December 2014 - kernelnewbies@kernelnewbies.org

How to handle Hotplug with UIO userspace driver
by Mandeep Sandhu Sept. 28, 2017

Sept. 28, 2017

Hi All, We're working on a custom PCIe based hardware, which can be hotplugged into the system. We're using the UIO approach of having a minimal kernel driver which basically handles the interrupt and provides a mmap of the device memory to userspace and userspace driver which mmap's the provided memory and manages the device. We're using systemd and udev rules to spawn off a user process whenever the hardware is plugged into the system. Now my question is, how can we gracefully inform the userspace process if someone pulls out the hardware (sitting on a PCI slot). Even with full hotplug support, there can always be a case of "surprise removal", where someone yanks the board without going through the whole "attention button" routine. While the kernel driver has no problem with hot-removal, I see that if I "unregister" our UIO driver in response to the removal, then the mapping for the hardware's physical memory would become "invalid" (done by UIO on unregsiter). This could possibly cause the user-space process to crash due to illegal memory access. Is there a "standard" way to handle this scenario, i.e for hotpluggable hardware using UIO? Basically, we'd want to "unregister" the UIO device only after the userspace process is informed of the removal so that it could unmap/close the device and exit gracefully. We could try doing it using some systemd trick, where we put a dependency on one of the UIO created files (/dev/uioX), so that when the file is destroyed, systemd could kill/shutdown the process. Although, there's no guarantee that the userspace process won't try to access the mmaped space during that interval, so this isn't a reliable solution either. Any hints, thoughts? Thanks., -mandeep

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How to get the inode - no path_lookup
by Rishi Agrawal Sept. 17, 2015

Sept. 17, 2015

Hi All, I had a module which used the path_lookup function to print the details of any file's inode. I now want to rewrite that module in order to show some juniors how to write some code in kernel. I am using 3.4.6 kernel, I tried finding out path_lookup but google showed that it has been removed. I tried the following code then which did not work . . . dentry = kern_path_create(AT_FDCWD, filename, &path, 1); if (IS_ERR(dentry)) { printk("Failed to obtain the dentry"); return; } its not returning dentry I again tried after seeing the implementation of vfs_stat function user_path_at(AT_FDCWD, filename, lookup_flags, &path); but this also fails. I am using a proc interface to pass the filename, and copying the filename into a kernel buffer. How can I get a copy of vfs inode for a file name. -- Regards, Rishi Agrawal

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Is it all right for Netfilter modules to use control buffers (skb->cb)?
by Alberto Leiva April 28, 2015

April 28, 2015

Hi I was assuming yes, since I've seen at least one module doing this. An example is nf_defrag_ipv6 (ipv6_defrag() calls nf_ct_frag6_gather() which calls NFCT_FRAG6_CB()). What confuses me is that the IPv6 subsystem seems to have its own control buffer: - ipv6_rcv() calls IP6CB(): http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L102 - then it calls the netfilter chain, which might include nf_defrag_ipv6: http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L186 - then ip6_rcv_finish() comes into play, which calls dst_input(), which might well call ip6_input(), which calls ip6_input_finish(). - ip6_input_finish() once again dereferences the IPv6 version of the control buffer: http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L219 If Netfilter modules are allowed to wash out control buffers, how does the IPv6 private data seems to be surviving the Netfilter chain? Thanks

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Looking for ARM SoC porting guidelines
by Mike Thompson Feb. 2, 2015

