DRIVER SPECIFIC IOCTLS(9)

💅 driver specific ioctls 💅

okay bestie, welcome to driver specific ioctls — that's the page, section 9 of the Kernel API Manual, and it's genuinely all about how render drivers get their own private IOCTLs. buckle up 🎀

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✨ the whole vibe (aka where we're at)

first things first, periodt: driver private IOCTLs should only be needed for drivers that support rendering. kernel modesetting is all standardized, and it gets extended through properties. there are a few exceptions floating around in some existing drivers — they define IOCTLs for use by the display DRM master — but ngl they all predate properties. old money, not new money 💀

so, if you do have a render driver, you always have to support it through driver private properties. there's a few steps needed to wire all the things up 🧚

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🌸 step one: define the struct

you need to define the structure for your IOCTL in your driver private UAPI header, include/uapi/drm/my_driver_drm.h:

struct my_driver_operation {
        u32 some_thing;
        u32 another_thing;
};

please make sure you follow all the best practices from Documentation/process/botching-up-ioctls.rst 🙏

⚠️ heads up bestie: drm_ioctl automatically zero-extends structures, so make sure you can add more stuff at the end — i.e. don't put a variable sized array there. don't do it. i'm serious. 😭

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🦋 step two: define your IOCTL number

then you need to define your IOCTL number, using one of DRM_IO, DRM_IOR, DRM_IOW or DRM_IOWR. it must start with the DRM_IOCTL_ prefix ✨

##define DRM_IOCTL_MY_DRIVER_OPERATION         DRM_IOW(DRM_COMMAND_BASE, struct my_driver_operation)

DRM driver private IOCTL must be in the range from DRM_COMMAND_BASE to DRM_COMMAND_END. no cap, that's the whole range, stay in it 💅

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💖 step three: wire up the handlers

finally you need an array of struct drm_ioctl_desc to wire up the handlers and set the access rights:

static const struct drm_ioctl_desc my_driver_ioctls[] = {
    DRM_IOCTL_DEF_DRV(MY_DRIVER_OPERATION, my_driver_operation,
            DRM_AUTH|DRM_RENDER_ALLOW),
};

and then assign this to the drm_driver.ioctls field in your driver structure. slay, you're basically done 👑

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🌈 one more thing

see the separate chapter on file operations<drm_driver_fops> (:ref: — yk the ref markup) for how the driver-specific IOCTLs are wired up 🫶

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⭐ SEE ALSO ⭐

• Kernel file ./drivers/gpu/drm/drm_ioctl.c getunique and setversion story(9)
• drm_noop(9)
• drm_invalid_op(9)
• drm_ioctl(9)
• drm_ioctl_flags(9)

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aaand that's the page!! 🎀 this iconic little number lives at ./drivers/gpu/drm, dated October 2026 — go off, kernel docs, you ate 💜🌸