February 2012 - kernelnewbies@kernelnewbies.org

Regarding Kernel development
by shubham Feb. 14, 2012

Feb. 14, 2012

Hi All, I want to learn how Linux kernel is developed . It would be great if somebody can guide from where I should start and what should I study. I want to understand it clearly so that I can develop some kernel modules too. I have knowledge of C language and as far as Linux knowledge is concerned , I am Red Hat Certified Engineer (RHCE). So, it would be great if anybody can guide me on Kernel Development for my understanding level. Thank you in advance. Best Regards, Shubham Sharma RHCE,SCJP

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Question on registering driver fops
by Ezequiel García Feb. 14, 2012

Feb. 14, 2012

Hi, I noticed that after registering a video driver with "video_register_device" the "open" function gets called. The registration is like: peasycap->video_device.fops = &v4l2_fops; peasycap->video_device.minor = -1; peasycap->video_device.release = (void *)(&videodev_release); video_set_drvdata(&(peasycap->video_device), (void *)peasycap); First question: who calls it and why does it open the device? Second question. This is a common kernel interface, driver fills a "fops" struct and then registers. This means I need to be *always* prepared to get a call on "open" (or other file operation) immediately after driver registration? Thanks, Ezequiel.

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Programming program individual hardware CPU performance counters with desired hardware events
by utsav kanani Feb. 14, 2012

Feb. 14, 2012

Hi, I searched a lot but was not able to find this information. What tool can i use to program individual hardware CPU performance counters with desired hardware events. For eg. say i want to program General Purpose Counter 3 on intel CPU to monitor Retired Instructions. Is this possible using Oprofile or Linux perf? If yes can you point me to the correct documentation or give an example on how to do this. If no is there any other tool available which is capable of going to that granularity. Thanks, UK

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Does the mq_timedreceive() fully implement the POSIX specification?
by K K Feb. 14, 2012

Feb. 14, 2012

Hi All, I am doing POSIX test on linux. And for mq_timedreceive() in POSIX spec 2008 Issue 7, Line 43787: The validity of the abstime parameter need not be checked if a message can be removed from the message queue immediately. But when I run test case mq_timedreceive/10-2 of POSIX suite (can be viewed at : http://ltp.git.sourceforge.net/git/gitweb.cgi?p=ltp/ltp.git;a=blob;f=testca…) mq_timedreceive() could get the message without wait, but the timeout is still validated. Do we intend to do so, or the implementation needs update? Thanks and Regards, Kai

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Get USB Device Class Type and Mount point
by Abhijit Pawar Feb. 14, 2012

Feb. 14, 2012

Hi , Is there any way we can get the class type of the attached USB device through struct usb_device in a kernel module? I tried getting the device descriptor and interface descriptor and then use them to get the Device and Interface Class and Subclass. struct usb_device_descriptor *descriptor = &usbDev->descriptor; struct usb_interface_descriptor *idesc = to_usb_interface(usbDev); Whenever I run my kernel module, I get 0 for DeviceClass and Interface Class. I checked the meaning and it says its the information and to be gathered through interface. Am I missing something here? What I want is to get the mount point / device path of the USB devices. Like for Storage Device I would like to get its mount path and for HID I would like to get its device path. Regards, Abhijit Pawar

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How to measure performance inside Kernel?
by Peter Senna Tschudin Feb. 14, 2012