Feb. 2, 2015

I'm working on a port the latest Linux kernel to a Nuvoton N32905 - a very inexpensive ARM9 based SoC similar to the Freescale iMX23 in many respects. My hope is that my code for this SoC will eventually reach the mainline kernel, but the whole process a fairly daunting task to a kernel newbie such as myself. I do have a six year old port of the 2.6.35 kernel to this SoC to help as a reference, but it's pretty much a rewrite to support devicetree, the common clock framework, and a myriad of changes needed to get a new SoC supported in the mainline kernel. Not to mention all the 2.6.35 code added for this chip isn't even close to the coding standards for the Kernel. So far I got the 3.17 kernel running pretty well the baseline peripherals such as clocks, interrupt controller, UART, GPIO & pin muxing along with hire level peripherals such as MMC/SD card support, MTD NAND and NOR flash and USB host support. As a guide, I've been using other ARM SoCs for examples and a number of very useful presentations on ARM SoC support from the Free Electron folks. Although presentations such as the "ARM SoC Linux Support Check-list" are very useful, they don't go into much detail. My question is two fold. Is there detailed information of all the things that need to be covered in the code to introduce a new ARM SoC into the kernel? Then, is there a description of how I start to go about contributing support for this chip into the Linux kernel? I honestly don't really know where to begin. Any advice or pointers would be sincerely appreciated. Mike Thompson BTW, my git reposity for this SoC is: https://github.com/mpthompson/linux/tree/n329

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ternary vs double exclamation
by Vinícius Tinti Jan. 8, 2015

Jan. 8, 2015

Hi, I was looking the kernel source code and there are a lot of places in which either "(expression) ? 1 : 0" or "(expression) ? 0 : 1" appear. As fair as I can tell both can be replaced by "!!expression" and "!expression". Moreover there it seems that using "!!" does not add a "nopl" instruction at the end of the call. Does anybody knows why? Anyway. Wouldn't be nice if kernel provides something like "boolean(x)" macro and "inv_boolean(x)" to do this operations? #ifdef MOD_IF int mod_if(int x) { return (x == 0) ? 0 : 1; } #endif #ifdef MOD_X int mod_x(int x) { return !!x; } #endif 0000000000000000 <mod_if>: 0: 31 c0 xor %eax,%eax 2: 85 ff test %edi,%edi 4: 0f 95 c0 setne %al 7: c3 retq 8: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) f: 00 0000000000000010 <mod_x>: 10: 31 c0 xor %eax,%eax 12: 85 ff test %edi,%edi 14: 0f 95 c0 setne %al 17: c3 retq Regards, Vinícius -- Simplicity is the ultimate sophistication

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Why the Email of @yahoo.com can't subscribe majordomo@vger.kernel.org ?
by lx Jan. 6, 2015

Jan. 6, 2015

hi all: I want to subscribe the majordomo(a)vger.kernel.org about netdev. But It don't work. The error message is: ######################## Sorry, we were unable to deliver your message to the following address. <majordomo(a)vger.kernel.org> <majordomo(a)vger.kernel.org>: Remote host said: 553 5.7.1 Hello [106.10.151.226], for your MAIL FROM address <lxgeek(a)yahoo.com> <lxgeek(a)yahoo.com> policy analysis reported: Your address is not liked source for email [MAIL_FROM] --- Below this line is a copy of the message. Received: from [106.10.166.120] by nm33.bullet.mail.sg3.yahoo.com with NNFMP; 29 Dec 2014 05:50:14 -0000 Received: from [106.10.167.174] by tm9.bullet.mail.sg3.yahoo.com with NNFMP; 29 Dec 2014 05:50:14 -0000 Received: from [127.0.0.1] by smtp147.mail.sg3.yahoo.com with NNFMP; 29 Dec 2014 05:50:14 -0000 X-Yahoo-Newman-Id: 541845.56363.bm(a)smtp147.mail.sg3.yahoo.com X-Yahoo-Newman-Property: ymail-3 X-YMail-OSG: 3s0rdfQVM1lPcl7PYi8obCNwfrPXi49mnORqxY8zxPBP3GZ WJNE8J4eollOsC50XIZRciC4O4E_2ak65nfZUWIcWZ5l3LThPaGj2NLnRRfb ae0Zm7lhLNH5GsT8BCLBOB4KuUeLqo7XFm1uOKu3noMmqMF5qFY9AFTLJRQ4 GBGBn2RLuVQD2SNVV8kbd0otyfm.jJK.AIyCnUymqqi45ux3R1Vatzzu4rhk N5BsL2uYUufhcjbKBl8vSClY4dcBN6gvwCT21AVlPYza0ekIPAS7Y2XPdAKK vJnRPbj8hTidiNVr__g8eTjWBLR7sMPrRZV351BsXsyUrfw4sQitpvobSko0 K_Jg9o_aFPuMgr5qCAS1DjKUVd2JcpGU5.XVp58mZXwi0rg1uePB4oRavhv_ Uj1zX4WBx8q7FSfWsbMGZvSZZpYLWuBrEKNHO34BXJqDr1HzW_ObaY.nzzxC SpuijN8aIG380ZIhgW7xcd6sYG2fFIb6Gxyfg0A6JFhvasNVdXbdcDvJGw_D zsYWQ706pxDyoznM1hIXt5B9NwImQu7NLtUPSxA-- X-Yahoo-SMTP: Ot9DgBiswBD6.gcTBOjHB_vvEA-- Message-ID: <54A0EB96.3060500(a)yahoo.com> <54A0EB96.3060500(a)yahoo.com> Date: Mon, 29 Dec 2014 13:50:14 +0800 From: lx <lxgeek(a)yahoo.com> <lxgeek(a)yahoo.com> User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: majordomo(a)vger.kernel.org Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit subscribe netdev ######################## How to fix it? Thank you.

