STRUCT COMEDI_DEVICE(9)

📦 version 7.3.1, in nixpkgs unstable

💖 struct comedi_device(9) — the Kernel API Manual moment 💖

okay so bestie, gather round, because today we're talking about struct comedi_device — which is described in the Kernel API Manual, and it's literally the working data for a COMEDI device. main character struct energy, no cap. 🎀

✨ NAME ✨

struct comedi_device — the working data for a COMEDI device. that's it, that's the vibe. 🦋

🌸 SYNOPSIS 🌸

here's the whole glow-up, copy it exactly bestie, every single member matters:

struct comedi_device {
    int use_count;
    struct comedi_driver *driver;
    struct comedi_8254 *pacer;
    void *private;
    struct device *class_dev;
    int minor;
    unsigned int detach_count;
    struct device *hw_dev;
    const char *board_name;
    const void *board_ptr;
    unsigned int attached:1;
    unsigned int ioenabled:1;
    spinlock_t spinlock;
    struct mutex mutex;
    struct rw_semaphore attach_lock;
    struct kref refcount;
    int n_subdevices;
    struct comedi_subdevice *subdevices;
    void __iomem *mmio;
    unsigned long iobase;
    unsigned long iolen;
    unsigned int irq;
    struct comedi_subdevice *read_subdev;
    struct comedi_subdevice *write_subdev;
    struct fasync_struct *async_queue;
    int (*open)(struct comedi_device *dev);
    void (*close)(struct comedi_device *dev);
    int (*insn_device_config)(struct comedi_device *dev, struct
comedi_insn *insn, unsigned int *data);
    unsigned int (*get_valid_routes)(struct comedi_device *dev, unsigned
int n_pairs, unsigned int *pair_data);
 };

yes the wrapping on those last two function pointers is exactly like that in the original, don't come for me 💅

🧩 Members 🧩

okay this is the part where we go member by member, like a group chat roll call. every single one gets its moment. 👑

