Vertical Blanking
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## Vertical Blanking

Vertical blanking plays a major role in graphics rendering. To achieve tear-free display, users must synchronize page flips and/or rendering to vertical blanking. The DRM API offers ioctls to perform page flips synchronized to vertical blanking and wait for vertical blanking.

The DRM core handles most of the vertical blanking management logic, which involves filtering out spurious interrupts, keeping race-free blanking counters, coping with counter wrap-around and resets and keeping use counts. It relies on the driver to generate vertical blanking interrupts and optionally provide a hardware vertical blanking counter. Drivers must implement the following operations.

-     int (*enable_vblank) (struct drm_device *dev, int crtc);
      void (*disable_vblank) (struct drm_device *dev, int crtc);

  Enable or disable vertical blanking interrupts for the given CRTC.

-     u32 (*get_vblank_counter) (struct drm_device *dev, int crtc);

  Retrieve the value of the vertical blanking counter for the given CRTC. If the hardware maintains a vertical blanking counter its value should be returned. Otherwise drivers can use the `drm_vblank_count` helper function to handle this operation.

Drivers must initialize the vertical blanking handling core with a call to `drm_vblank_init` in their `load` operation. The function will set the struct drm_device `vblank_disable_allowed` field to 0. This will keep vertical blanking interrupts enabled permanently until the first mode set operation, where `vblank_disable_allowed` is set to 1. The reason behind this is not clear. Drivers can set the field to 1 after `calling drm_vblank_init` to make vertical blanking interrupts dynamically managed from the beginning.

Vertical blanking interrupts can be enabled by the DRM core or by drivers themselves (for instance to handle page flipping operations). The DRM core maintains a vertical blanking use count to ensure that the interrupts are not disabled while a user still needs them. To increment the use count, drivers call `drm_vblank_get`. Upon return vertical blanking interrupts are guaranteed to be enabled.

To decrement the use count drivers call `drm_vblank_put`. Only when the use count drops to zero will the DRM core disable the vertical blanking interrupts after a delay by scheduling a timer. The delay is accessible through the vblankoffdelay module parameter or the `drm_vblank_offdelay` global variable and expressed in milliseconds. Its default value is 5000 ms. Zero means never disable, and a negative value means disable immediately. Drivers may override the behaviour by setting the drm_device `vblank_disable_immediate` flag, which when set causes vblank interrupts to be disabled immediately regardless of the drm_vblank_offdelay value. The flag should only be set if there's a properly working hardware vblank counter present.

When a vertical blanking interrupt occurs drivers only need to call the `drm_handle_vblank` function to account for the interrupt.

Resources allocated by `drm_vblank_init` must be freed with a call to `drm_vblank_cleanup` in the driver `unload` operation handler.

### Vertical Blanking and Interrupt Handling Functions Reference

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