March 2013 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

DMA attributes with dma_sync_single()
by Moritz Fischer March 20, 2013

March 20, 2013

Hi there, I was wondering whether setting DMA attributes like DMA_ATTR_WRITE_BARRIER will have an effect if I don't dma_unmap_single_attr() an area but merely dma_sync_single() it after a transfer from the device to the host memory. My problem is that I get an interrupt (MSI) indicating that an (independent) DMA transfer is done, where the interrupt flags get pushed (via a DMA write) to host memory. >From time to time, the data of the other DMA transfer does not show up in host memory in time with the interrupt. I am trying to force coherency by using the DMA_ATTR_WRITE_BARRIER on this status push (of the interrupt flags), to force the other writes (device to host) to complete. Does that seem reasonable? Can anyone with insight shine some light on that? It's a PCIe device, I'm running Fedora 18 with 3.8.3-201.fc18.x86_64. Cheers, Moritz

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Memory allocations in linux for processes
by Niroj Pokhrel March 20, 2013

March 20, 2013

Hi all, As I have read, whenever we execute a program, the memory is allocated in different sections viz stack, text and data segment. But, since we have used loader and linker the three allocations will happen for them too. But when I ran a program and did pmap pid, I saw several other fields which I have no idea of. What is the [anon] and what is it doing here ? Why the memory is being implemented for it and there are so many of them ? And also the stack ( 3rd last element), is it the process stack?? Can somebody please explain. Thanking you all in advance. 5448: ./a.out 0000000000400000 4K r-x-- /home/n.pokhrel/Personal/mywork/a.out 0000000000600000 4K r---- /home/n.pokhrel/Personal/mywork/a.out 0000000000601000 4K rw--- /home/n.pokhrel/Personal/mywork/a.out 00007f57f5e18000 1512K r-x-- /lib/libc-2.11.1.so 00007f57f5f92000 2044K ----- /lib/libc-2.11.1.so 00007f57f6191000 16K r---- /lib/libc-2.11.1.so 00007f57f6195000 4K rw--- /lib/libc-2.11.1.so 00007f57f6196000 20K rw--- [ anon ] 00007f57f619b000 128K r-x-- /lib/ld-2.11.1.so 00007f57f6396000 12K rw--- [ anon ] 00007f57f63b8000 8K rw--- [ anon ] 00007f57f63ba000 4K r---- /lib/ld-2.11.1.so 00007f57f63bb000 4K rw--- /lib/ld-2.11.1.so 00007f57f63bc000 4K rw--- [ anon ] 00007fffa2f37000 84K rw--- [ stack ] 00007fffa2fa0000 4K r-x-- [ anon ] ffffffffff600000 4K r-x-- [ anon ] total

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About time delay in kernel threads at LOCAL_OUT netfilter hook
by Rifat Rahman March 20, 2013

March 20, 2013

Hello there, I am in a situation where I am mangling RTP data in kernel space. I have written a netfilter module which is responsible for encryption, padding and ptime modification. The thing is that, encryption, padding works just fine. Ptime modification involves two steps. When large packets are incoming to the server (say 60 ptime, 60 bytes of RTP payload with g729 codec), they are splitted with ptime 20. The SIP server is Asterisk. When asterisk (the server) is sending packets of 20 ptime, they are merged to the desired big ptime(say 60). All the things went well. I looked up packets whether checksumming, RTP timestamping and sequencing is done perfectly. And it is done quite well. The whole process without large ptime is satisfactory. The thing is that, after contacting with asterisk mailing list, I came to know that I have to make some delay before sending each large packet. But, I am sending packets outside of the box in LOCAL_OUT hook. I wrote a thread to make queue the packets until some delay conditions is satisfied. And then trying to send packets from that thread. But, eventually what I found is that, if I set delay to zero, then everything is fine, the packets are queued and sent as expected. But, if I set delay>0, the thread does not send packets. I used dst_output() in include/linux/dst.h for sending packets. But, this function works only in LOCAL_OUT hook. When I am adding some delay in a thread, the thread just misses the hook and the packet is never sent. I tested that, if I add the delay condition in the target function, the whole kernel faces the delay, so it is of no help. I have to do this thing inside a thread. But how to not miss that hook? Can anyone suggest any work around regarding this? -- Rifat Rahman

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Design Patterns in Linux Kernel: Fancy Tricks With Linked Lists
by Arlie Stephens March 20, 2013

