February 2012 - kernelnewbies@kernelnewbies.org

Re: function of major/minor device number
by Alexandru Juncu Feb. 12, 2012

Feb. 12, 2012

2012/2/12 hz hanks <hankshz(a)gmail.com>: > Hey Alexandru Juncu > > Thank you so much! You are so helpful for me. > > As far as I'm concerned, the point that confuses me is the definition > of ONE device driver. For example, I would consider a file of some > source codes, that manipulate the GPIO and use misc_register to > register as miscdevice, as one device driver. However, I know that it > will share the major device number 10 with other device drivers such > as AD/DA. So how kernel works like that? In other words, how kernel > works with major/minor device number? For example, will the kernel > allocate some specific resources to drivers with major number 4 > because it knows those drivers are for Serial ports. How about the > minor number, will the kernel care about that? Hello! First of all, you should always reply to the list, because when there are 100 people rather than one, you have a bigger chance of getting an answer. Is hard to define ONE device driver. Linux is a monolithic kernel. It's one big code. But linux is also modular and you can have dynamically loaded modules. But it's very flexible... you can have a module (a .ko file) that is a driver for two devices (not going to find this, but it could happend). You usually have one .ko for one device driver for one device. This would mean one major. For example, a device driver for all the SATA devices. [0] http://en.wikipedia.org/wiki/Monolithic_kernel > > Thank you again! > > 2012/2/12 Alexandru Juncu <alex.juncu(a)rosedu.org>: >> 2012/2/12 hz hanks <hankshz(a)gmail.com>: >>> 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! >> >> Hello! >> >> I would redirect you to the /dev fs to do a long listing of its >> contents and notice the pattern. >> Devices with the same major, are usually handled by the same device >> driver. If that device driver needs to handle several devices, it uses >> an array of minors. >> >> Example: >> >> crw-rw---- 1 root dialout 4, 64 2011-12-12 16:25 ttyS0 >> crw-rw---- 1 root dialout 4, 65 2011-12-12 16:25 ttyS1 >> crw-rw---- 1 root dialout 4, 66 2011-12-12 16:25 ttyS2 >> crw-rw---- 1 root dialout 4, 67 2011-12-12 16:25 ttyS3 >> >> Notice that all the Serial ports have the same major (4) but different >> minors (64-67). If you do a "cat /proc/devices" you will notice that >> the major 4 is registered by ttyS device driver. "ttyS" is the name >> variable in the register_chrdev_region call. >> >> A device driver can register zero, one ore more majors. And for each >> of that major, it can a number of minors (the count parameter in >> register_chrdev_region). >> >> Two devices having the same minor but different majors is just a >> coincidence, they have nothing in common. It's just an obvious thing >> because a major has more minors so there will be more minors that >> majors. >> >> Hope this helps.

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Writing module in kernel 3.1.6
by Swapnil Gaikwad Feb. 12, 2012

Feb. 12, 2012

Hi, I am new to kernel programming, I have written one module to get vfs inode using path_lookup function in kernel 2.6.35. Now I am trying to compile same module under kernel 3.1.6 then i got some errors like ioctl undefined, path_lookup not present i.e error: implicit declaration of function ‘path_lookup’ . I have searched and used unlocked_ioctl insted of ioctl. But I am struggling to find function equivalent to path_lookup as in new kernel this function dose not exist. Can anyone suggest me proper function which dose the same job as path_lookup in 3.1.6? -- Regards, Swapnil Gaikwad.

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Confusing in Address
by hz hanks Feb. 12, 2012

Feb. 12, 2012

Hi, all I'm confused with different addresses in Linux, such as virtual address and hardware address. For example, which kind of address one can get when using "&num" in application layer programming? How about that in kernel, such as in device driver? Which kind of address one can get when using "void _ _user *to" in device driver? And which kind of address the kernel is working with when using "mmap"? And which kind of address we are talking about when divide the memory into user space and kernel space? It's quite tedious but thank you so much for helping me!

