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Query: UBIFS: How to detect empty volumes
by Pintu Agarwal June 27, 2021

June 27, 2021

Hi, I have one basic query related to UBIFS volumes on a system with NAND partition. In short, how to detect a particular empty volume inside a system partition while flashing the partition image? Suppose I have one big system partition that consists of 4-5 ubi volumes inside it with varying sizes. Lets say: -- System Partition (ubi image) - rootfs volume (ro, squashfs) - data volume (rw, ubifs) - firmware volume (ro, ubifs) - some-other volume (ro, squashfs) Consider that all these could be flashed together as part of system.ubi image in a single shot from the bootloader. Now, suppose, one of the volume image (say firmware) is missing or remains empty (as you know we can have empty volumes). So, during system image flashing, we wanted to detect if one of the volume (firmware) is empty. Since this is an important volume, so we wanted to detect if this volume is empty/missing we will abort flashing the system partition. As there is no point in booting the system without this partition. So, I am exploring options, how can this be detected ? I mean is there any kind of magic number or header information which we can read to detect a particular empty volume ? Can we get any information from just "system.ubi" image to indicate about the volume information ? Also it could be possible that 2 or more volumes are empty, but we are only concerned about one particular volume (firmware), so how to detect particular volume ? If anyone has any thoughts about this requirement, please share your opinion. Thanks, Pintu

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Wikibook Linux kernel
by Constantine Shulyupin June 25, 2021

June 25, 2021

Dear Linux kernel documentation writers and readers: Writing Linux documentation is a huge complex collaborative process. To make it better I invite you to contribute to https://en.wikibooks.org/wiki/The_Linux_Kernel . Main benefits of the book: - Readers friendly architecture - Matrix table of contents - Convenient common wikimedia format - Hosted on wikibooks.org - Updateable by everyone. I've designed the front page matrix table of contents and composed draft articles with outline, links to kernel documentation, sources and other resources. Sometimes visitors of the book contribute to it. My vision is to have a complete updated wikibook about Linux kernel written by many experienced developers. Please vist page https://en.wikibooks.org/wiki/The_Linux_Kernel/About for the book desripton. Welcome to contribute to https://en.wikibooks.org/wiki/The_Linux_Kernel! Sincerely yours Constantine Shulyupin -- Constantine Shulyupin

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Adding a custom layer 3 protocol driver
by Cedric Velandres June 25, 2021

June 25, 2021

Hi everyone, We're currently developing a new communication using custom layer 2 and 3 that we'll be using in a cubesat and I'm looking on how to create the layer 3 driver for linux (similar to af_inet.c but the headers are completely different). I've read something similar in a thread about a previously asked question here, but it didn't really answer my questions. And so, my questions are: 1. Will it suffice to patch linux/socket.h and insert the PF_xxxx before PF_MAX then use the added PF_xxxx as family in struct net_proto_family? 2. Is there another way to do the above without patching and only in an out-of-tree module? 3. How do I expose the new AF_xxxx to userspace? It seems like macros for AF_xxxx are placed in bits/socket.h, but I haven't found a way to patch it. Thanks, Cedric

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why does an in-tree loadable module taint the kernel
by jim.cromie＠gmail.com June 16, 2021

June 16, 2021

serio_raw is apparently tainting the kernel when its modprobed. why ? other modules load properly, no code changes to this module bash-5.1# dmesg | grep -i taint [ 6.517150] serio_raw: module verification failed: signature and/or required key missing - tainting kernel [ 7.449072] CPU: 0 PID: 203 Comm: systemd-udevd Tainted: G E 5.13.0-rc6-lm1-00004-g28dc6f490a7f #106

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USB sound card issues
by Muni Sekhar June 15, 2021

June 15, 2021

Hi all, I am using a USB sound card(snd-usb-audio), and it fails while doing the audio recording "arecord: pcm_read:2032: read error: Input/output error". What could be the problem? and is there a workaround for this? $ arecord -l **** List of CAPTURE Hardware Devices **** card 1: BT_AG [BT_002], device 0: USB Audio [USB Audio] Subdevices: 1/1 Subdevice #0: subdevice #0 $ arecord --device hw:1,0 --channels 1 --format S16_LE --rate 32000_Hz x.wav Recording WAVE 'x.wav' : Signed 16 bit Little Endian, Rate 32000 Hz, Mono arecord: pcm_read:2032: read error: Input/output error -- Thanks, Sekhar

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Getting backtrace of a process
by hemanth nandish June 15, 2021

June 15, 2021

I am trying to get create a custom error log of a process when it causes a segmentation fault. We can modify the* /proc/sys/kernel/core_pattern* file to pipe the core information to a handler binary. I am trying to use the *libunwind *library within this handler to also get the backtrace information but it is failing. As per my understanding, the *do_coredump* kills the faulty process inside *coredump_wait* function before invoking the userspace handler, This might be the reason the libunwind fails to gather the backtrace as it uses PTRACE calls. Is there any way to overcome this without modifying the kernel code?

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CMA: why DMA_ATTR_FORCE_CONTIGUOUS is not used in v4l2 or DRM cma helper
by ayaka June 11, 2021

June 11, 2021

Hello All I am looking for the CMA allocator in current kernel, when I check the videobuf2-dma-contig.c and drm_gem_cma_helper.c, they just call dma_alloc_wc()/dma_alloc_attrs() without any flag that request a contiguous pages. I could hardly find a driver use the flag DMA_ATTR_FORCE_CONTIGUOUS, what I need to do if I want to allocate a CMA and DMA capable buffer for a driver(not all the buffers, because it may have a iommu in a different direction) ? And anything I could that limit a driver that only allocate the pages for the DMA in a address base(for example only the low 4G memory) ? Sincerely ayaka

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SNDRV_PCM_IOCTL_READI_FRAMES returns EIO
by Muni Sekhar June 10, 2021

June 10, 2021

Hi all, I noticed recording(arecord) audio data on a USB sound card fails at SNDRV_PCM_IOCTL_READI_FRAMES is giving EIO. Can someone please clarify when the kernel returns EIO for Ioctl SNDRV_PCM_IOCTL_READI_FRAMES? -- Thanks, Sekhar

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Data present in vm_area_struct
by Deepanshu Kartikey June 7, 2021

June 7, 2021

How can I access the "data" present in vm_area_struct i.e virtual memory page. As I want to learn how the data is picked from the vm page add put it to the physical ram. Thanks

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Missing panic logs in /var/log/kern.log
by manty kuma June 7, 2021

June 7, 2021

I just triggered a panic, expecting that the logs will be visible in `/var/log/kern.log` after reboot, but there are no logs present there. Considering I have no access to the serial port, how do I know what went wrong? Is there anything we can do to make sure that the kernel log contains all the messages until before reboot? # sysctl kernel.panic kernel.panic = 30 If more information is needed, please let me know. Best Regards, Manty

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