STRUCT FF_DEVICE(9)

💅 struct ff_device(9) — the kernel API manual, but make it make sense

okay so bestie, welcome to the struct ff_device page from the Kernel API Manual ✨ this is literally the force-feedback part of an input device — rumble, shake, the whole vibe 🎮💥 let's get into it.

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🌸 NAME 🌸

struct ff_device — the force-feedback part of an input device. that's it, that's the whole thing, she's the haptic main character 👑

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✨ SYNOPSIS ✨

struct ff_device {
    int (*upload)(struct input_dev *dev, struct ff_effect *effect,
struct ff_effect *old);
    int (*erase)(struct input_dev *dev, int effect_id);
    int (*playback)(struct input_dev *dev, int effect_id, int value);
    void (*set_gain)(struct input_dev *dev, u16 gain);
    void (*set_autocenter)(struct input_dev *dev, u16 magnitude);
    void (*destroy)(struct ff_device *);
    void (*stop)(struct ff_device *);
    void *private;
    unsigned long ffbit[BITS_TO_LONGS(FF_CNT)];
    struct mutex mutex;
    int max_effects;
    struct ff_effect *effects;
    struct file *effect_owners[];
};

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🎀 Members 🎀

• upload — called to upload a new effect into the device. yeah, this one's on the must-have list, no skipping 🫶
• erase — called to erase an effect from the device. optional but honestly iconic of her
• playback — called to request the device start playing the specified effect. also mandatory, bestie
• set_gain — called to set the specified gain
• set_autocenter — called to auto-center the device
• destroy — called by the input core when the parent input device is being destroyed 💀
• stop — called by the input core when the parent input device is being unregistered
• private — driver-specific data, and it gets freed automatically, so don't you dare double-free it 🙈
• ffbit — bitmap of force-feedback capabilities truly supported by the device (not emulated like the ones in input_dev->ffbit). yes, truly. she's not pretending 💅
• mutex — mutex for serializing access to the device. one at a time, besties
• max_effects — maximum number of effects supported by the device
• effects — pointer to an array of effects currently loaded into the device
• effect_owners — array of effect owners; when the file handle owning an effect gets closed, the effect is automatically erased. self-cleaning queen ✨

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🦋 Description 🦋

Every force-feedback device must implement upload and playback methods — those two are non-negotiable, periodt. erase is optional. And set_gain and set_autocenter only need to be implemented if the driver sets up the FF_GAIN and FF_AUTOCENTER bits.

⚠️ okay this part is so important: note that playback, set_gain and set_autocenter are called with dev->event_lock spinlock held and interrupts off, which means they may not sleep. not even a lil nap. don't do it 😭

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🔥 SEE ALSO 🔥

• Kernel file ./include/linux/input.h
• struct input_value(9)
• struct input_dev(9)
• struct input_handler(9)
• struct input_handle(9)
• input_set_events_per_packet(9)

go read those too, iykyk 📚💜

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and that's the whole page, bestie 🌈 this livable little doc lives at ./include/linux and it's dated October 2026 — so like, future vibes fr fr. stay hydrated, don't sleep in a spinlock, and slay 💅🎀🦋