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gdb debug get_unmapped_area
by Dengke Du Sept. 10, 2018

Sept. 10, 2018

Hi When I use gdb debug get_unmapped_area on qemumips64, I found a strange phenomenon: file: mm/mmap.c function: get_unmapped_area line: addr = get_area(file, addr, len, pgoff, flags); This return -22, I want to find where return -22, then I step in get_area: (gdb) s shmem_get_unmapped_area (file=0x900000000888b000, uaddr=18446744073709547520, len=4096, pgoff=0, flags=17) at /usr/src/kernel/mm/shmem.c:2043 2043 /usr/src/kernel/mm/shmem.c: No such file or directory. (gdb) n arch_get_unmapped_area_topdown (filp=0x900000000888b000, addr0=18446744073709547520, len=4096, pgoff=0, flags=17) at /usr/src/kernel/arch/mips/mm/mmap.c:142 142 /usr/src/kernel/arch/mips/mm/mmap.c: No such file or directory. (gdb) n 143 in /usr/src/kernel/arch/mips/mm/mmap.c (gdb) n arch_get_unmapped_area_common (filp=0x900000000888b000, addr0=18446744073709547520, len=4096, pgoff=0, flags=17, dir=DOWN) at /usr/src/kernel/arch/mips/mm/mmap.c:59 59 in /usr/src/kernel/arch/mips/mm/mmap.c (gdb) n 66 in /usr/src/kernel/arch/mips/mm/mmap.c Why it jump three three function? shmem_get_unmapped_area arch_get_unmapped_area_topdown arch_get_unmapped_area_common

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ATA link errors in custom PATA driver
by Nathan Williams Sept. 5, 2018

Sept. 5, 2018

Hi, I have a Altera socfpga platform that implements a very basic ATA PIO0 Compact Flash controller in the FPGA and I have a very basic PATA Linux driver that does MMIO to the FPGA registers and configures 3 GPIOs for power, reset and card detect. It's basically a copy of pata_platform.c with a check_status function to read the card detect GPIO and if a card is detected, it switches on the power GPIO and deasserts the reset GPIO. The driver and FPGA CF controller work fine together for me except for an intermittent issue where on boot I occasionally see the following: [ 5.836242] ata1: link is slow to respond, please be patient (ready=0) [ 10.816241] ata1: device not ready (errno=-16), forcing hardreset [ 16.016250] ata1: link is slow to respond, please be patient (ready=0) [ 20.876239] ata1: SRST failed (errno=-16) [ 26.076243] ata1: link is slow to respond, please be patient (ready=0) [ 30.936243] ata1: SRST failed (errno=-16) [ 36.136243] ata1: link is slow to respond, please be patient (ready=0) [ 65.956240] ata1: SRST failed (errno=-16) [ 70.976238] ata1: SRST failed (errno=-16) [ 70.981059] ata1: reset failed, giving up This is occurs when there is no CF card plugged in, so I don't understand why the driver appears to be trying to communicate with a Compact Flash device? I'm a little stuck trying to figure out how to properly handle a hot pluggable CF card, so any advice on this would be much appreciated. Nathan

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Implementing tight time constraints for read/write with USB serial (ftdi_sio)
by Thomas Bracht Laumann Jespersen Sept. 5, 2018

Sept. 5, 2018

Hello all! I'm working on a project where we want to integrate into a production line and listen on connected devices, and the challenge is that our setup needs to be fast enough. Let me try to explain the setup. There are some 80+ devices on the network. They are connected in a multidrop talking PROFIBUS at a speed of 1.5MB. I have an RS485<->USB serial cable that uses the ftdi_sio driver to listen on the network. It is identified as a FT232RL. Just listening and de-constructing the PROFIBUS communication blocks that are coming through is not a problem. The problem is when I need to reply to messages in the network. At first I was seeing reads from the serial port taking either ~.002ms or ~16ms, which I assume is due to buffering on the chip. I then learned about setserial and tried adjusting the latency to 1ms. This gives me read times at around ~4ms on average (but sometimes going above 10ms). I'm looking for ways to disable/minimize the buffering on the FTDI chip, but I also suspect that the ftdi_sio driver does some buffering. The challenge is that my application needs to act within 100 tbits, ie in around ~.066ms. That is the window in which I need to begin transmitting a reply. Is there a way to achieve this with the ftdi_sio driver or do I need dedicated hardware for this? I guess my question is: Is it possible to achieve sub-millisecond response times in a "standard" Linux (ie a desktop variant)? Or is it necessary to look into real-time Linux or other specific kernel configuration? As an aside, I'll mention that I've looked into libftdi as an alternative (could it be?), but since it specifically unloads the kernel driver I'm not sure I should be asking about that here :-) To be honest, I'm not sure if kernelnewbies is the right place to ask, if not, I'll be happy to take my questions elsewhere :-) All the best, -- Thomas