March 20, 2013

Hi Folks, I'm trying to understand the linux way of doing things, so as to write code which will be easier for experienced linux kernel people to understand. Yesterday I wound up in a 3 engineer discussion about something conceptually simple, that just didn't want to be done with the tools at hand; what's worse, the only ways I could see to do it were (a) do my own linked lists or (b) write some routines that relied kind of heavily on the never-mentioned-in-documentation details of linux/list.h I'll probably go with option (b), as that seems less insane, but I thought I might as well ask here as well. Here's the problem. I have a collection of resources. Call them A,B,C,D but note that resources get added at run time, so I don't know how many to expect. I want to go through them in a round robin fashion, but not all at once. On the first request, I use A. On a later request I use B, etc. This seems to me to map naturally to a circular linked list, with a "cursor" - a pointer of some kind to the next one I want to use. To make my specific situation more interesting, I actually have multiple cursors - that may or may not happen to point to the same node at any given time, but usually will not. This is where I wish I could draw pictures in ascii emails ;-) Perhaps the following will come through halfway sanely: C1 C2 C3 V / V A<->B<->C-<->D ^------------^ list.h implements a circular linked list - see for example http://www.makelinux.net/books/lkd2/app01lev1sec2 - so I thought this would be easy and natural. But then I discovered there was no such thing as a list_next(). OK, I can write it - Something like: cursor->resource = list_entry(cursor->resource->link.next, struct resource, link); But the fact there was no interface made me ask advice from colleagues. And that's when the debate erupted. First of all, it's unclear whether that would even work. If the list is initialized following the standard pardigm, there may be a "head" element embedded in the list, which all the existing interfaces know to ignore. I.e. LIST_HEAD(resources); add_resource(...) new_resource = kmalloc(sizeof(struct resource), GFP_KERNEL); new_resource->ID = ... INIT_LIST_HEAD(&new_resource->link); list_add(&new_resource->link, &resources) It looks as if the circular list is likely to include the variable named 'resources', so new_resource->link.next might point to resources, and list_entry(new_resource->link.next, struct resource, link) might point to whatever unrelated variable happens to be somewhere near 'resources'. It's certain that the implementation makes no attempt to clarify what kind of list it's using ... not when we had 3 experienced engineers draw 2 different conclusions, one changing his mind midway. Obviously I can figure out what the link.h implementation actually does, and code accordingly. That's my plan for this morning. I'm just not sure it's going to produce code that other engineers will be comfortable maintaining. And if they aren't comfortable, there will be uneccessary bugs. So, how would a dyed-in-the-wool linux kernel engineer approach this task? I.e. what's the natural design pattern here, within linux? I think it's pretty clear that the one I've come up with seems so unnatural to the authors of list.h that they never even imagined it. Thanks for any insights, -- Arlie (Arlie Stephens arlie(a)worldash.org)

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Wake_lock in linux kernel
by Ben Wu March 20, 2013

March 20, 2013

Dear All: I now study power driver in linux source code, but didn't find some doc about liux,all the doc was for android, can some help me?BTW,how /sys/power interactive with linux kernel?? Many thanks

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do_initcalls
by ishare March 19, 2013

March 19, 2013

for (call = &__initcall_start; call < &__initcall_end; call++) (*call)(); I want to know where are these function point assigned by specified function ? thanks!

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Re: Design Patterns in Linux Kernel: Fancy Tricks With Linked Lists
by Victor Buciuc March 19, 2013

March 19, 2013

On Tue, Mar 19, 2013 at 8:50 PM, Victor Buciuc <victor.buciuc(a)gmail.com> wrote: > On Tue, Mar 19, 2013 at 5:34 PM, Arlie Stephens <arlie(a)worldash.org> wrote: >> Hi Folks, >> >> I'm trying to understand the linux way of doing things, so as to write >> code which will be easier for experienced linux kernel people to >> understand. Yesterday I wound up in a 3 engineer discussion about >> something conceptually simple, that just didn't want to be done with >> the tools at hand; what's worse, the only ways I could see to do it >> were (a) do my own linked lists or (b) write some routines that relied >> kind of heavily on the never-mentioned-in-documentation details of >> linux/list.h I'll probably go with option (b), as that seems less >> insane, but I thought I might as well ask here as well. >> >> Here's the problem. I have a collection of resources. Call them >> A,B,C,D but note that resources get added at run time, so I don't know >> how many to expect. I want to go through them in a round robin >> fashion, but not all at once. On the first request, I use A. On a >> later request I use B, etc. This seems to me to map naturally to a >> circular linked list, with a "cursor" - a pointer of some kind to the >> next one I want to use. To make my specific situation more >> interesting, I actually have multiple cursors - that may or may not >> happen to point to the same node at any given time, but usually will >> not. >> >> This is where I wish I could draw pictures in ascii emails ;-) >> Perhaps the following will come through halfway sanely: >> >> C1 C2 C3 >> V / V >> A<->B<->C-<->D >> ^------------^ >> > > Hi, > > Another option might be to not iterate through the list but to simply > move the element you're processing to the end of the list using > list_rotate_left[1]. > And you could use list_next_entry[2] to get the next entry after the > head. You'll > have to check that the list is empty, tough. > > This would cost you some extra operations but you'll use the existing interface. > > [1] http://lxr.linux.no/linux+v3.8.3/include/linux/list.h#L214 > [2] http://lxr.linux.no/linux+v3.8.3/include/linux/list.h#L361 > > Regards, > Victor Buciuc. Forgot to send to the list. Regards, Victor Buciuc.

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for interested folks, i'll announce new kernel tutorials via twitter
by Robert P. J. Day March 19, 2013

March 19, 2013

as i don't want to seem like i'm spamming this list with notes about new, posted kernel tutorials, people who want to stay on top of these are welcome to follow me on twitter at @rpjday. everything written and posted will be freely available (creative commons license), and even after it's incorporated into future training courses, even those courses will be released under the CC so you'll never lose access to any of it. so feel free to play along. rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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Kernel warning messages- protocol 0000 is buggy, dev eth0
by Sengottuvelan S March 19, 2013

March 19, 2013

Hi I am getting kernel messages = "protocol 0000 is buggy, dev eth0" in my kernel while calling dev_queue_xmit(skb);. The packet is successfully tranmitted on the wire but these messages are keep coming on the console. I am not sure what is the exact modification needed to avoid these kernel messages. Can you someone help on this. Exact code I have in my driver code: tx_len is length of the packet to be transmitted tx_packet is the pointer to packet container. skb = dev_alloc_skb(tx_len); if (skb == NULL) return -1; skb_put(skb,tx_len); memcpy(skb->data, tx_packet, tx_len); skb->dev = dev_my_device; skb->len = tx_len; dev_queue_xmit(skb); dev_put ( dev_my_device); return 0; Thank you. -- Regards, S. Sengottuvelan.

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only for test
by Ben Wu March 19, 2013

March 19, 2013

FYI

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