I am looking for source code of interrupt service routine and searching net_bhi(); and netif_rx(); interrupt routine in the linux kernel. The above both api are the packet receiving of udp in the linux kernel. I want to modify interrupt routine as - I should calculate the timestamp when the interrupt occurs. So please someone help where is the location for the above file ??
Each network device (driver, really: the software that knows how to operate the device) will have its own interrupt service routine. The driver registers that routine's address with request_irq (in essence, "when this interrupt fires, call me here").
In the case of a network driver, the driver's interrupt routine will typically do little other than invoke a tasklet or softirq. This is to avoid running for long periods of time in a state that may block other critical interrupts.
In most modern network drivers, a softirq is actually triggered through a framework called NAPI. The driver will have registered its NAPI poll routine with netif_napi_add, and at interrupt time, the driver calls napi_schedule to communicate that its poll routine needs to run.
Once its tasklet or NAPI poll routine is invoked, the driver will access device registers to see why the device interrupted. If new packets are available, the driver will usually forward them to the linux TCP/IP stack with netif_rx or a variant thereof.
So, you'll have to choose where/how to record your timestamp. It would be easiest to do so in the tasklet or NAPI poll routine, but that may be some (possibly many) microseconds after the packet actually arrived. (Some delay between packet arrival and timestamp recording will be unavoidable in any case without specialized hardware.)
I am not sure about the path. But you must find it in /usr/src/linux-...
But if you want to have a time stamp printed with the interrupt, you can actually catch the interrupt using signal handlers, and then use gettimeofday() API to print the time.
Related
Is there any Hardware emulator which can generate hardware interrupt on Linux. I am looking to write device drivers that could process hardware interrupts, read or write into hardware memory, deferred work, top and bottom halves processing, etc. Basically, looking to learn complete device driver end to end. But what hurdle is - how to simulate hardware. Do I really need some hardware that could generate an interrupt. I went through book LDD3, but there they are using skull - a chunk of kernel space memory emulating as a hardware, but this cannot generate an interrupt, or it can? pls, throw some light.
The skull driver of LDD3 doesn't generate interrupts, because there's no actual hardware to generate them.
Device driver interrupts are a mechanism that allows the cpu to begin attending some other task because the action being performed will be handled by an asynchronous interrupt.
For example, a floppy disk drive interrupt's the cpu as each byte of a disk transfer is readin if no dma is in use. If DMA is being used, the disk will transfer directly to ram the bytes of the transfer until a full block (or a set of them) is actually transferred. Then some hardware interrupt will come in.
A serial interface interrupts your computer in a programmed basis. When a single character arrives, when a specific character arrives (let's say a \r char).
LDDP shows you how linux device drivers work..... but as the book cannot assume you have any concrete device, it is not capable of selecting a proper hardware to serve as usable (strange, as normally every pc has a parallel port or a serial port) I think LDDP3 has some driver using the parallel port, but you must continue reading the book before you start with interrupting hardware.
Asynchronous interrupts must be programmed into the device (the device must know that it has to generate an interrupt at the end of the transfer) so they have to be activated. For the interrupt to be properly catched, an interrupt handler must be installed before the first interrupt happens, or you'll get in an state in which no interrupt comes ever, because it arrived and was lost. And finally, interrupts have to be acknowledged. Once you have stored the data comming from the device, they have to be reactivated, so another interrupt can happen again. You need to learn that you have to protect your processes from accessing the data structures shared with an interrupt handler and how to do this. And all of this is explained in the book.... but you must read it, and don't stop in the skull driver which is the first driver developed in the book.
By the way, the kill(2) and sigaction(2) system calls of user mode are a very close approach into the world of hardware interrupts, because they are asynchronous, you can block them to occur, before entering a critical zone, and you can simulate them by kill(2)ing your process externally from another program. You will not see the difference, but instead of having a full system crash, you only get a hung process to kill.
