- Chapter 6 activity 6.2 drivers ed hidden message how to#
- Chapter 6 activity 6.2 drivers ed hidden message driver#
- Chapter 6 activity 6.2 drivers ed hidden message code#
- Chapter 6 activity 6.2 drivers ed hidden message simulator#
The function is defined for every platform, and it always returns 0 on Get_cycles, and was introduced during 2.1 Headers offer an architecture-independent function that you can use Some of the other platforms offer similar functionalities, and kernel These lines, for example, measure the execution of the instruction To access the whole register if the time lapse you are benchmarking Will overflow a 32-bit counter once every 8.5 seconds you won't need Variable and is sufficient in most cases. Variables the latter reads the low half of the register into a 32-bit The former atomically reads the 64-bit value into two 32-bit "machine-specific registers''), you can use one of these macros: It is a 64-bit register that counts CPU clockĬycles it can be read from both kernel space and user space. Introduced in x86 processors with the Pentium and present in allĬPU designs ever since. The most renowned counter register is the TSC (timestamp counter), Get the work done without claiming exclusive ownership of the register You can always measure differences using unsigned variables, you can Strongly discourage resetting it, even when hardware permits. Whether or not the register can be zeroed, we
May be 64 or 32 bits wide - in the latter case you must be prepared May not be readable from user space, it may or may not be writable, and it
The details differ from platform to platform: the register may or Is steadily incremented once at each clock cycle. Most modern processors thus include a counter register that To count clock cycles as an easy and reliable way to measure time Various levels of cache memories, CPU manufacturers introduced a way Unpredictability of instruction timing in CPU designs caused by the In response to the extremely high speed of modern processors, the pressingĭemand for empirical performance figures, and the intrinsic Given the inherent unpredictability of instruction timing on most systems (due to instruction scheduling, branch prediction, and cache memory), this clock counter is the only reliable way to carry out small-scale timekeeping tasks. Most modern CPUs include a high-resolution counter that is incremented every clock cycle this counter may be used to measure time intervals precisely. Resources, selecting precision over portability. High precision in your figures, you can resort to platform-dependent If you need to measure very short time intervals or you need extremely HZ, by virtue of our complete trust in the kernelĭevelopers, who have certainly chosen the best value.
Timer interrupt is to keep the default value for All in all, however, the best approach to the The value of HZ to get better response times theyĪre willing to pay the overhead of the extra timer interrupts toĪchieve their goals. People using Linux for hard real-time tasks have been known to raise Who want systems with a different clock interrupt frequency. It is possible to change the value of HZ for those Jiffies overflows, but it is good to be aware of
Chapter 6 activity 6.2 drivers ed hidden message driver#
Driver writers do not normally have to worry about That the kernel operates properly when jiffies Volatile, and will possibly overflow after a long time ofĬontinuous system operation (but no platform features jiffy overflow Is initialized to 0 when the system boots, and is thus the number ofĬlock ticks since the computer was turned on. Despite what your preferred platform uses, no driver writer shouldĮvery time a timer interrupt occurs, the value of the variable
Chapter 6 activity 6.2 drivers ed hidden message simulator#
Platforms, but some platforms use 1024, and the IA-64 simulator usesĢ0. Linux versions define HZ to be 100 for most Value of HZ, which is an architecture-dependent Regular intervals this interval is set by the kernel according to the Timer interrupts are generated by the system's timing hardware at Issues are covered in Chapter 9, "Interrupt Handling".
Chapter 6 activity 6.2 drivers ed hidden message code#
Interrupts are asynchronous events that are usually fired byĮxternal hardware the CPU is interrupted in its current activityĪnd executes special code (the Interrupt Service Routine, or ISR) The mechanism the kernel uses to keep track of time intervals. The first point we need to cover is the timer interrupt, which is Scheduling asynchronous functions to happen after a specified time Dealing with time involves theįollowing, in order of increasing complexity:ĭelaying operation for a specified amount of time On some of the kernel resources that are available, starting with how In the next few chapters we'll fill in information The kernel makes a number of facilities available to ease the task of Timing, memory management, hardware access, and more. The necessary operations they have to deal with issues such as Real-world drivers, however, need to do more than implement
Chapter 6 activity 6.2 drivers ed hidden message how to#
Linux Device Drivers, 2nd Edition By Alessandro Rubini & Jonathan CorbetĪt this point, we know the basics of how to write a full-featured char Linux Device Drivers, 2nd Edition: Chapter 6: Flow of Time
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