Feb. 14, 2012

Dear list, I'm looking for a way to compare the performance of two different codes inside Kernel. I was able to do some comparison on user land but I want to test the specific portion of code inside Kernel. At line 1195 of drivers/media/video/videobuf2-core.c: /* * Reinitialize all buffers for next use. */ for (i = 0; i < q->num_buffers; ++i) q->bufs[i]->state = VB2_BUF_STATE_DEQUEUED; With: /* buf2 */ /* * Reinitialize all buffers for next use. */ buf_ptr_end = q->bufs[q->num_buffers]; for (buf_ptr = q->bufs[0]; buf_ptr < buf_ptr_end; ++buf_ptr) buf_ptr->state = VB2_BUF_STATE_DEQUEUED; To test on user land I've created two separate C source codes and compiled with gcc -O2, then used the "perf" tool on the entire application. With num_buffers = 131072: $ perf stat -e cycles,stalled-cycles-frontend,stalled-cycles-backend,cache-references,cache-misses -r 2048 ./buf1 Performance counter stats for './buf1' (2048 runs): 16,538,039 cycles #0.000 GHz (+-0.06%)[80.23%] 6,917,411 stalled-cycles-frontend#41.83% frontend cycles idle(+-0.14%)[80.25%] 4,686,384 stalled-cycles-backend #28.34% backend cycles idle(+-0.14%)[80.28%] 148,990 cache-references (+-0.38%)[80.24%] 71,180 cache-misses #47.775 % of all cache refs (+-0.22%)[88.14%] 0.005234340 seconds time elapsed $ perf stat -e cycles,stalled-cycles-frontend,stalled-cycles-backend,cache-references,cache-misses -r 2048 ./buf2 Performance counter stats for './buf2' (2048 runs): 14,740,563 cycles #0.000 GHz (+-0.04%)[77.89%] 5,187,716 stalled-cycles-frontend#35.19% frontend cycles idle(+-0.14%)[77.81%] 3,383,748 stalled-cycles-backend # 101,894 cache-references (+-0.23%)[84.60%] 66,647 cache-misses #65.408 % of all cache refs (+-0.14%)[90.52%] 0.004661826 seconds time elapsed (+-0.06%) But I want to repeat the tests on specific portion of code, not on entire application. Is there a safe way of do something like: start_bench ( ?? ); /* start measurement */ buf_ptr_end = q->bufs[q->num_buffers]; for (buf_ptr = q->bufs[0]; buf_ptr < buf_ptr_end; ++buf_ptr) buf_ptr->state = VB2_BUF_STATE_DEQUEUED; end_bench ( ?? ); /* end measurement */ And is this the correct approach for testing the performance of specific portion of Kernel code? Thank you! Peter -- Peter Senna Tschudin peter.senna(a)gmail.com gpg id: 48274C36

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Recompile the Library of the Linux Kernel
by LI LEI Feb. 13, 2012

Feb. 13, 2012

Hello, everyone, I am a beginner to learn about the linux kernel. I met the following problem today: I created a C file called "drv_event.c", and put it into the location "/lib", since I want to link it to the library of the kernel, i.e., *lib.a*. I also put the header file "drv_event.h" into the directory "/include/linux". I recompile the directory "/lib", and my own object "drv_event.o" was successfully linked into the *lib.a*. However, I called one of the functions defined in the "drv_event.c" in my driver, and I compiled this driver, at the link time, I got the warning which shows that the function called is undefined. It seems although I do recompile the lib.a successfully, but I did not put the new one into the right place. I tried "make install lib" and also "make install" in the kernel source. However, I still got the same warning at the link time. So does someone can help me to get rid of this problem? Thanks so much. Best, Li

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difference between ACLs and SElinux
by bharat dhaker Feb. 13, 2012

Feb. 13, 2012

Hi all, I want to know the differences between ACLs and SElinux. Does anyone know which file-systems supports SElinux? -- Bharat Dhaker Senior Software Engineer Memory Solutions Group Samsung India Software Operations, Bangalore Phone- 7676390877

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cross compile linux
by chromaticwt zac Feb. 12, 2012

Feb. 12, 2012

what is the proper way to cross compile the linux kernel from one arch to another? specifically, i386 -> ppc.

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function of major/minor device number
by hz hanks Feb. 12, 2012

Feb. 12, 2012

Hi, all I have a question about major/minor device number. I know that one can use int register_chrdev_region(dev_t from, unsigned count, const char *name); to register a driver with a unique pair of major/minor device number. But what's the difference between two drivers: in one case they have same major device number but different minor device number; on another case they have different major device number but same minor device number. When I searched the Internet, it said that major device number is to identify the driver while the minor device number is to identify specific hardware. But I'm still confused because it seems legal to share the major device number with different drivers. Please help me！Thank you!

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