March 2015 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Mouse driver
by Ronit Halder March 7, 2015

March 7, 2015

Hi,I want to write mouse driver for linux ( usb mouse). I am reading "Linux device driver" and "Linux kernel development". But i don't how to start writing the driver. So,Please help. Best regards, Ronit.

2 1

How to check if a file is a present in a directory(directly/indirectly)
by noyb noybee March 6, 2015

March 6, 2015

I am building a LKM in which I need to check whether a file is inside a particular directory(directly or inside sub-directories). Is there a function for this? Do I need to use the lookup function in inode_operations for this or is there a better method? Regards, winged_elite

3 2

question scheduler
by Meyer Lansky March 6, 2015

March 6, 2015

Hello all continue to study the scheduler is O(1 ) there was one difficulty with the string 2 on 3, in that these blocking occurs after lines ? 1 rcu_read_lock(); 2 pcred = __task_cred(p); 3 match = (cred->euid == pcred->euid || cred->euid == pcread->uid); 4 rcu_read_unlock(); I ask because in the comments there is not described . Thank.

1 0

menuconfig: can not select scsi device support
by robert rottermann March 6, 2015

March 6, 2015

Hi there, I am working trough a book on embeded systems (embeded linux by J.Quade) There in the chapter about building a new kernel is suggested to select Device Drivers-> SCSI device support -> a number of subslections. Now, when I start menuconfig I find Device Drivers-> SCSI device support, but selecting it produces an empty page. Why is that so? Do I have to add other elements to the configuration process? I am using linux-3.19. Thanks robert

4 3

locking a device for exclusive use by userspace process (or kernel thread)
by Yann Droneaud March 4, 2015

March 4, 2015

Hi, I have a device and I have to write a driver that exposes the following three operations to kernel modules AND to userspace programs: - LOCK LOCK operation is used to get exclusive access to the device. If exclusive access is already granted, the caller will wait for it be released. - PROCESS PROCESS can be issued only after a successful LOCK operation. PROCESS operations can be issued multiple times between LOCK and UNLOCK. - UNLOCK UNLOCK operation release the exclusive access to the device. This operation can be issued only after a successful LOCK operation. In kernelspace, the lock would be implemented using a semaphore (with an initial count of 1). That's the easy part. In userspace, it's expected LOCK and UNLOCK can be issued in different threads within one process, and, if, while holding the lock, a process die, either being killed or by exit(), an implicit UNLOCK should be done. I've initially thought I could represent the lock by a file descriptor, map LOCK to open() and UNLOCK() to close() (with the added benefit that opened file descriptor will be closed on exit): operation userspace kernelspace LOCK open() ->open() PROCESS ioctl() ->ioctl() UNLOCK close() ->release() open() would block if the lock is already held by some other process (and would not block if O_NONBLOCK or O_NDELAY is used). Seems a great scheme. But that doesn't fit well with Unix / POSIX, as both fork() and dup{,2,3}() or fcntl(, F_DUPFD*,) creates new references to open()'ed device: - A child will inherit the file descriptor for the device after parent fork(). As the child has a file descriptor for the device, the parent couldn't UNLOCK by calling close() since the device won't be released until the child also close() its copy of the file descriptor. It should be noted that even if the child has the device opened by the vertue of inheritance, it does not own the lock, so it should not be allowed to issue PROCESS operation. But at the same time, it prevents the real owner from releasing the lock (... I'm seeing dead locks). So who really own the lock ? It seems awkward to allow an exclusive lock to be shared between two different process even if they are parents. - In the very same process, multiple references to the device could be created with dup(). So close()ing the file descriptor returned by open() won't UNLOCK the device, as the device will only be released once the last reference would be close()'d. It seems awkward to allow an exclusive lock to be "duplicated" within a process. I've tried to (ab)use ->flush() to implement UNLOCK operation so that a call to close() on the device's file descriptor would UNLOCK, even if a duplicated file descriptor exists in the same process or in a child. But this has the unfortunate consequence of allowing to UNLOCK when closing a duplicated file descriptor (and closing the file descriptor in the child could also mean UNLOCK). So in the end, I'm probably not going to use open() for LOCK and close() for UNLOCK. I think I have to use dedicated LOCK and UNLOCK ioctls. But I failed to find a way to be able to release the lock when the process owning the lock die / exit. Perhaps I need a mechanism to be notified of the death of the process owning the lock. Do you have such mean in mind ? Or any other reasonable solution to implement the three operations of such driver ? Regards. -- Yann Droneaud OPTEYA

3 4

Google Summer of Code 2015
by Galactuz Prime March 4, 2015

March 4, 2015

Hi all, My name is Pavel, and I am looking for some projects and mentors to start working on Google Summer of Code 2015 . I saw some of them from Linux Foundation and it says that asking here is the best way to get helped. I have knowledge in c/c++, python and I am also learning kernel development on Eudyptula challenge. I don't have an specific project in mind, but I would like to help from now and keep helping maintaining after gsoc, if possible. Thanks for the attention. Best regards, Pavel.

2 1

ACCESS_ONCE usage inside llist_add_batch function
by Cihangir Akturk March 4, 2015

March 4, 2015

Reading the lib/llist.c file in the kernel sources, I came across the llist_add_bach function defined like this; bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last, struct llist_head *head) { struct llist_node *first; do { new_last->next = first = ACCESS_ONCE(head->first); } while (cmpxchg(&head->first, first, new_first) != first); return !first; } One thing bugging my mind is the ACCESS_ONCE macro. Is it really needed here ? I mean I would write this function with ACCES_ONCE moved outside the loop like as follows; bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last, struct llist_head *head) { struct llist_node *first, *old; old = ACCESS_ONCE(head->first); for (;;) { first = old; new_last->next = old; old = cmpxchg(&head->first, first, new_first); if (old == first) break; } return !first; } I think that it may be faster to just use the return value of cmpxchg. But I am not sure about this. Is my understanding correct ?

4 4

I want to remove my current kernel after upgradtion of new kernel
by Anurudh Tiwari March 4, 2015

March 4, 2015

Hi, I am upgrading my machine with new version of os which have newer version of the kernel. I have to remove my current kernel(which is going to be old kernel in next reboot) after up-gradation because i don't want to get prompt to choose the kernel at booting time. I tried few methods: 1 . Try to remove using command ( sudo apt-get purge uname -r ) -- but after it gives a pop up (dialog box) to select yes or no.(But i don't to do that because system is automated, user should not interact with system). My intention is remove to old kernel at very first time when it reboot(so no prompt will show to select the kernel by grub).. Please if you have any best solution for this. please share. Thanks Anurudh

5 7

Beginner in Kernel Development
by sahil aggarwal March 3, 2015

March 3, 2015

Hi all, I am using linux since past 9-10 months and have good programming skills in C/C++. Since past few weeks i am looking for good source to dive in kernel development but could not find the starting point and proper order of learning to be followed to join the community. I will be grateful if someone could point me in right direction. Thanks Regards Sahil

4 8

Perf API vs Ftrace
by sahil aggarwal March 3, 2015

March 3, 2015

Hi all Do anybody have experience using Perf API and Ftrace.? I am writing an application to profile a process and using Ftrace. But came to know that Perf provide API too so looking for some facts to benchmark efficiency. Thanks Sahil Regards

2 1