December 2019 - kernelnewbies@kernelnewbies.org

How to make /dev/ttyACM0 (and friends) exclusive?
by Jeffrey Walton Oct. 11, 2020

Oct. 11, 2020

Hi Everyone, I'm trying to trackdown a problem using my dialup modem. I have a program that opens the modem and watches caller id's. It flashes the hook when a telemarketer calls. It works well until... When another program opens the modem then my program starts reading intermittent responses intended for the other program. I cannot seem to open the device in exclusive mode. The current open is (I also tried with O_EXCL): int modem = open(device_path, O_RDWR | O_NOCTTY | O_SYNC); I'm not sure if it because udev configures the device incorrectly or the kernel driver is configured incorrectly. Or maybe it is impossible to do. Does anyone know how I can make the device exclusive? Thanks in advance.

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interrupt handler not getting called after resume
by Pintu Agarwal Dec. 30, 2019

Dec. 30, 2019

Hi All, I have one general query. If an interrupt handler is NOT getting called (for one device) after the system resume (from snapshot image), then what could be the issue? Note: * The suspend worked perfectly fine. * The runtime suspend/resume of the system is also working fine. If anybody have experienced this situation and fixed it, please let us know. It will be a great input for further debugging. Regards, Pintu

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Sample Block Multiqueue Driver
by cdupuis1＠gmail.com Dec. 28, 2019

Dec. 28, 2019

Hi Folks, While there are some good references for block drivers such as loop and null_blk, I've not see a good bare bones block multiqueue example driver. This may not mean that one doesn't exist but since I personally have not seen one, I've created one called blk_example which I've posted on my github account: https://github.com/cdupuis1/nvme_test/tree/master/blk_example. I've tried to make as bare bones a driver that actually still will work as a small block device, /dev/blk_example. The memory footprint of the backing store is relatively small, 2000 sectors or 1MB, but is resizeable with the num_sectors module parameter. It was helpful as a learning experience for me so I'd thought I would share in case anyone else happened to be looking for such a sample driver. Comments or improvements are welcome. Thanks, Chad

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Kernel assisted keychain?
by Jeffrey Walton Dec. 23, 2019

Dec. 23, 2019

Hi Everyone, Does the kernel have a Keychain API to save secrets locally? I'm specifically looking for kernel assistance and a API to use across different OSes. Jeff

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the difference between /drivers/spi and /drivers/mtd
by Tomek Domek Dec. 19, 2019

Dec. 19, 2019

Hi I have in device tree node: qspi: somename@0 { compatible = "nameOfDriverFrom/drivers/spi"; reg = <someAddr size>; clocks = <reference to clock>; clock-names = "some names"; #address-cells = <1>; #size-cells = <0>; status = "disabled"; }; and then I have such thing: &qspi { pinctrl-names="default"; pinctrl-0 = <&reference to some pin controller>; status = "okay"; spi-max-frequency = <11000000>; flash@0 { compatible = "spi-flash (this is driver from /drivers/mtd/)"; #address-cells = <1>; #size-cells = <1>; reg = <0x0>; spi-max-frequency = <22000000>; }; }; Why just one node isn't enough? And what frequency driver have at all - 11MHz or 22MHz? What is the difference between drivers defined in /drivers/spi and /drivers/mtd? Does it mean that driver in /drivers/mtd can use somehow driver or the code from /drivers/spi? Or maybe it works in opposite direction?

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Free RAM in Linux .
by Neel chakraborty Dec. 17, 2019

Dec. 17, 2019

I was studying the Linux Kernel Source , and came across this doubt . Does Linux use all of the physical memory (RAM) I have ? In both the outputs of /proc/meminfo and free -h , shows that 1.4 gigs is used and 1.6 gigs is cached , and the rest is "free" out of 32 Gigs . The available ram is the cached ram + reclaimable ram + free ram , right ? I went through the source code of /fs/proc/meminfo.c and it's polling ram usage from the sysinfo struct , so I browsed through /linux/include/uapi/Linux/sysinfo.h , over there freeram is available memory size . But again in meminfo.c , we are seeing MemFree as i.freeram ( here i is assigned to the structure of type sysinfo ) . so is free ram = available memory ? But then , I have free ram of 27 Gi , and available memory of 28 Gi , why is that ? And also , does the linux kernel use the amount of ram which is not used by applications as paging cache ? Say I have 4 gigs of ram , and Firefox is using 1 gig of it , the rest of RAM is used for disk/page caching or is it just unused and left there ? Thank you

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How to get the preprocessor output as part of the compilation process?
by Frank A. Cancio Bello Dec. 12, 2019

Dec. 12, 2019

Hi, I know that with gcc -E you can get the output of the preprocessor, but what I have to do to get that output for every source code file in the Linux Lernel as part of the compilation process? thanks frank a.

