February 2013 - kernelnewbies@kernelnewbies.org

test jiffies on ARM SMP board
by buyitian Feb. 25, 2013

Feb. 25, 2013

i am confused about my test. in one device driver, i put below code: printk("start to test test jiffies\n"); local_irq_save(flags); jf1 = jiffies; // read jiffies first time // hold cpu for about 2 seconds(do some calculation) jf2 = jiffies; // read jiffies after 2 seconds local_irq_restore(flags); printk("jf1:%lu, jf2:%lu\n", jf1, jf2); and the output is as below: <4>[ 108.551124]start to test test jiffies <4>[ 110.367604]jf1:4294948151, jf2:4294948151 the jf1 and jf2 are the same value, although they are read between 2 seconds interval, i think this is because i disabled local interrupt. but the printk timestamp is from 108.551124 to 110.367604, which is about 2 seconds. and on my platform, printk timestamp is got from the function read_sched_clock: static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read; and function jiffy_sched_clock_read() is to read from jiffies. it seems that the jiffies is frozen when local irq is disabled, but after local_irq_restore(), the jiffies not only start to run, but also recover the lost 2 seconds. is the jiffies updated from another cpu when irq is disabled on local cpu? is there some internel processor interrupt between cpu1 and cpu0 after local irq is re-enabled so that jiffies recover the lost 2 seconds?

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Intro and some questions
by NeonJohn Feb. 24, 2013

Feb. 24, 2013

Greetings, I just discovered this list while beating my head against the wall (kernel noobs do that a lot, I hear). I thought I'd introduce myself. I'm mainly an EE but with fairly extensive Unix and Linux user space coding and with embedded system programming. I'm a newborn infant when it comes to the kernel. I'm working with the BeagleBone and Robert Nelson's Angstrom derivative https://github.com/RobertCNelson/stable-kernel I intend to embed the BB in our new product if I can solve a small problem. The BB has 2 SPI ports, 1 with 1 chip select and the other with 2. Only one port is supported in the standard kernel. I found some info that guided me into activating the new port and having all three devices enumerated in /dev. What I have not yet been able to do is figure out how to enable that second chip select line. So my first question is, where do I start looking for the existing code? I'm hoping that I'll be lucky enough to reuse that code, changing a bit or two in the CPU register. Thanks, John -- John DeArmond Tellico Plains, Occupied TN http://www.fluxeon.com <-- THE source for induction heaters http://www.neon-john.com <-- email from here http://www.johndearmond.com <-- Best damned Blog on the net PGP key: wwwkeys.pgp.net: BCB68D77

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Re: how to receive msgs in a channel?
by horseriver Feb. 24, 2013

Feb. 24, 2013

On Sun, Feb 24, 2013 at 11:47:50AM +0000, Mark Cunningham wrote: > Just type and press enter. By default it goes to the room not one person. > I do as you say. after I type enter , it said: Cannot send to channel I am join as a guest .does this matter? > Mark > > On Sun, Feb 24, 2013 at 2:02 AM, horseriver <horserivers(a)gmail.com> wrote: > > > > > > I can see a list of users in that room ,but receive none msgs for a long time . > > how to send message to this room ?not just to one member. > > > > thanks! > >

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V4L2 Framework
by Kaushal Billore Feb. 24, 2013

Feb. 24, 2013

I have some doubt regarding Linux kernel V4l2 API's. When capture application calls Reqbuff ioctl to allocate n no of buffer which would belongs to v4l2 layer and display application calls the Reqbuff ioctl to allocate N no of buffer which would also belongs to device memory. Question: 1. V4l2 maintains the generic layer for all devices in which buffers can be allocated by any device and can be handle by any device? 2. If not then while capturing the data from capture device can capture device allocated buffer gets filled and while displaying the same data there memory copy happens between capture buffer and output buffers? 3. If not then I want to capture data from capture device and display onto display device through the v4l2 framework layer. Awaiting for responce! Thanks in advance Kaushal

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test mailing list
by 9981.0 Feb. 23, 2013

Feb. 23, 2013

just for test, ignore it if you receive it!

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Re: Sending an IP packet
by Adel Qodmani Feb. 22, 2013

Feb. 22, 2013

On Fri, Feb 22, 2013 at 5:40 PM, <Valdis.Kletnieks(a)vt.edu> wrote: > In that case, you *really* want to go look at how TCP and SCTP and other > protocols handle ICMP integration. You want an API that integrates your > ICMP > handling with the rest of the protocol stack, because otherwise you'll > end up with an unmaintainable mess. Also, it will be about 436 times > easier > to extend your protocol to work correctly over IPv6. :) > > I am gonna definetly suggest that to the team and see what their reply is; thanks for a nice suggestions :) > Go look at net/ipv4/udp.c, functions __udp4_lib_err() and __udp_lib_rcv(), > particularly the latter's use of icmp_send(). You'll want to extend > icmp_send() > to handle your additional control information. > Checking the icmp_send() code, it seems to be doing exactly what I need to do but there's a little problem, icmp_send uses a function called skb_rtable which returns the routing table entry(hopefully I got it right :P) but that function is declared as static(the skb_rtable) and I didn't find a higher-level function that wraps it and is actually exported. Using icmp_send as it is wouldn't be as useful because my aim is to use ICMP(or actually any type of traffic, I wouldn't mind anything as long as it does the job and makes sense from an implementation prespective), to use ICMP, to send the information data I am exchanging to let my protocol work and that data might be 200 bytes long sometimes. Regards, Adel

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Sending an IP packet
by Adel Qodmani Feb. 22, 2013

Feb. 22, 2013

Hey, My question is quite simple, I have an sk_buff that I want to transmit, the sk_buff is an ICMP message and so far, I've built the headers and set up everything. The problem is, when I want to do such a thing, I typically use a function called: dev_queue_xmit which takes the skb and transmits the packet using the net_device specified in the skb; but time time I can't set the net_device, I need the packet to go through the routing code and get it sent normally; so what function can I use to that? Some notes: *The kernel I am working on is 3.2.0-37 *I am working using Modules, so symbols that are not exported are unreachable for me *I set the IP header using htons, so my header data is already in network byte-order. Regards, Adel

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setup code
by horseriver Feb. 22, 2013

Feb. 22, 2013

hi I am confused on booting from grub's rescue mode . As we know ,before the kernel start ,the setup code, which is from setup.S ,will be executed at front. If boot from rescue mode , will grub also load setup code and execute it ? or grub execute its own code doing the same work as setup ? then jump to kernel-start directly? thanks!

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ptrace and kernel threads
by Kevin Wilson Feb. 22, 2013

Feb. 22, 2013

H i all, You will get an error when trying to call ptrace system call on kernel threads. In code, you have the following check: static int ptrace_attach(struct task_struct *task, long request, unsigned long addr, unsigned long flags) { ... retval = -EPERM; if (unlikely(task->flags & PF_KTHREAD)) goto out; ... see: http://lxr.free-electrons.com/source/kernel/ptrace.c#L233 Question: what is the reason that kernel threads cannot be traced while userspace threads can be traced ? rgs, Kevin

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How to solve module collision
by Jimmy Pan Feb. 22, 2013

Feb. 22, 2013

Hi, I would like to write a driver for my usb mouse. It functions normally with kernel driver, nonetheless, I want to study usb system. When the kernel executes the probe function, what would happen if there exists two suitable drivers. And how can I override the existed kernel driver? I guess that the the interface buffer would be emptied since either one of the drivers grab it, if the kernel mouse driver contends for usb data with my driver, what should I do? Or if both of us strive to control the device, what can be done to avoid this. Thanks, Jimmy

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