September 2017 - kernelnewbies@kernelnewbies.org

Reviews on linux-kernel
by Christoph Böhmwalder Sept. 26, 2017

Sept. 26, 2017

Hi all, Here's a question that the various "how to help" documents about the linux-kernel mailing list couldn't help me answer: Is anyone "allowed" to review patches on the mailing list? Is it considered appropriate for any random person to review a change and approve/disapprove of it (given, of course, they have an understanding of what's going on in and around the patch), or is this right reserved for subsystem maintainers and other "known" people? Let's say I saw a trivial patch on the linux-wireless mailing list, replacing a local function by a globally defined one (e.g. ARRAY_SIZE). If I know and understand what the patch does, is it appropriate for me to put an "Acked-by" or "Reviewed-by" under it, even if I'm technically not a part of this community? Thanks in advance for your answers! -- Regards, Christoph

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How can I use -O3 compiler optimization flag?
by Ketan Singh Sept. 25, 2017

Sept. 25, 2017

I am building a kernel for arm64 and I want to add -O3 kernel optimization flag to check if this can give any performance improvement. Someone already made a path to do this https://github.com/graysky2/kernel_gcc_patch and reported little improvement in some applications but that patch is only for x86.

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Any successful story of debugging linux 4.13 with qemu 2.10 and gdb 8.01?
by jjDaNiMoTh Sept. 22, 2017

Sept. 22, 2017

Hello all, As the title says, any of you have successfully tried to debug Linux 4.13 with QEMU? My problem is that it is not possible to break (even with hbreak) in any function of the kernel, from the most used (schedule or spin_lock) to the most obvious (uptime_proc_show when /proc/uptime is read). To be more specific, QEMU correctly stops if I put "-s -S" on the command line, and perfectly continue the execution of the kernel when I connect gdb using 'target remote :1234'. However, any breakpoint is simply ignored. I have followed any possible tutorial and any possible answer to similar user questions. I also tried to do the same on a 16.04 version of Ubuntu, but the result is always the same. In particular, I have followed https://stackoverflow.com/questions/11408041/how-to- debug-the-linux-kernel-with-gdb-and-qemu, using both a busybox image and a raw filesystem image of Archlinux. There is any known problem in debugging Linux? How can I "debug" this debug process? Thank you! -- Stay open, be free.

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[IO Scheduler] : Query regarding Multi-Queue Block IO Queuing
by Narendra Pal Singh Sept. 21, 2017

Sept. 21, 2017

Hello, I have a server with bunch of SSD and HDD. i want to enable blk-mq for SSDs only as it gives negative performance for rotational disks. (Please correct me here if i required) Is it possible to enable blk-mq for selective disks ? -- Best Regards, Narendra Pal Singh

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Kernel Compiling Issue
by Herb Garcia Sept. 20, 2017

Sept. 20, 2017

I recently learned how to compile the kernel on my Debian PC. What I'm running into is the default kernel runs well with no issues. With that said, when I will run menu oldconfig. To keep the current settings. When Alall said and done. When I reboot my PC. I get this error: failed to. Start load/save screen backlight brightness Any suggestions appreciated ~H.P. Garcia

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Maximum value of offset in scatterlist structure
by Harsh Jain Sept. 18, 2017

Sept. 18, 2017

Hi, Since scatter/gather structure deals with page address. By defination offset in page should be 0 <= offset< PAGE_SIZE. But in kernel code when ever we increment sg->offset, we didn't not check for upper limit like in function " __skb_to_sgvec". " Is n't it wrong? Regards Harsh Jain

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regmaps with different register sizes
by Hendrik v. Raven Sept. 15, 2017

Sept. 15, 2017

Hi, I currently try to develop a device driver for the National Instruments X-series DAQ cards. These are PCIe cards with real-time capable analog and digital i/o modules. All of these are configured through many registers in combination with dma. I thought of using regmap-mmio to configure all these registers. However the register size is not homogenous throughout the device. Depending on the submodule they use 8, 16, 32 or 64 bit registers. Would it be a problem to use several regmaps in parallel, one per register size? Assuming this would work I've seen the regmap_fields API. It would be nice to use as it would hide the different regmaps inside the regmap_field struct. However the device has many registers and even more fields (didn't count, but many hundred). Is it ok to allocate this many regmap_fields dynamically? I am wondering as all information except the regmap pointer is static. Another question for the same device: The device has the actual output modules and a custom DMA chip (NI CHInCh). As the DMA seems to be a special component of the chip, should I still try to integrate it into the dmaengine framework? And if so, how do I register the modules to have the pci device shared between DMA and DAQ components? Thanks, Hendrik

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how to instrument/debug module loading
by Cliff Brake Sept. 14, 2017

Sept. 14, 2017

Hi, I'm trying to better understand the kernel module loading mechanism (for example when you plug in a USB device and a kernel module autoloads). It appears that depmod creates a module.alias map that maps USB info to modules. Is there any way to instrument or observe the actual loading process? I've looking into udev debug logging, and udevadm, but have not found anything yet that shows when this device is installed, this module loads. I'd also be interested in understanding which program is run to do the loading. I assume udev runs this, but again, I'd like to understand the details better. It seems some of the documentation is out of date -- kernel:Documentation/usb/hotplug.txt refers to /proc/sys/kernel/hotplug and /sbin/hotplug -- neither exist on my system. Appreciate any pointers to documentation, etc. My test system is Arch Linux with systemd. Thanks, Cliff

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What all are the drivers those get compiled with the kernel?
by Dipanjan Das Sept. 14, 2017

Sept. 14, 2017

There are numerous drivers present in the 'drivers' directory of Linux kernel. The kernel source for any Android mobile device [e.g. https://android.googlesource.com/kernel/msm/+/android-msm-2.6.35] is, too, not an exception. Definitely all the drivers are not relevant for the chipset which the mobile device is built on. It leads me to believe that *not* all the drivers which are present in the kernel source for a mobile device are built and shipped on commercial phones. Given the kernel source, how can one figure out what all are the drivers those get compiled for that particular mobile device? -- Thanks & Regards, Dipanjan

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Default file_operations methods?
by Martin Galvan Sept. 12, 2017

Sept. 12, 2017

Hi everyone, I'm testing out a small kernel module and wondered what the default file_operations methods are. I know about no_llseek for llseek, but what about the rest? Is there anything like no_llseek for e.g. read, write and so on? The reason I'm asking this is that my module should only allow opening, closing, and calling a couple ioctls. Reading, writing and such wouldn't make sense here.

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