I want to compare the performance when we copy skb from kernel to userspace using DMA and normal skb_copy_datagram_iovec(). I create a module to test. May anyone show me how we can create a big buffer from userspace and pass it to my module (as tcp_recvmsg() is passed an iovec from userspace). Any suggestions are appreciated. Thanks in advance!
Write a Linux character device driver and implement the writev method. For details check out Linux Device Drivers latest edition.
Related
I'm writing a simple block dev driver to overcome some limitations with porting a previously hardware based RAID array to linux's software raid (mdadm).
This driver will create it's own block device, but proxy r/w requests to 1 or more other block devices (much like mdadm does already).
What is the best way for one kernel mode driver to read and write to another kernel mode (block device) driver?
[EDIT1]:
Ok, looking through the mdadm kernel module code - it looks like we need to do as the kernel does - use generic_make_request to the other disk drivers that handle the disks in the 'volume'. This avoids any user-mode filesystem block devices (/dev/xyz) to kernel mode device driver translations and keeps the I/O completely in kernel mode.
Now... How to obtain the bio handle from the several /dev/xyz strings passed to my module....
[EDIT2]:
Looking at this the wrong way, need to give my driver Major/Minors (translate the /dev/xyz in usermode and hand the dev_t value via ioctl to the driver, from there it can reference the driver.
Well on my way here, but still open to suggestions/recommendations.
The answer was to modify the BIO and re-send it off as I've done in this post:
https://unix.stackexchange.com/questions/171800/hp-smartarray-raid5-recovery-on-linux/171801#171801
I have booted a ubuntu on a ZedBoard. I want to transfer data between fpga and linux. For example, I want to write or read a register from linux. What is best way for doing it? I have not any idea.
Thanx.
First of all, you need to specifically say what you want to do, for example. if you want to access the IO signals on the FPGA, you need to first add the GPIO module to your system, synthesize and implement it.
Then you use the Linux GPIO Driver to access the port as it is explained in this page:
Linux GPIO Driver
The GPIO driver fits in the Linux GPIO framework which is not a char
mode driver. Yet it does provide access to the GPIO by user space
through the sysfs filesystem. This allows each GPIO signal to be read
and written in similar manner to a char mode device. The interface is
somewhat documented in the kernel tree at Documentation/gpio.txt. The
following text is aimed to augment, not replace the existing
documentation.
For other, more complex interfaces you need to create your own driver or use one of the drivers that are available and modify it to fit your needs.
I am a beginner of kernel programming. I just need some inspiration. I know I can write some functions in the kernel source, rebuild and reboot the kernel. The codes might be some hardware driver controlling the hardware. But how can our user space program use those functions?
I know through syscall the user space program can communicate with kernel space, and the loadable kernel module can also use the functions defined in kernel source code. But how can our user program achieve this?
PS： Now I am learning qemu-kvm. I know qemu is a user space program and kvm is kernel. I just want to figure out how qemu program uses kvm.
I know this is a very basic linux kernel programming problem but it confuses me for a long time. Can someone give me a hint? :>
You shouldn't insert a new syscall if you are programming a driver. New syscalls are usually a bad idea, you should have a very good reason to do it, and a hardware driver is not good one. You have to register your driver as char device, block device or network device. I recommend you the "Linux Device Driver" book (which is legally available on the Internet) to see examples of different type of drivers.
And about your question of how you can call a function in the kernel from userspace... there is no direct way to do it, you can't link userspace code with the kernel like you do with a library. First, you have to register your function as syscall and then call the syscall with the syscall() function.
Here is a good howto explaining it: http://www.tldp.org/HOWTO/html_single/Implement-Sys-Call-Linux-2.6-i386/
I'm working an a system with embedded Linux (Kernel 2.6.31).
It is a AT91SAM9G20 chip inside, and some of the Pins are forwarded to the outside.
Now I want to use them as GPIO Inputs.
I read the gpio.txt documentation about using the GPIOs via filesystem, and that works very well 'til here. I connected some switches to the gpio-pins and I can see the result in /sys/class/gpio/gpioX/value. But now I'd like to react on a change without busy-waiting in a loop. (i.e echo "Switch1 was pressed").
I guess I need interrupts here, but I couldn't find out how to use them without writing my own kernel driver. I'm relatively new to Linux and C (I normally program in Java), so I'd like to handle the Interrupts via sysfs too. But my problem is, that there is no "edge"-file in my GPIO directory (I guess because this is only since Kernel version 2.6.33+). Is that right? Instead of "edge" I've got a uevent file in there, which is not described in gpio.txt.
In the gpio.txt documentation there was a Standard Kernel Driver mentioned: "gpio_keys". Is it possible to use this for my problem?
I guess it would be better to work with this driver than allowing a userspace program to manipulate kernel tasks.
I found a lot of codesnippets for writing my own driver, but I wasn't even able to find out which of the 600 gpio.h files to include, and how to refer to the library (cross compiler couldn't find the gpio.h file).
Sorry for newbie questions, I hope you could give me some advices.
Thanks in advance
See this for an example on how to do that. Basically, the thing you're missing is the usage of the select or poll system calls.
i'm writing a char device in linux - xubuntu ,
and i'm wondering if i have to implement ioctl OR
maybe i can use the regular read write funcs??
thanks all,
Amit
The primary interface for a character device is the file functions, with ioctl serving extra, optional functionality.
If you're new to kernel module programming, you might want to check out the Linux Kernel Module Programming Guide.
The standard read/write functions are part of standard fops(file operations) structure in a character device driver model.If someone needs an additional functionality (which is user defined) and which is application dependent so you can go with ioctl() call.