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gpio: userspace tools
by Subhashini Rao Beerisetty Sept. 3, 2018

Sept. 3, 2018

Hi All, I’m using v4.4 kernel and I see that gpiochipN are created under /sys/class/gpio. Latest kernel has three tools lsgpio, gpio-hammer and gpio-event-mon, all these tools opens the device node under /dev/gpiochipN. Does v4.4 kernel has any similar tools to control GPIO pins by using /sys/class/gpio/gpiochipN? If not, can someone point me how the GPIO pins were controlled in the older kernel(which supports /sys/class/gpio)? Thanks

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Queries on bottom halves
by Abhinav Misra Sept. 3, 2018

Sept. 3, 2018

Hi, I have few queries regarding bottom halves- 1. Interrupts use their own per cpu stack for their execution.Which stack does the bottom halves use for their execution ? 2. Why can't we sleep in bottom halves ? BR,Abhinav

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Question about DMA mapping.
by phind.uet＠gmail.com Aug. 31, 2018

Aug. 31, 2018

Hi everyone, Thank for your time, I'm new to linux kernel. I am reading LDD3 chapter 15, Dirrect Memory Access section. I see that when I call function /dma_map_single/ and /dma_unmap_single/, I need to pass a direction as a parameter. I wonder what is purpose of it? Does It relate to something like permission? Can I call /dma_map_single /with DMA_BIDIRECTIONAL and then call /dma_unmap_single /with DMA_TO_DEVICE? Will it cause any problem? Thank you very much.

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suggestions for good in-kernel examples of simple ethernet/PHY drivers?
by rpjday＠crashcourse.ca Aug. 30, 2018

Aug. 30, 2018

while i'm actually learning about the intricacies of the kernel networking stack, i was looking for what would be really good examples of existing network drivers that could be used as examples for future tutorials, as nothing is as informational as seeing something that actually works. there are, of course, a ton of net drivers already in the kernel, but many of them are fairly complex, and might define custom methods to handle individual quirks and so on, so what i'm interested in is pointers to examples that are nice and simple and straightforward that could be used for a walkthrough of net driver design. thoughts? rday p.s. i'm already aware of the mass of documentation under Documentation/networking -- any pointers to online net driver design and tutorials would be appreciated. and, yes, i'm also digging through LDD3, https://lwn.net/Kernel/LDD3/.

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How to alloc memory in kernel on a specific numa node
by bing zhu Aug. 28, 2018

Aug. 28, 2018

Dear Sir / Ma'am: Greeting ,Thank you for your time,I'm trying to alloc some memory on a specific numa node (like cpu 0) ,but i'm new to linux kernel ,can someone help me with some information ?please . Thanks anyway

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First Patch: Memory Limit Exceeded
by Anika Murarka Aug. 27, 2018

Aug. 27, 2018

Hi, I am Anika , and I have been trying to submit my first patch for Linux Kernel. I had completed till the make command and rebooted my vm after that. While rebooting the following message popped :- " End kernel panic not syncing out of memory and no killable processes" . I tried the whole procedure again , yet again same error occurred. Please let me know what can be done. Thanks, Regards, Anika Murarka

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Merge Window closed, submitting patches?
by John Whitmore Aug. 26, 2018

Aug. 26, 2018

Recently got into the habit of making very simple coding style changes in staging. It's basic, but it is a gentle introduction to the process involved in submitting patches to the kernel. So the merge window is now closed and I have submitted a few patch sets, the first not knowing that the window was closed, and the second wrongly thinking that the window had reopened. My question is about what to do during the closed window? I don't think I should submit patches as I assume I'll create a backlog for the maintainer who'll possibly be flooded when the window re-opens. Similarly I could create patches but not send them until the window re-opens, but again I'll just be creating a backlog. So I guess I should just be patient and find something else to do for a while, but thought I'd ask. I have tried to find the answer to this but haven't found one. jwhitmore

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