May 2018 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Softlockup and Hardlockup sample test module
by Ivid Suvarna May 6, 2018

May 6, 2018

Hi All, Could you please provide some sample test module for causing a Softlockup and Hardlockup in latest kernel for SMP for testing purposes. I assume there are some Kconfigs to be enabled as well for this. I had tried with the module where I put a busy loop inside spinlock but was not able to cause any lockups. Maybe this is because of SMP which schedule the job to other CPU. "How do I make a task to run on single CPU only?" I only get RCU stalls sometimes instead of softlockup when I use busy loop inside a spinlock. Is there any explanation for difference between these stalls and softlockup. Thanks, IVID

4 4

When does of_match_device() return NULL ?
by Himanshu Jha May 6, 2018

May 6, 2018

Hi, We know that the only way to probe an OF driver is when a match is found in the device table. And hence checking for of_match_device is redundant and instead it is better to use of_device_get_match_data() to get the matched data in the probe function. For instance: https://lkml.org/lkml/2018/4/30/66 static int mmio_74xx_gpio_probe(struct platform_device *pdev) { - const struct of_device_id *of_id; struct mmio_74xx_gpio_priv *priv; struct resource *res; void __iomem *dat; int err; - of_id = of_match_device(mmio_74xx_gpio_ids, &pdev->dev); - if (!of_id) - return -ENODEV; - priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL); if (!priv) return -ENOMEM; + priv->flags = (uintptr_t)of_device_get_match_data(&pdev->dev); + res = platform_get_resource(pdev, IORESOURCE_MEM, 0); dat = devm_ioremap_resource(&pdev->dev, res); if (IS_ERR(dat)) return PTR_ERR(dat); - priv->flags = (uintptr_t) of_id->data; - -------------------------------------------------------------------------- In the above example we know of_match_device() is useless since the probe occurs *only* when a match is found and therefore checking again is redundant. But now my question is if this is the case, then when does of_match_device() return NULL ? When is the error handling required ? There are places where it is done as "safe-play" coding as suggested to me by Jonathan previosuly https://lkml.org/lkml/2018/3/16/1245 -- Himanshu Jha Undergraduate Student Department of Electronics & Communication Guru Tegh Bahadur Institute of Technology

1 0

Update required packages to compile Linux Kernel
by Jefferson Capovilla May 5, 2018

May 5, 2018

Hi, I followed the tutorial available in this link: https://kernelnewbies.org/OutreachyfirstpatchSetup and noticed that it's necessary to install three more packages during step "Install some packages". The missing packages are: *bison flex libelf-dev* Could someone please update this info there? -- -- Thanks, Jeff

2 1

serial: uart_port->type
by Muni Sekhar May 4, 2018

May 4, 2018

Hi All, For the uart_port structure’s “type”, I see the permitted types are none, 8250, 16450, 16550, 16550A, 16650, 16650V2, 16654, 16750, 16850, 16950, and 16954 etc. For FPGA based UART hardware(it has 512 bytes FIFO and supports up to 4Mbps speed) what should be the ‘type’ value? -- Thanks, Sekhar

1 0

serial: start_tx & buffer handling
by Muni Sekhar May 4, 2018

May 4, 2018

Hi All, I’m trying to understand how user mode buffer is written to low level serial hardware registers. For this I read the kernel code and I came to know that from user mode write() API lands into kernel’s tty_write() ("drivers/tty/tty_io.c") and then it calls a uart_write() ("drivers/tty/serial/serial_core.c"). In uart_write(), the buffer is copied to circ_buf and then it calls low level serial hardware driver’s start_tx() (struct uart_ops .start_tx). But here I could not find how the buffer kept in circ_buf is copied to serial port’s TX_FIFO registers? Can someone take a moment to explain me on this? -- Thanks, Sekhar

3 5

good practice to use gpio as wakeup source
by Arthur LAMBERT May 4, 2018

May 4, 2018

Hi, I am working on arm custom board with kernel 4.1.X at this moment. I used to handle usb (gpio1) et push button (gpio0) event thanks to gpio through userspace in my code with '/sys/class/gpio'. I would like to use the suspend/resume feature with 'echo mem > /sys/power/state'.It is working great. But currently I am not able to wake up the board in suspend mode with usb or push button event. After some research I add this in my device tree : + gpio_usb { + compatible = "gpio-keys"; + wakeup_usb { + label = "wakeup-usb-gpio"; + gpios = <&gpio1 1 1>; + linux,code = <29>; + gpio-key,wakeup; + }; + }; + + gpio_button { + compatible = "gpio-keys"; + wakeup_button { + label = "wakeup-button-gpio"; + gpios = <&gpio1 0 1>; + linux,code = <29>; + gpio-key,wakeup; + }; + }; + It works ! I am able to wake up from sleep mode by unplugging/plugging board or pushing the push button. Problem is that now I cannot use anymore my gpio in userspace. gpio-keys kernel driver request the both gpio. When I try to export them in /sys/class/gpio the gpio is busy. Anyway I was able to retrieve the gpio in /dev/input. I was able to write a piece of code in userspace to process event in /dev/input/inputX. Problem now is that I am able to get event on these gpio but not initial state. When the userspace code is starting on boot, I need to know current usb plug/unplug status to put state machine in right state. But /dev/input is always able to send event and not current status. I tried to remove the gpio-keys compatible in device tree and just keep wakeup keyword. But by removing that wakeup from suspend with usb and push button is not working anymore. What is my next move ? Do I have completely wrong ? Do I have to patch gpio-keys driver to add a char device to read the gpio state ? Thanks ! Arthur.

1 0

serial: custom baud rate
by Muni Sekhar May 3, 2018

May 3, 2018

Hi All, >From include/asm-generic/termbits.h , I see baudrate can be one of the standard values: 50, 75, 110, 134, 150, 200, 300, 600, 1200, 1800, 2400, 4800, 9600, 19200, 38400, 57600, 115200, 230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000, 4000000. If I need to set a custom baud rates(e.g. 14400, 128000, 256000), does Linux serial framework has any supporting method? -- Thanks, Sekhar

1 0