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Framebuffer (framebuffer)

Framebuffer display.

Device Type​

DEVICE_TYPE_FRAMEBUFFER

Struct​

struct framebuffer_t {
    char * name;
    int width, height, pwidth, pheight;
    void (*setbl)(struct framebuffer_t * fb, int brightness);
    int (*getbl)(struct framebuffer_t * fb);
    struct surface_t * (*create)(struct framebuffer_t * fb);
    void (*destroy)(struct framebuffer_t * fb, struct surface_t * s);
    int (*present)(struct framebuffer_t * fb, struct surface_t * s, struct dirtylist_t * l, void (*cb)(void *), void * data);
    void (*wait)(struct framebuffer_t * fb);
    void * priv;
};

Present Contract​

present unifies synchronous and asynchronous modes. Completion means the source surface is no longer referenced by hardware:

  • cb == NULL: synchronous mode, blocks until completion and returns 0
  • cb != NULL: asynchronous hint. Returns 1 if the transfer is in flight, cb(data) will be invoked exactly once on completion (interrupt context allowed); returns 0 if it has already completed during the call, cb will never be invoked

cb is an asynchronous-completion callback, not an unconditional completion notification. A driver must not invoke cb when returning 0, which avoids re-entering the caller while it is still in the submission path or holding a lock. A caller that wants the same completion handler in both modes should use this pattern:

if(!framebuffer_present_submit(fb, s, l, present_done, data))
	present_done(data);

The caller handles synchronous completion and the driver reports asynchronous completion, so present_done(data) executes exactly once in total.

wait is required and blocks until the in-flight present completes; drivers without asynchronous support may implement it as an empty function. For page-flip style drivers (DRM, double-buffered LCDC) the completion event is naturally aligned with vertical sync.

Key API​

FunctionDescription
search_framebuffer(name)Find framebuffer by name
register_framebuffer(fb, drv)Register framebuffer
unregister_framebuffer(fb)Unregister framebuffer
framebuffer_get/set_backlight(fb, bl)Get/set backlight
framebuffer_present_submit(fb, s, l, cb, data)Submit a present, blocks synchronously until completion when cb is NULL
framebuffer_present_wait(fb)Wait for the in-flight present to complete

Description​

Video framebuffer display interface. Manages pixel buffer, supports surface creation and dirty-rectangle based presentation, with DMA asynchronous refresh support.