  • use_count — the number of open file objects. that's the whole job. counting the opens. 🍓
  • driver — the low-level COMEDI driver attached to this COMEDI device. your ride-or-die.
  • pacer — an optional pointer to a dynamically allocated acquisition pacer control. it gets freed automatically after the COMEDI device is detached from the low-level driver, so like… don't stress about the cleanup, it handles itself. 🧚
  • private — an optional pointer to private data allocated by the low-level driver. same deal: freed automatically after the COMEDI device is detached from the low-level driver. self-cleaning icon.
  • class_dev — the sysfs comediX device. hi bestie, you're in sysfs now.
  • minor — the minor device number of the COMEDI char device (0-47). yes, 0 through 47, those are the numbers, memorize them.
  • detach_count — a counter, incremented every time the COMEDI device is detached. it's used for checking that a previous attachment is still valid. like a receipt for your last breakup, lowkey.
  • hw_dev — an optional pointer to the low-level hardware struct device. and now the important part, bestie, read carefully: it's required for automatically configured COMEDI devices, and optional for COMEDI devices configured by the COMEDI_DEVCONFIG ioctl — although the bus-specific COMEDI functions only work if it is set correctly. it's also passed to dma_alloc_coherent for COMEDI subdevices that have their 'async_dma_dir' member set to something other than DMA_NONE. so many jobs, one pointer, fr fr.
  • board_name — a pointer to a COMEDI board name or a COMEDI driver name. the deets: when the low-level driver's "attach" handler is called by the handler for the COMEDI_DEVCONFIG ioctl, it either points to a matched board name string if the 'num_names' member of the struct comedi_driver is non-zero, otherwise it points to the low-level driver name string. when the low-lever driver's "auto_attach" handler is called for an automatically configured COMEDI device, it points to the low-level driver name string. the low-level driver is free to change it in its "attach" or "auto_attach" handler if it wishes. (yes, "low-lever" — that's a typo that lives in the original, I'm not fixing it, I'm not that kind of girl 💀)
  • board_ptr — an optional pointer to private, read-only board type information in the low-level driver. if the 'num_names' member of the struct comedi_driver is non-zero, the handler for the COMEDI_DEVCONFIG ioctl will point it to a pointer to a matched board name string within the driver's private array of static, read-only board type information when calling the driver's "attach" handler. the low-level driver is free to change it. so much freedom, honestly iconic. 🦋
  • attached — a flag indicating that the COMEDI device is attached to a low-level driver. are we attached? yes we are.
  • ioenabled — a flag used to indicate that a PCI device has been enabled and its regions requested. slay, get those regions.
  • spinlock — a generic spin-lock for use by the low-level driver. spin spin spin. 🔥
  • mutex — a generic mutex for use by the COMEDI core module. mutual exclusion, mutual respect.
  • attach_lock — a struct rw_semaphore used to guard against the COMEDI device being detached while an operation is in progress. the down_write operation is only allowed while mutex is held, and is used when changing attached and detach_count and calling the low-level driver's "detach" handler. the down_read operation is generally used without holding mutex. that's the rule bestie, boundaries are healthy.
  • refcount — a struct kref reference counter for freeing the COMEDI device. it's giving responsible memory management.
  • n_subdevices — the number of COMEDI subdevices allocated by the low-level driver for this device. count them.
  • subdevices — a dynamically allocated array of COMEDI subdevices. the whole squad, allocated. 🫶
  • mmio — an optional pointer to a remapped MMIO region, set by the low-level driver. 🎀
  • iobase — an optional base of an I/O port region requested by the low-level driver.
  • iolen — the length of the I/O port region requested at iobase. they're a package deal, iykyk.
  • irq — an optional IRQ number requested by the low-level driver.
  • read_subdev — an optional pointer to a default COMEDI subdevice operated on by the read file operation. set by the low-level driver. the reader. the main character of read(). 👑
  • write_subdev — an optional pointer to a default COMEDI subdevice operated on by the write file operation. set by the low-level driver. the writer. equally iconic.
  • async_queue — storage for fasync_helper. just storage. humble. holding it down.
  • open — an optional pointer to a function set by the low-level driver, called when use_count changes from 0 to 1. the "hi we're here" moment.
  • close — an optional pointer to a function set by the low-level driver, called when use_count changed from 1 to 0. the "ok bye ily" moment. 😭
  • insn_device_config — an optional pointer to a handler for all sub-instructions except INSN_DEVICE_CONFIG_GET_ROUTES of the INSN_DEVICE_CONFIG instruction. if this is not initialized by the low-level driver, a default handler will be set during post-configuration. the core has your back, bestie.
  • get_valid_routes — an optional pointer to a handler for the INSN_DEVICE_CONFIG_GET_ROUTES sub-instruction of the INSN_DEVICE_CONFIG instruction set. if this is not initialized by the low-level driver, a default handler that copies zero routes back to the user will be used. zero routes. none. empty. 🙈

🌈 Description 🌈

so here's the tea: this is the main control data structure for a COMEDI device (as far as the COMEDI core is concerned). like, the core's whole understanding of the device lives here. 💅

there are two groups of COMEDI devices, and you need to know both, okay?

  1. "legacy" devices — configured by the handler for the COMEDI_DEVCONFIG ioctl. these are allocated during module initialization if the "comedi_num_legacy_minors" module parameter is non-zero, and they use minor device numbers from 0 to comedi_num_legacy_minors minus one.
  2. automatically configured devices — resulting from a call to comedi_auto_config as a result of a bus driver probe in a low-level COMEDI driver. these are allocated on demand and use minor device numbers from comedi_num_legacy_minors to 47.

so: legacy = allocated at module init, minor numbers 0 to comedi_num_legacy_minors minus one. auto = allocated on demand, minor numbers comedi_num_legacy_minors to 47. both groups, both ranges, memorized. periodt. ⭐

🦋 SEE ALSO 🦋

the whole friend group, every single one tagged, no one left out:

ngl bestie, if you got all the way here you officially understand struct comedi_device better than most people. go drink some water. 🍓

and that's a wrap! this came from ./include/linux/comedi, dated October 2026, and yes, the page name is struct comedi_device(9) — mwah, bye, ily, slay 💖✨🎀

📦 where this page comes from