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Missing FDT description in DTB
by Tomek Domek Dec. 6, 2019

Dec. 6, 2019

Hi I have FSBL and SSBL. SPL uboot starts successfully and u-boot.img is loaded successfully but it hangs on 'boot_from_devices' function and 'fit_find_config_node': fit_find_config_node: Missing FDT description in DTB No matching DT out of these options: Firmware image with one or more FDT blobs boot_from_devices error SPL: failed to boot from all boot devices I have suspicion that I wrongly create u-boot.img. My system build do this by executing: ./tools/mkimage -f auto -A arm -T firmware -C none -O u-boot -a 0x100000 -e 0 -n "someName" -E -d u-boot-no-dtb.bin u-boot.img >/dev/null The same issue occurs when I use u-boot-dtb.bin version. The value 0x100000 is the value stored in CONFIG_SYS_TEXT_BASE. And this uboot and spl is somekind of experimental software which is in the middle of creation. Could anyone try to guide what might be possible the reason of the issue as I am a bit new in u-boot development? BR Tomek

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interrupt handler not getting called after resume
by Pintu Agarwal Dec. 3, 2019

Dec. 3, 2019

Hi All, I have one general query. If an interrupt handler is NOT getting called (for one device) after the system resume (from snapshot image), then what could be the issue? Note: * The suspend worked perfectly fine. * The runtime suspend/resume of the system is also working fine. If anybody have experienced this situation and fixed it, please let us know. It will be a great input for further debugging. Regards, Pintu

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I2C bus driver TIMEDOUT because of PM autosuspend
by Primoz Beltram Dec. 3, 2019

Dec. 3, 2019

I am analysing a problem with I2C bus driver where the problem shows up as I2C bus completely blocked. The LX driver in question is /drivers/i2c/busses/i2c-xiic.c. Problem is difficult to reproduce, it happens very rarely. So far I saw that the main precondition is to have very heavy I2C traffic on bus. In my case this is achieved/reproduced via netdev driving SFP LEDs via /sys/class/leds/ (via gpio-pca953x). I generate traffic with iperf3. Network traffic is on 10Gbps EMAC. LX kernel is 4.14.0. What I saw from debugging this problem is that I2C bus get blocked when wait_event_timeout() completes because of timeout. The timeout handling in this driver is probably not robust enough (bus should not remain blocked), but at this moment this are just my speculations (don't know enough details). Looking the driver code and data on oscilloscope, I saw that SCL in single I2C data transfer sequence can be interrupted for very long delays, e.g up to hundredths of usec (SCL is 100kHz). I started to suspect that PM autosuspend delay could play some role here. There are only two delays in driver code, first in wait_event_timeout and second in set autosuspend delay. Case is a bit strange because in very busy I2C traffic, PM autosuspend should not be triggered at all. Additionally, if I lower PM timeout, e.g. from 1000 (default) to 100, I hit the problem sooner (waits for problem hit are in order of n*10minutes). It looks to me that PM autosupend is playing some role here. Power management options in my .config: # CONFIG_SUSPEND is not set # CONFIG_PM is not set CONFIG_ARCH_SUSPEND_POSSIBLE=y I intentionally did not put all detail descriptions of embedded system and test setup here (long list), because the main reason of this post is: The workaround that works for me/customer (at the moment) is to disable PM autosuspend in the driver code, either by incerementing PM delay from 1000 to 10000 or by disabling autosuspend (comment out call to pm_runtime_put_autosuspend() in xiic_xfer()). But, I would like to expose/discuss this issue (maintainer of the code, or others). The reason/source of the problem can be much more complex and in some other place. So my question is who should I contact, is this the M: in the MAINTAINERS list, the MODULE_AUTHOR, ...? How to proceed. WBR Primoz

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