June 2014 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Building a software serial driver
by Torrie Fischer June 25, 2014

June 25, 2014

Greetings fellow newbies. I'm toying around with a Raspberry PI and trying to create a driver that takes a single GPIO pin and turns it into a read-only TTY device because using the userspace GPIO sysfs API is too slow for me. Here's my current code: https://github.com/tdfischer/gpio_serial It is based off of the tiny_serial example driver in the LDD3 repo: https://github.com/martinezjavier/ldd3/blob/master/tty/tiny_serial.c It builds and loads just fine, and even creates a /dev/ttyGPIO0 device. However, reading from it returns -EIO. Running strace on cat shows that it opens up the device successfully, but fails on the first call to read(). Any idea what I'm missing here? I'm fairly certain it is something obvious.

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Getting started with kernel programming..
by Murtaza Amravatiwala June 24, 2014

June 24, 2014

I have been reading about Linux kernel from LLD3 and famous book from Robert Love. Now I want to get into the kernel code. How should I start contributing to the main kernel? Where should I start from? And If I am planning to take kernel programming as my career what should I keep track of? Please help me.

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bio_endio error codes
by Alvin Abitria June 23, 2014

June 23, 2014

Hello Gurus, Please disregard my earlier mail due to poor formatting and readability. I'm developing a block driver using the make_request method, effectively bypassing existing scsi or request stack in block layer. So that means im directly working with bios. As prescribed in linux documentation and from referring to similar drivers in kernel, you close a session with a bio with the bio_endio function. I usually invoke bio_endio during successful I/O completion, meaning with an error code of zero. But there are cases that this is not fulfilled or there are error cases. My question is, what are the valid error codes that can be used with it? My initial impression is that other than zero as error code, bio_endio will fail. I've read somewhere that -EBUSY is not recognized, and I tried -EIO but my driver crashed. I got a panic in some dio_xxx function leading from bio_endio(bio,-EIO). I would like to block subsequent bios sent to me after reaching my queue depth and with no tags left, and so I want to use bio_endio with an error code. What are those error codes, and will they work for my intended function? Or is there a better way aside from bio_endio? Thanks! Sent from Samsung Mobile

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TASK05
by Akam June 23, 2014

June 23, 2014

Hello, I am stuck on Task05. Can you help me with more pointers for this task ? So I can read and understand and write my own code. I am sorry, I am lazy. I tried following LDD chapter mentioned but I am not able to get it into the code, so as to get it working. Either I am not trying hard enough or I am not understanding it as I am lacking some basics to get right clarity from reading chapter 14 of LDD. Can somebody give a push to my stuck threads so I get it right? Thanks in advance. Sorry if I sound vague. -- cheers Akam

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how to test the vanilla kernel
by shubham k June 22, 2014

June 22, 2014

Hi Gurus, I have virtual m/c with ubuntu linux-kernel 3.2.xx. Suppose I made any fix in latest kernel say 3.16-rcx and want to test those changes. How i should go about it? I am thinking to compile the modified kernel 3.16-rcx and install it like standard procedure (make, make modules, make modules_install, make install) and boot the ubuntu in runlevl=3, to avoid GUI as it may not work with latest vanilla kernel and test kernel with existing root filesystem. Is it the right approach or need to create separate boot partition and root filesystem? If not please let me know the right way to test kernel. my host config x-86 core i5. thanks in advance, shubham

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Are Small Changes in kernel accepted as patches
by karthik June 21, 2014

June 21, 2014

Hey There, I was just going through the linux-next source and found a small change in one of the files. (Unnecessary else) Now should I make this a patch and submit it or is it too small to submit? Regards Karthik Nayak

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How to get the average/max process scheduling latency
by Teto June 20, 2014

June 20, 2014

Hi, I would like to retrieve the average latency between 2 schedulings of an application. The purpose is to model the service time of the application (ie. How fast it can empty the TCP buffer). I could not find an answer yet. I've found that on latest kernel latency is on average around 3us with a max at 64us (I consider a moderate load). Is that correct ? Regards Matt

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issues dealing with kobjects
by Raghavendra June 20, 2014

