August 2017 - kernelnewbies@kernelnewbies.org

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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[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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USB keyboard can NOT wakeup from S3( suspend to RAM )
by jingyun＠cpu-os.ac.cn Sept. 5, 2017

Sept. 5, 2017

hi, there thx in advance; I encounter a strange (maybe) bug: During the suspending of my linux box(which runs debian 9), I hit the key of the USB interfaced keyboard; after making sure the box enter suspend state for a while ( which I can tell from the power LED blinks), I can NOT wakeup it by hit that keyboard any more. But if I do NOT hit the key during the suspending, I CAN wake it up by hitting the keyboard. After that I tried Ubuntu 16.04(runs 4.4 version kernel ) and Ubuntu 14.04 ( 3.13 kernel). And I found all of them got the same problem; Do not know if this is the right place to ask. Just a try. Thanks again. Jim

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Access RAM data externally (dual port ram)
by Jay Aurabind Aug. 30, 2017

Aug. 30, 2017

Hi, For a learning experiment, I'd like to access data in the RAM from an external interface. Please advise me how to accomplish this in Linux. The SoC is connected to external RAM that has two ports. Only one port is connected to the SoC. The other port is connected to an FPGA which is used as a hardware accelerator. FPGA has a high bandwidth access directly to the SoC as well. Once the SoC triggers an operation on the FPGA (via I/O mapped memory), it is expected to read the input data from the RAM, process it and write back the results data to RAM. Prior to triggering the task, SoC will have to tell in advance where in the RAM the required input data is (data's physical address in RAM). This is done so that the latency can be avoided in copying the input data around (RAM <-> SoC <-> FPGA) because the input data may come via network or from a local peripheral to the RAM. I'd like to reduce the latency by not making the data go back to SoC and then to FPGA. Because even after reaching FPGA, I might need to copy this data to FPGA's RAM. This will unnecessarily increase the latency to trigger operations on FPGA. This unnecessary copy overhead can be avoided if FPGA can directly read the SoC's RAM and if both FPGA and SoC are using the same physical RAM. When the userspace application runs (responsible for triggering the operation), it will pass the address if input data to the kernel driver which converts process's virtual address to physical address (address in the RAM) and pass this information to FPGA. The FPGA can then directly read the physical RAM address and start its job. Once done, FPGA writes the results directly to RAM and notifies the SoC about the location of the result in RAM, which is through the kernel driver which converts RAM's physical address converted to virtual and mapped to the process address space. I guess this sounds reasonable when the input data is in the heap. If its in the stack, I'd have to copy it to the heap I suppose. I've learned that memory mappings for userspace processes may change, so the physical address may not be valid indefinitely. But the solution of using mlock() sounds reasonable to me. I'd like to know the feasibility of the idea, and security implications, if any. -- Thanks and Regards, Jay

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Regarding install uvcvideo driver for Iball chd 20.0 webcam
by vaibhav chaudhari Aug. 29, 2017

Aug. 29, 2017

Dear sir, I want to configure iball chd 20.0 webcam on ubuntu 14.04 with kernel version of 3.13.0-24-generic. for that i want to install uvcvideo driver please suggest me how to install uvcvideo driver. when i have try modinfo uvcvideo command it gives depends videodev,videobuf2-core,videobuf2-vmalloc.Plsease help me for the same. Thanks Vaibhav Chaudhari

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why set tcp check CSUM_MANGLED_0?
by 孙建希 Aug. 29, 2017

Aug. 29, 2017

all: I am reading tcp checksum code. uh->check = csum_tcpudp_magic(src, dst, len, IPPROTO_UDP, csum); if (uh->check == 0) uh->check = CSUM_MANGLED_0; #define CSUM_MANGLED_0 ((__force __sum16)0xffff) uh->check will be 0, but will never be 0xFFFF. Why set uh->check to CSUM_MANGLED_0?

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Want some help for starting contribution to linux-kernel projects
by Harsha Sharma Aug. 25, 2017

Aug. 25, 2017

hello! I am trying to fix some of bugs from https://bugzilla.kernel.org/buglist.cgi?component=man-pages&list_id=959835&… but where to make the required changes. Can anyone please tell what bugs I can start fixing to contribute in linux-kernel projects?? I am comfortable in C programming and linux-command line Please help . I am really looking forward to this.

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wrapping Kconfig "source" directives in "if" statements?
by Robert P. J. Day Aug. 25, 2017

Aug. 25, 2017

just curious as to Kconfig style, i noticed the following in drivers/pps/. first, once PPS is selected, the subsequent two config variables predictably "depend" on PPS: menuconfig PPS tristate "PPS support" ---help--- ... snip ... config PPS_DEBUG bool "PPS debugging messages" depends on PPS help ... snip ... config NTP_PPS bool "PPS kernel consumer support" depends on PPS && !NO_HZ_COMMON help ... snip ... obviously, those two dependencies on PPS could be removed and those two config variables could then be wrapped in: if PPS ... endif but with only two config entries, it's like, meh, no big deal, who cares? but that Kconfig file finishes by sourcing two pps subdirs: source drivers/pps/clients/Kconfig source drivers/pps/generators/Kconfig each of which appears to totally depend on PPS; here's the (stripped) clients/ Kconfig file: comment "PPS clients support" depends on PPS config PPS_CLIENT_KTIMER tristate "Kernel timer client (Testing client, use for debug)" depends on PPS config PPS_CLIENT_LDISC tristate "PPS line discipline" depends on PPS && TTY config PPS_CLIENT_PARPORT tristate "Parallel port PPS client" depends on PPS && PARPORT config PPS_CLIENT_GPIO tristate "PPS client using GPIO" depends on PPS same with the pps/generators/Kconfig file. so, rather than all of those "depends on PPS" lines, would it be equivalent to have the top-level Kconfig file just look like: menuconfig PPS tristate "PPS support" if PPS config PPS_DEBUG bool "PPS debugging messages" config NTP_PPS bool "PPS kernel consumer support" depends on !NO_HZ_COMMON source drivers/pps/clients/Kconfig source drivers/pps/generators/Kconfig endif and remove all the lower-level Kconfig "PPS" dependencies? i'm pretty sure that would be equivalent, would it not? or is there some subtlety that prevents that? 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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