August 2013 - kernelnewbies@kernelnewbies.org

Re: Getting Started
by Valdis.Kletnieks＠vt.edu Aug. 5, 2013

Aug. 5, 2013

On Wed, 31 Jul 2013 10:32:59 +0530, you said: > I am a computer science student. I want to contribute to the open source > projects, debugging. As it is my first time, i need some guidance. (a) You're better off replying to the list, as you then get answers from others besides me. Redirecting back to the list. (b) If you want to actually *help*, and have it set on being the kernel rather than any one of thousands of deserving userspace projects, your best bet is 1) learn to build and install a self-compiled kernel. 2) just get a copy of the 'linux-next' tree 3) Update and build kernels every few days 4) watch them fail (and fail they will - I have at least 3 bugs I've tripped over in the past week to report still) 5) Use 'git bisect' to identify the patch that caused the failure, and report it to the appropriate people. Quite frankly, the kernel needs less half-baked patches from novices, and more qualified testers. And you can get up to speed on testing a heck of a lot faster than you can learn all the ins and outs of kernel code hacking. And if you're ambitious, you can always add "(6) include a patch fixing the problem" once you get better at it...

3 3

How kernel handle interrupts
by Woody Wu Aug. 5, 2013

Aug. 5, 2013

Hi, List Where is the Kernel code that handles external interrupts? I want to have a look at it but haven't found out where it is. Actually, I have some basic questions about interrupt handling in Linux. 1. After Kernel's ISR received an interrupt, I believe it will invoke a handler defined in a device driver if any. But it should be the device driver's responsibility or kernel ISR's responsibility to clear (or acknowledge) the interrupt? 2. My device, an AX88796B network controller, asserting the interrupt line in a level-triggered manner. Now I met problem with the device that might caused by the CPU interrupt mode is not set as level-triggered by edge trigger. My CPU is Samsung S3C2410, an ARM920T powered one. Does anyone know usually where and how should I do this kind of setting? Thanks in advance. -- woody I can't go back to yesterday - because I was a different person then.

4 14

Where to call my init() from (without LKM)?
by George White Aug. 4, 2013

Aug. 4, 2013

Hi, For a small side-project I decided to make some changes directly to the kernel code, so that it offers some extra functionality I need. I have not done this before in such capacity and only put together some modules here and there in the past. Since now I'm not creating a module, instead I'm hacking the kernel directly, I have some questions: Most of my code is implemented in a new file I created under /kernel/myfile.c and added in the main Makefile (other parts of the code are spread around the existing code as edits). Question A: Is that a good idea? Alternatives? When developing modules, you have the module_init where you declare a function to run when the module inits and set up everything. In my code, I have a similar init function, that basically sets up some procfs endpoints so that users can configure parts of my system with boolean options (enable/disable functionality). Question B: Where is the best place to I call this init function from? Question C: Is using procfs the best option for setting up config options for my system? Best, George

2 1

Why count the hash value in this way?
by lx Aug. 4, 2013

Aug. 4, 2013

2 1

pulling in patches mentioned in mails (linux-kernel@vger.kernel.org)
by kernel neophyte Aug. 4, 2013

Aug. 4, 2013

Hi, I am sorry if this has been asked before, I tried to look around but did not get an solution and hence the mail. In this mail from Mel Gorman: https://lkml.org/lkml/2013/7/15/362 He mentions a set of patches: https://lkml.org/lkml/2013/7/15/292 https://lkml.org/lkml/2013/7/15/383 https://lkml.org/lkml/2013/7/15/322 Could someone please teach me 1) how do I find out where is the tree he is working on (i.e. is he working on linux-next or ... ) 2) Where do I find these patches mentioned ? Currently I cut copy paste in a vim manually and its really frustrating, could some one please point out the right way of pulling these patches. via git ? Thanks a lot for your time in advance. -Neophyte PS: This is my first ever mail in public, please pardon my grammatical errors or choice of words.

2 1

Best way to go about learning about video capture in the kernel
by Nicholas Othieno Aug. 3, 2013

Aug. 3, 2013

Hi everyone, What is the best way to go about learning about video capture in the kernel. This especially concerns trying to write a device driver for a camera module. Thanks. Nick

1 0

Kernel with KGDB enabled does not wait for remote server
by Nicholas Othieno Aug. 3, 2013

Aug. 3, 2013

Hi guys, I am trying to get remote debugging over serial working on an ARM platform using KGDB. I compiled the kernel with the following options: CONFIG_DEBUG_KERNEL=y CONFIG_DEBUG_INFO=y CONFIG_KGDB_SERIAL_CONSOLE=y And had the following parameters passed by uboot to the kernel: bootargs_mmc=setenv bootargs ${bootargs} ip=off root=/dev/mmcblk0p1 rootwait kgdboc=ttymxc0,115200 kgdbwait When I boot the kernel, it acknowledges that it got the correct kernel parameters: Kernel command line: console=ttymxc0,115200 ip=off root=/dev/mmcblk0p1 rootwait kgdboc=ttymxc0,115200 kgdbwait but it never stops to wait for a gdb server. What could the problem be? Any ideas? Thanks. Nick

