July 2016 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

where is __percpu_seg initialized?
by 落痕 March 28, 2018

March 28, 2018

Hi all: I knew the kernel implement the per-cpu variable via __percpu_seg. For example on x86 architecture, it's defined as: #ifdef CONFIG_X86_64 #define __percpu_seg gs #define __percpu_mov_op movq #else #define __percpu_seg fs #define __percpu_mov_op movl #endif But I'm curious where __percpu_seg is initialized?? I fail to find the relative code. Thanks.

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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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What are need to makes humidifier?
by 윤영석 April 1, 2017

April 1, 2017

Hi, I want to make own humidifier. What sensor need when making humidifier?? humidity indicator (detecting humidity), ultrasonic waves maker?? Please shares your wisdom to me. Thanks. Best regards.

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Are these books outdated?
by Aleksander Alekseev Aug. 15, 2016

Aug. 15, 2016

Hello I'm a full-time *nix C developer. I have a pretty good idea how operating systems work. Still I would like learn more about Linux internals in particular, write a few device drivers, maybe some patches to kernel itself, etc. Here are a few books I've found: * Linux Kernel Development, 3rd Edition (2010) * Understanding the Linux Kernel, 3rd Edition (2005) * Linux Device Drivers, 3rd Edition (2005) Could you tell me please, are these books considered worth reading in year 2016 or they are completely out of date? Perhaps there are some newer books and/or tutorials you could recommend? -- Best regards, Aleksander Alekseev

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doubt on schedule_work() - work task getting scheduled lately
by Muni Sekhar Aug. 5, 2016

Aug. 5, 2016

Hi All, I have a doubt regarding the workqueue scheduling. I am using the workqueue for processing the Rx Interrupt data. I am calling schedule_work() on receiving the Rx interrupt from hardware. I calculated the time between calling the schedule_work() and workqueue task actually getting executed, Here I see many cases of less than 100 us(It is fairly good). But sometimes I see 10’s of ms and a lot in the 100’s of ms. I have seen over 0.5 secs too. I would like to know why does sometimes kernel takes longer time(in milli seconds) to schedule it? Is there any way to reduce this time gap? My code: static void my_workqueuetask(struct work_struct *work) { printk("In %s() \n",__func__); mutex_lock(&bh_mutex); //Do something here mutex_unlock(&bh_mutex); } static irqreturn_t my_irq_handler(int irq, void *dev) { ……; if(Rx Interrupt) schedule_work(&rx_work); return IRQ_HANDLED; } -- Thanks, Sekhar

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Dynamic Loading and unnloading of Device tree(dtb)
by Raul Piper Aug. 4, 2016

Aug. 4, 2016

Hi, Where can I find the example for the dynamic Loading of the device tree and its corresponding device driver. How can I build the dts with or without the kernel source. I am getting this error whenever I am trying to build the below sample dts file using the dtc compiler. dtc -O dtb -o leds-ns2.dtb leds-ns2.dts Error: leds-ns2.dts:1.1-2 syntax error FATAL ERROR: Unable to parse input tree Below is the content of the leds-ns2.dts #include <dt-bindings/leds/leds-ns2.h> ns2-leds { compatible = "lacie,ns2-leds"; blue-sata { label = "ns2:blue:sata"; slow-gpio = <&gpio0 29 0>; cmd-gpio = <&gpio0 30 0>; modes-map = <NS_V2_LED_OFF 0 1 NS_V2_LED_ON 1 0 NS_V2_LED_ON 0 0 NS_V2_LED_SATA 1 1>; }; }; Thanks, R

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Minimul Config file for Dev Environment
by Jitendra July 31, 2016

July 31, 2016

Hi Folks, I usually compile the linux kernel by copying current system .config file. But the problem it compile bunch things that I don't which is required. My aim is setup filesystem environmrnt setup on my laptop. Could somebody point the minimal .config file that will save from me reading all the options and compile the kernel quickly and test it. PS: I google it out, but most .config files are very old. I am hoping somebody helping me for current mainline kernel. --- Jitendra

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Ethernet driver - without DMA ?
by Ran Shalit July 29, 2016

July 29, 2016

Hello, Can we write Ethernet driver without using dma ? But still using sk_buff APIs like done in most drivers ? Thanks, Ran

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LinuxChange Page has incorrect intra page links
by Craig Andersen July 29, 2016

July 29, 2016

Not sure if this is where to report HTML errors. The https://kernelnewbies.org/LinuxChanges intra page links do not work and have incorrect references. Example: Incorrect linkage. <li><a href="#head-fbc5af93967e6335e4be8d4162e4e4964428b130 <https://kernelnewbies.org/LinuxChanges#head-fbc5af93967e6335e4be8d4162e4e49…>">perf trace calls stack</a></li> References <h2 id="head-4fbd15090c2d8d0d322a037a2df3c49fb7041489">1.5. perf trace calls stack</h2>

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Help understanding block layer sample in LDD3
by François July 29, 2016

July 29, 2016

Hi there, I've been reading LDD3's chapter 16 on block devices a few times, and have made toys block layers module. Now I've been looking at the up-to-date examples provided by martinez javier[1], but still there's a fundamental concept I fail to understand. Considering only the RM_FULL and RM_SIMPLE cases, a request queue is created, bound with a lock, and associated with a request function. In the simple case, that function "sbull_request" processes each request from the request queue, and delegates the work to "sbull_transfer", which basically performs some arithmetic and does the actual data copy. This function is given a sector, a number of sectors, a pointer to a buffer, and a read or write parameter extracted from the request using blk_rq_pos(), blk_rq_cur_sectors(), req->buffer and rq_data_dir() respectively. On the other hand, the same mechanism is used, but a different function is associated: "sbull_full_request". That function also extracts requests and delegates the actual work to "sbull_xfer_request" which iterates on the request's bio, calls "sbull_xfer_bio" which itself iterates on the bio's segments and finally, calls the same "sbull_transfer" function. What I fail to understand is: how (with the same initialization) the behaviour of the module using those two somehow different mechanism is equivalent. One has to understand the full complexity of the underlying data structure (requests having bio, having segments) while the other only reads the containing structure (the struct request) and do the same job, without iterations. Bonus point, to give some context: I'm writing an asynchronous block-layer which has to split requests into custom subrequest. I'm wondering which approach (between those two) I should pickup. Thanks for reading so far, and for any hints :) [1] https://github.com/martinezjavier/ldd3/blob/master/sbull/sbull.c -- François

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