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Boot up fail with linux-next source code tag next-20141226
by Sunil Shahu Jan. 6, 2015

Jan. 6, 2015

Hi, I am trying to build and boot linux-next kernel tree. My original kernel with Ubuntu 12.04 LTS was 3.5.0-23-generic. I checked out code with tag next-20141226. I generated a ".config" file with "make localmodconfig". I installed modules and kernel.On restart PC do not boot up. I then checked out tag v3.16-rc3 and booted. It worked pretty good and booted properly. I had few question from this exercise: 1) Why my PC didn't boot up with linux-next kernel?2) Do I need to apply any specific patches to boot mainline linux-next kernel on my Ubuntu 12.04 PC?3) How do people test mainline kernel-next? Do they apply their OS specific patch on linux-next tree or test linux-next on some specific OS? Thanks,Sunil.

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Question on checpatch.pl warning -- possible unnecessary 'out of memory' message.
by Bas Peters Jan. 5, 2015

Jan. 5, 2015

Dear all, I'm fixing up my first driver (drivers/isdn/gigaset) to get some experience submitting patches. When I run checkpatch.pl on some of the code, I get the following warning: ./drivers/isdn/gigaset/capi.c:337: WARNING: Possible unnecessary 'out of memory' message The corresponding code is: cskb = alloc_skb(CAPI_DATA_B3_CONF_LEN, GFP_ATOMIC); if (!cskb) { dev_err(cs->dev, "%s: out of memory\n", __func__); return; } All instances of this warning refer to usage of alloc_skb using the GFP_ATOMIC flag. Why does checkpatch show this warning? Is it not good practice to _always_ check whether allocation succeeded or not? I hope someone can explain the nature of this warning and, if right, what it should look like. With kind regards, Bas Peters

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Doubt In Char Drivers
by me storage Jan. 3, 2015

Jan. 3, 2015

Hi All, I am following LDD3 for Char Drivers. So we writing a kernel module(char driver) after that creating a char file which is representation of Character Device. So my doubt is how we will communicate with real hardware device let suppose how to communicate with our monitor . Can any one please explain how we will communicate with real hardware for read/write calls? And what is mknod is doing internally when we are creating char/block devices? Thanks in Advance

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Re: question about pr_info (newbie)
by Larry Martin Dec. 30, 2014

Dec. 30, 2014

Hi, Kevin - This gets me all the time. I think your system is working correctly. This is the log entry showing your system call: > Dec 29 11:24:40 kv kernel: [67479.245642] Hello world! And this is your actual output: > Dec 29 11:24:40 kv kernel: Hello world! If you reduce your log level you will lose the first message but not the second. BR, Larry

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