported Os, such as vxWorks in this instance, requires replacing the current OS-specific calls, such as the Linux-specific code shown in Effigy ii.11c, with the vxWorks specific code and utilizing the CBIO library throughout the JFS source.
Figure 2.11d. Example of vxWorks File Organisation Interface
Figure 2.11e. vxWorks CBIO Library13
In vxWorks, calling some of the CBIO APIs is role of the process of setting upward a file system, such as dosFS on a hard disk, floppy bulldoze or any other storage medium accessed equally a block device under vxWorks.
As shown in Effigy ii.11f, when utilizing the CBIO APIs in vxWorks an example process is every bit follows:
Figure ii.11f. Logical Layers of CBIO-based vxWorks System13
Pace ane. Configure vxWorks to support the:
•
Cake Device
•
CBIO Library
•
File System, i.e., dosFS.
Effigy 2.11g. Example of Configuring vxWorks12
Stride 2. Create the Block Device.
Figure ii.11h. Example of Creating Block Device in vxWorks12
Stride 3. Create the CBIO Block Driver Wrapper. The CBIO block driver wrapper layer wraps the block driver with a CBIO API compatible layer using the cbioWrapBlkDev() function.
Figure 2.11i. CBIO Cake Device Wrapper in vxWorks1
Pace four. Create the CBIO Cache Layer
Figure two.11j. CBIO Cache Layer Using vxWorks CBIO Library
.
Step 5. Implement CBIO Partition Director.
Effigy 2.11k. CBIO Partition Layer Using vxWorks CBIO Library12
An case of source lawmaking using the CBIO APIs in vxWorks is shown in Figure 2.11l.
Figure two.11l. vxWorks CBIO APIs Source Code Case
Peng Zhang , in Industrial Command Technology, 2008
7.3.i.3 PCI I/O and PCI Memory Addresses
The devices to communicate with their device drivers running in the kernel on the CPU utilise these two address spaces. For example, the DEC chipset 21141 fast Ethernet device maps its internal registers into PCI I/O infinite. Its device driver then reads and writes those registers to control the device. GUI drivers typically use large amounts of PCI memory infinite to comprise GUI information. Until the PCI organization has been prepare and the device's access to these accost spaces has been turned on using the Control field in the PCI configuration header, nothing tin access them. Information technology should be noted that simply the PCI configuration code reads and writes PCI configuration addresses; the device drivers only read and write PCI I/O and PCI memory addresses (this is again left to the device driver's policies).
David Kleidermacher , in Real World Multicore Embedded Systems, 2013
I/O virtualization within microkernels
As discussed before, virtual device drivers are normally employed by microkernel-style operating systems. Microkernel-based hypervisors are as well well suited to secure I/O virtualization: instead of the typical monolithic approach of placing device drivers into the hypervisor itself or into a special-purpose Linux guest operating system (the dom0 method described earlier), the microkernel-based hypervisor uses small, reduced-privilege, native processes for device drivers, I/O multiplexors, health managers, ability managers, and other supervisory functions required in a virtualized environment. Each of these applications is provided only the minimum resources required to reach its intended function, fostering robust embedded organisation designs. Effigy 7.xiv shows the arrangement-level architecture of a microkernel-based hypervisor used in a multicore networking application that must securely manage Linux control-plane functionality aslope high-throughput, depression-latency data-plane package processing within virtual device drivers.
Figure 7.xiv. Virtual device drivers in microkernel-based organisation virtualization architecture.
Without virtualization, the above application could be implemented with a dual Linux/RTOS configuration in which the control- and data-plane operating systems are statically bound to a set of independent cores. This is referred to equally an asymmetric multiprocessing (AMP) approach. One advantage of virtualization over an AMP sectionalisation of labor is the flexibility of irresolute the allocation of control- and data-plane workloads to cores. For example, in a normal mode of operation, the builder may just want to employ a single core for control and all other cores for data processing. Yet, the arrangement can be placed into management style in which Linux needs four cores (SMP) while the data processing is temporarily express. The virtualization layer can handle the reallocation of cores seamlessly under the hood, something that a static AMP system cannot back up.
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