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How to find the child processes?
by Surenkumar Nihalani Feb. 11, 2012

Feb. 11, 2012

Hi guys, I need help, again. I am writing a kernel module. I have the pid and tid from the current structure. How do I figure our the number of threads in the given process id? If possible, how do I get the individual tids within the pid? Thanks, Suren Nihalani.

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/proc help
by Surenkumar Nihalani Feb. 9, 2012

Feb. 9, 2012

Hi guys, I want to find the number of threads owned by the caller process who tried to read from my proc file, how do I find the stuct responsible for it? Also, I want to know the thread id of my caller. I am writing a kernel module. Please help. Thanks, Suren.

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Re: Google Summer of Code 2012
by Peter Senna Tschudin Feb. 8, 2012

Feb. 8, 2012

Hi Greg, :-) > kernelnewbies has not been involved in GSoC before, there are different > groups handling kernel related projects with GSoC. PSU and the Linux > Foundation are two of them. I asked here due: http://www.linuxfoundation.org/collaborate/workgroups/gsoc/2011-gsoc-kernel… Any idea about how to get involved in Linux Foundation Google Summer of code? Thank you! -- Peter Senna Tschudin peter.senna(a)gmail.com gpg id: 48274C36

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page replacement policy in the latest Linux kernel
by Zheng Da Feb. 8, 2012

Feb. 8, 2012

Hello, I'm trying to understand the page replacement policy in the latest Linux kernel and reimplement a similar one in the user space. As far as I can understand from ULK and the code, there are two lists: active list and inactive list. Here I only consider the case of the *page cache*: When a page is added to the page cache (by add_to_page_cache_lru), it is also added to the inactive list and its reference bit is set (by mark_page_accessed). When a page is accessed again, it is added to the active list and the reference bit is cleared (by mark_page_accessed). When a page is accessed and it's in the active list, the reference bit is set if it's not set (by mark_page_accessed). When the system is out of free pages, it starts to shrink the cache. When a page is in the active list, if the page hasn't been referenced for a period of time (page_referenced() returns 0, and for the pages in the page cache, it's always 0), it will be moved to the inactive list (in shrink_active_list); When a page is in the inactive list, if the page hasn't been referenced (it's always true for pages in the page cache), reclaim the page (in shrink_inactive_list); When the system is out of free pages, it always starts to reclaim pages in the inactive list first, unless the number of pages in the active list is larger than that in the inactive list. Please correct me if any part is wrong for the page cache. Thanks, Da

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blktrace vs ftrace
by Matthias Brugger Feb. 8, 2012

Feb. 8, 2012

Hi all, I'm trying to analize I/O behavior of Qemu on the host. I used blktrace and the blk tracer for ftrace but I see different I/O behavior. I use 4 threads in the VM each reading sequentially a file. Readahead is turned off. With blktrace reads are transmitted in a sequential way, which means the next read will be executed when the first one has completed. In comparison with ftrace, up to 4 reads are issued to the disk in parallel. I suppose the different behaviors are due to the overhead introduced by the tracers. My doubt is, which tracer has less influence on the I/O behaviour and therefore reflects the system behaviour without tracing. Thanks a lot, Matthias --- http://motzblog.wordpress.com/

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Contribute to linux kernel
by Mayank Agarwal Feb. 8, 2012

Feb. 8, 2012

Dear Experts, I want to contribute to linux kernel in camera drivers or in any of the multimedia related driver development linux kernel job. Please send me the details how can i start doing this. Thanks and Regards, Mayank

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Google Summer of Code 2012
by Peter Senna Tschudin Feb. 8, 2012

Feb. 8, 2012

Hello, Are there plans for 2012 GSoC Kernel works? Is this the right place for asking? Regards, Peter -- Peter Senna Tschudin peter.senna(a)gmail.com gpg id: 48274C36

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