1 0

RE: Conceptual questions about device driver
by Rajat Sharma Aug. 3, 2013

Aug. 3, 2013

BTW I have not seen filesystem implementations doing write ordering for direct IO unless a request merge is possible (different user credentials), since you have mentioned you are from filesystem background, do you know any such implimentation? Rajat ------------------------------ From: Rajat Sharma Sent: 03-08-2013 09:25 To: neha naik; Greg Freemyer Cc: kernelnewbies Subject: RE: Conceptual questions about device driver If filesystem has to guarantee write ordering, it has to serialize write request. So, only when 'A' is written to the block (completion received from block driver) filesystem can dispatch 'B' not before that. Remember that there could be failures too. So if 'A' is failed, request for 'B' is pending, would a filesystem fail that too? I guess not. Usually page cache absorbs such overwrites first than issues a single merged IO to block device, but what about direct IO? If applications need consistency guarantee against such failures, it should serialize writes. So to summarize, write ordering effort from each layers Block driver: none Filesystem: best effort Application: full Rajat ------------------------------ From: neha naik Sent: 03-08-2013 02:40 To: Greg Freemyer Cc: Rajat Sharma; kernelnewbies Subject: Re: Conceptual questions about device driver Thanks for the responses. I have one more question for Greg. I come from filesystem background and not device driver so i may be a bit confused about the write order fidelity. I know that filesystems guarantee that. Looking from filesystem perspective, no write will be allowed on the same block until the first write finishes. So, if 'B' is written after 'A' you can always guarantee that you will see 'B' at the end of the two writes. Now imagine not having a filesystem, and doing a write directly on the device. Do device drivers honour it. Should they? I imagine device driver as a kind of queue. So any writes are always queued up one after the other so that it gives write order fidelity whether it wants to or not. Am i missing something here. Regards, Neha On Fri, Aug 2, 2013 at 1:56 PM, Greg Freemyer <greg.freemyer(a)gmail.com>wrote: > On Fri, Aug 2, 2013 at 1:32 AM, Rajat Sharma <fs.rajat(a)gmail.com> wrote: > > On Fri, Aug 2, 2013 at 2:25 AM, neha naik <nehanaik27(a)gmail.com> wrote: > >> Hi, > >> I have some conceptual questions about device driver : > >> > >> 1. Write order fidelity should be maintained when submitting requests > from > >> device driver to disk below. > >> However, acknowledging these requests it is okay if we don't > necessarily > >> maintain that order, right? > >> > > > > Yes it should not matter as long as application can rely on data being > > written is in order of submission. > > But it can't ..... unless the write cache is turned off and it is > known the the cache is truly off. > > There is no guarantee of write order in the block stack. Not between > the filesystem and the driver. Not between the driver and the drive. > > There are at least 2 elevators shuffling the order of writes to > optimize performance. > > Rajat, did you get confused? Or were you trying to say something else? > > Greg >

1 0

RE: Conceptual questions about device driver
by Rajat Sharma Aug. 3, 2013

Aug. 3, 2013

If filesystem has to guarantee write ordering, it has to serialize write request. So, only when 'A' is written to the block (completion received from block driver) filesystem can dispatch 'B' not before that. Remember that there could be failures too. So if 'A' is failed, request for 'B' is pending, would a filesystem fail that too? I guess not. Usually page cache absorbs such overwrites first than issues a single merged IO to block device, but what about direct IO? If applications need consistency guarantee against such failures, it should serialize writes. So to summarize, write ordering effort from each layers Block driver: none Filesystem: best effort Application: full Rajat ------------------------------ From: neha naik Sent: 03-08-2013 02:40 To: Greg Freemyer Cc: Rajat Sharma; kernelnewbies Subject: Re: Conceptual questions about device driver Thanks for the responses. I have one more question for Greg. I come from filesystem background and not device driver so i may be a bit confused about the write order fidelity. I know that filesystems guarantee that. Looking from filesystem perspective, no write will be allowed on the same block until the first write finishes. So, if 'B' is written after 'A' you can always guarantee that you will see 'B' at the end of the two writes. Now imagine not having a filesystem, and doing a write directly on the device. Do device drivers honour it. Should they? I imagine device driver as a kind of queue. So any writes are always queued up one after the other so that it gives write order fidelity whether it wants to or not. Am i missing something here. Regards, Neha On Fri, Aug 2, 2013 at 1:56 PM, Greg Freemyer <greg.freemyer(a)gmail.com>wrote: > On Fri, Aug 2, 2013 at 1:32 AM, Rajat Sharma <fs.rajat(a)gmail.com> wrote: > > On Fri, Aug 2, 2013 at 2:25 AM, neha naik <nehanaik27(a)gmail.com> wrote: > >> Hi, > >> I have some conceptual questions about device driver : > >> > >> 1. Write order fidelity should be maintained when submitting requests > from > >> device driver to disk below. > >> However, acknowledging these requests it is okay if we don't > necessarily > >> maintain that order, right? > >> > > > > Yes it should not matter as long as application can rely on data being > > written is in order of submission. > > But it can't ..... unless the write cache is turned off and it is > known the the cache is truly off. > > There is no guarantee of write order in the block stack. Not between > the filesystem and the driver. Not between the driver and the drive. > > There are at least 2 elevators shuffling the order of writes to > optimize performance. > > Rajat, did you get confused? Or were you trying to say something else? > > Greg >

1 0

Conceptual questions about device driver
by neha naik Aug. 2, 2013

Aug. 2, 2013

Hi, I have some conceptual questions about device driver : 1. Write order fidelity should be maintained when submitting requests from device driver to disk below. However, acknowledging these requests it is okay if we don't necessarily maintain that order, right? 2. Also i want to understand what the device driver does say if in a multiple paged bio, some of the pages get written and some don't, we send the error in the bio. But what about the pages it has already written??? It can't possibly do anything about it, right? Regards, Neha

5 7