Linux already contains all the interrupt handling for network data. don't have to do anything regarding this. Data arrives, Linux will process it (in the kernel) and pass it to the process waiting for the data. do not write interrupt handlers for network devices. You don't have to write an interrupt handler, because all the interrupt handlers needed are already provided by Linux. Just have your program read from the opened socket.
I want to know the time at which the kernel starts executing after the interruption. could some one help me how to know the time at which the kernel starts executing ??
how to copy the time when the interrupt occurs and send it back as a response to the client.
This time will change according to what the machine is currently doing, if it is in a critical section where the interruptions are masked it will wait. Hopefully these critical sections are short.
You can use a logic analyser to look at that (I did it a long long time ago on a Windows NT machine - pentium 100 MHz, and the usual interrupt latency was a few micro seconds, while with an IDE drive busy at the same time it was often 100 ms). I bet that with a recent machine and linux standard kernel it should always be a few microseconds, less than 30, but that's just a guess. Real time linux kernel will have a consistent response time.
I'm implementing a custom serial bus driver for a certain ARM-based Linux board (a custom UART driver, actually). This driver shall enable communication with a certain MCU on the other end of the bus via a custom protocol. The driver will not (and actually must not) expose any of its functions to the userspace, nor it is possible to implement it in userspace at all (hence, the need for the custom driver instead of using the stock TTY subsystem).
The driver will implement the communication protocol and UART reads/writes, and it has to export a set of higher-level functions to its users to allow them to communicate with the MCU (e.g. read_register(), drive_gpios(), all this stuff). There will be only one user of this module.
The calling module will have to wait for the completion of the operations (the aforementioned read_register() and others). I'm currently considering using semaphores: the user module will call my driver's function, which will initiate the transfers and wait on a semaphore; the IRQ handler of my driver will send requests to the MCU and read the answers, and, when done, post to the semaphore, thus waking up the calling module. But I'm not really familiar with kernel programming, and I'm baffled by the multitude of possible alternative implementations (tasklets? wait queues?).
The question is: is my semaphore-based approach OK, or too naïve? What are the possible alternatives? Are there any pitfalls I may be missing?
Traditionally IRQ handling in Linux is done in two parts:
So called "upper-half" is actual working in IRQ context (IRQ handler itself). This part must exit as fast as possible. So it basically checks interrupt source and then starts bottom-half.
"Bottom-half". It may be implemented as work queue. It is where actual job is done. It runs in normal context, so it can use blocking functions, etc.
If you only want to wait for IRQ in your worker thread, better to use special object called completion. It is exactly created for this task.
I'am working on a real-time control system that calculates the control signals in a buffered fashion (a user-mode program) and outputs to the usb device the array through isochronous transfers. The usb device them reports the execution progress through interrupt transfer, so that pc software can then calculate and push the next control array.
The software runs based on raw win32 api, C based. (C++ used only on not time sensitive parts of the program, such as interface, 3D models...).
I would like to know if there is a way to register a callback function in response to a interrupt transfer?
From what I understand, although we are talking about interrupt transfers, the USB device still has to be polled using libusb_interrupt_transfer:
Interrupt transfers are typically non-periodic, small device
"initiated" communication requiring bounded latency. An Interrupt
request is queued by the device until the host polls the USB device
asking for data.
Excerpt from https://www.beyondlogic.org/usbnutshell/usb4.shtml#Interrupt
I currently have a client listening for packets in its own thread. I was told to try to implement an ISR so that the packet received from the recv() call can be handled immediately, instead of waiting for that thread to get scheduled.
EDIT: this is in windows now, but it will ported to a DSP later.
ISRs by definition run in kernel space. Unless you are in an embedded system without memory protection, you will need to add kernel code to your project. Furthermore, to reimplement recv, it will need to handle IP and TCP or UDP as necessary to extract the data from the ethernet packets.
The overhead of rescheduling and switching to a thread is minimal, and needs to happen anyway unless the packet is handled entirely in the kernel. Most operating systems have a highest-priority thread setting, sometimes called "real-time," which causes user space code to run with minimal delay after the driver receives data. This is often used for audio/video I/O as well as networking.