June 20, 2014

Hello all, I am facing a small issue dealing with kobjects. I am writing a simple i2c driver for which I would like to export a few sysfs attributes(files). The files are many, so I've decided to pack them into a directory in sysfs (inside the i2c device) and so I thought of kobjects. My private data struture is something like this : struct my_private { struct i2c_client *client; ... struct kobject kobj; }; In my probe function, I am doing something like this : int my_probe(struct i2c_client *client, ...) { struct my_private *dev; dev = devm_kzalloc(...); pr_info("%x", dev); /* The address that I got is : 0xdbf94210 */ .... /* Init. and add kboject */ kobject_init(&dev->kobj, client->dev.kobj.ktype); kobject_add(&dev->kobj, &client->dev.kobj, "my_dir"); /* Export sysfs group */ sysfs_create_group(&dev->kobj, &my_attr_grp); .... } My show function for one of the attribute is something like this : ssize_t show(struct kobject *kobj, ... ) { struct my_private *dev = container_of(kobj, struct my_private, kobj); pr_info("%x", dev); /* The address that I got is : 0xdbf94208 */ .... } I tried to probe and remove the device mutilple times. Every time I am getting a difference of 2bytes for the 'dev' pointer between probe and show functions. Can anyone explain me where am I going wrong or is there any better way to create directories in sysfs? I am building this module against 3.13.2 kernel. Thank you. Raghavendra. ------------------------------------------------------------------------------------------------------------------------------- [ C-DAC is on Social-Media too. Kindly follow us at: Facebook: https://www.facebook.com/CDACINDIA & Twitter: @cdacindia ] This e-mail is for the sole use of the intended recipient(s) and may contain confidential and privileged information. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies and the original message. Any unauthorized review, use, disclosure, dissemination, forwarding, printing or copying of this email is strictly prohibited and appropriate legal action will be taken. -------------------------------------------------------------------------------------------------------------------------------

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Understanding #ifdef in .h files
by Harold André June 20, 2014

June 20, 2014

Hi, I try to understand how #ifdef in .h files works. I read Greg Kroah-Hartman's Coding style paper http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/mgp0… And as he says, i try to do a simple example but it does not work. I try with a small piece of C outside the kernel. I have 3 files. test_ifdef.h: #ifdef TEST_FUNCTION void test(int *value); #else static inline void test(int *value) { } #endif test_ifdef.c: #include "test_ifdef.h" void test(int *value) { *value += 1; } main.c: #include <stdio.h> #include "test_ifdef.h" int main(int argc, char *argv[]) { int i = 3; printf("i = %d\n", i); test(&i); printf("i = %d\n", i); return 0; } And when i compile: $ gcc -Wall -g main.c test_ifdef.c -o test_ifdef -DTEST_FUNCTION $ ./test_ifdef i = 3 i = 4 $ gcc -Wall -g main.c test_ifdef.c -o test_ifdef test_ifdef.c:14:6: error: redefinition of ‘test’ void test(int *value) ^ In file included from test_ifdef.c:12:0: test_ifdef.h:17:20: note: previous definition of ‘test’ was here static inline void test(int *value) { } ^ $ I understand why it does not compile. But: - How it can work in the kernel code ? - Is-it possible to do this in code outside the kernel ? Thank you. Harold

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pow calculation in kernel
by 张晗 June 20, 2014

June 20, 2014

Hi: I am doing a research on D2TCP(http://dl.acm.org/citation.cfm?id=2342388), I just want to implement it into the linux kernel. When calculating the penalty function, it is p = a^d, where 0< a < 1 and 0< d < 1. Since the kernel only offers integer, so in my code, so I let a multiply 2^10. But I have no idea of calculating a^d when 0< d < 1. May be I want a approximate algorithm or other methods. Can you help me ? thanks~ ------------------ 祝好，张晗 ---------- HanZhang School of Computer Science Tsinghua University, Beijing, 100084, P.R.CHINA Mobile: +86 156-526-59782 E-mail: zhanghan(a)csnet1.cs.tsinghua.edu.cn

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