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* This completes task list in https://github.com/apache/nuttx/issues/11127. * This preserves selected content from cwiki and moves it to new docs. * Most pages are simple copy-paste with a simple RST formatting updates, with minor updates. * Content update / reorganization will follow later on when needed. * Files added (or updated title from cwiki -> current docs): * Documentation/implementation: * index. * cancellation_points. * Asynchronous vs. Synchronous Context Switches -> context_switches.rst. * ARMv7-M Hardfaults, SVCALL, and Debuggers -> hardfatuls.rst. * chip.h FAQ -> chip_h.rst. * Debug Output (SYSLOG) Issues -> syslog.rst. * Detaching File Descriptors -> file_descriptors.rst. * device_nodes.rst. * Dynamic Clocking -> power_management.rst. * ENOTTY ioctl() Return Value -> ioctl.rst. * memory_configurations.rst. * kernel_modules_vs_shared_libraries.rst. * NAKing USB OUT/IN Tokens -> usb.rst. * naming_arch_mcu_board_interfaces.rst. * naming_os_internals.rst. * nuttx_tasking.rst. * oneshot_timers_and_cpu_load.rst. * nuttx_initialization_sequence.rst. * short_time_delays.rst. * Signal Handler Tour -> signal_handlers.rst. * smp.rst. * syslog.rst. * Task Exit Sequence -> nuttx_tasking.rst. * tasks_vs_threads.rst. * tls.rst. * tickless_os.rst. * Why Can't Kernel Threads Have pthreads -> kernel_threads_vs_pthreads.rst. * Documentation/components/filesystem: * smartfs.rst. Signed-off-by: Tomasz 'CeDeROM' CEDRO <tomek@cedro.info>
94 lines
3.9 KiB
ReStructuredText
94 lines
3.9 KiB
ReStructuredText
.. _device-nodes:
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============
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Device Nodes
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============
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Linux Device Nodes
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==================
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I used to have good Linux expertise a decade or so ago.
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But my current Linux knowledge is dated and rusty.
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I don't know anything about udev, SystemD, devtmpfs, sysfs, or any of that.
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So this is my simplified understanding.
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Device files work quite a bit differently in Linux and NuttX.
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A device node in Unix/Linux only really contains the only type of the device
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and its major and minor device numbers, i.e., it just holds data.
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So creating the device node does not install or create the driver;
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it simply writes a tiny file containing some special data.
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Nothing happens until you try to open the device.
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If something in the operating system has not initialized and registered
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a driver for that type and major/minor numbers,
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then you fail to open the device.
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So the device nodes and the device drivers are decoupled in Linux/Unix
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and there is a rendezvous that must occur later for the device node
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to actually refer to the device.
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"(..) Linux maps the device special file passed in system calls
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(say to mount a file system on a block device) to the device's
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device driver using the major device number and a number of system
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tables, ...The major number is actually the offset into the
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kernel's device driver table, which tells the kernel what
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kind of device it is (whether it is a hard disk or a serial
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terminal) (..)"
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-- Source:
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www.linux-tutorial.info/modules.php?name=MContent&pageid=94.
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Normally, when you create a Linux file system, you also create all of the
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standard device nodes. But most of these do not map to real devices.
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If you try to access most of the devices under ``/dev`` in Linux, they will
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fail because the underlying driver that maps to that major/minor number
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has not been initialized.
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NuttX Device Nodes
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==================
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NuttX does not use major/minor device numbers and there are no device
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"system tables" to associate major/minor numbers to a driver implementation.
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NuttX simplifies this be removing the "man in the middle": When you register
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the driver, you also create the device node.
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.. important:: The device node IS the driver registry.
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This is a tremendous simplification and one of the things that
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makes NuttX usable in the constrained MCU environment.
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In NuttX, device nodes are not really files at all.
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They are special entries in the NuttX root pseudo-filesystem.
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See :ref:`NuttX Pseudo File System <nuttx-pseudofs>` for more details.
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Usage Differences
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=================
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.. important:: Only devices drivers can create device nodes and the existence
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of the device node means that the device has been initialized,
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registered, and is ready for use (with the exception of some
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removable devices that may not actually be ready).
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You cannot create device nodes from applications!
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You could argue that this simplification is a deviation from my Unix/Linux
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roadmap and would have to agree that you are right.
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But it is also the kind of enabling simplification that makes a tiny
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Unix-like operating system feasible on these lower end MCUs.
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In Linux standard device drivers are initialized and registered as with NuttX.
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A (privileged) application can create a device node, but cannot initialize
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or register a device driver directly (as far as I know).
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I believe that if you want to instantiate an uninitialized, unregistered
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device driver you would have to install a kernel module containing
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the driver (which would probably also create the device nodes corresponding
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to the driver).
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boardctl()
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==========
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NuttX does support a sneak interface to support interactions with board-level
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OS logic. That sneak interface is ``boardctl()`` (see :ref:`board-ioctl` and
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:ref:`nuttx-initialization-sequence` for more details).
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That interface could potentially be used to force initialization of device
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drivers by application code. That discussion is to be provided.
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