NXP G2D GPU

The NXP G2D library accelerates fills and image blits on i.MX MPU platforms, hardware independently. LVGL integrates it as a draw unit.

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On NXP i.MX MPU platforms, the acceleration can be done hardware independently by the NXP G2D library. LVGL integrates it as a draw unit, so several drawing features are offloaded to the G2D engine while the CPU is available for other operations. Linux is required to block the LVGL drawing thread and switch to another task or suspend the CPU for power savings.

Basic configuration

Select the NXP G2D engine in lv_conf.h: Set LV_USE_DRAW_G2D to 1.
Enable G2D asserts in lv_conf.h: Set LV_USE_G2D_ASSERT to 1. There are a few G2D assertions that can stop the program execution in case the LV_ASSERT_HANDLER is set to while(1); (Halt by default). Otherwise, only an error message is logged via LV_LOG_ERROR.
LV_G2D_HASH_TABLE_SIZE sets the maximum number of buffers mapped for the draw unit, including frame buffers and assets. Drawing asserts once the table is full.

Basic initialization

G2D draw initialization is done automatically in lv_init once the G2D is enabled as a draw unit, no user code is required:

#if LV_USE_DRAW_G2D
  lv_draw_g2d_init();
#endif

During G2D initialization, a new draw unit lv_draw_g2d_unit_t will be created with the additional callbacks, if LV_USE_DRAW_G2D is set to 1:

lv_draw_g2d_unit_t * draw_g2d_unit = lv_draw_create_unit(sizeof(lv_draw_g2d_unit_t));
draw_g2d_unit->base_unit.evaluate_cb = _g2d_evaluate;
draw_g2d_unit->base_unit.dispatch_cb = _g2d_dispatch;
draw_g2d_unit->base_unit.delete_cb = _g2d_delete;

and an additional thread _g2d_render_thread_cb() will be spawned in order to handle the supported draw tasks.

#if LV_USE_G2D_DRAW_THREAD
  lv_thread_init(&draw_g2d_unit->thread, "g2ddraw", LV_THREAD_PRIO_HIGH, _g2d_render_thread_cb, 2 * 1024, draw_g2d_unit);
#endif

If LV_USE_G2D_DRAW_THREAD is not defined, then no additional draw thread will be created and the G2D drawing task will get executed on the same LVGL main thread.

_g2d_evaluate() will get called after each task is being created and will analyze if the task is supported by G2D or not. If it is supported, then a preferred score and the draw unit id will be set to the task. A score equal to 100 is the default CPU score. Smaller score means that G2D is capable of drawing it faster.

_g2d_dispatch() is the G2D dispatcher callback, it will take a ready to draw task (having the DRAW_UNIT_ID_G2D set) and will pass the task to the G2D draw unit for processing.

_g2d_delete() will cleanup the G2D draw unit.

Features supported

Supported draw tasks are available in src/draw/nxp/g2d/lv_draw_g2d.c:

switch(t->type) {
    case LV_DRAW_TASK_TYPE_FILL:
        lv_draw_g2d_fill(u, t->draw_dsc, &t->area);
        break;
    case LV_DRAW_TASK_TYPE_IMAGE:
        lv_draw_g2d_img(u, t->draw_dsc, &t->area);
        break;
    default:
        break;
}

Fill area with color (w/o radius, w/o gradient) + optional opacity.
Blit source image ARGB8888 over destination ARGB8888 + optional opacity.
Scale source image ARGB8888.

Known limitations

G2D can only rotate at 90° angles.
Rotation is not supported for images unaligned to blocks of 16x16 pixels. G2D is set to process 16x16 blocks to optimize the system for memory bandwidth and image processing time. The output engine essentially truncates any output pixels after the desired number of pixels has been written. When rotating a source image and the output is not divisible by the block size, the incorrect pixels could be truncated and the final output image can look shifted.
Recolor or transformation for images w/ opacity or alpha channel can't be obtained in a single G2D pipeline configuration. Two or multiple steps would be required.
Buffer address must be aligned to 64 bytes: set LV_DRAW_BUF_ALIGN to 64 in lv_conf.h. No stride alignment is required: set LV_DRAW_BUF_STRIDE_ALIGN to 1 in lv_conf.h.

Project setup

Add the G2D related source files (and corresponding headers if available) to the project:

src/draw/nxp/g2d/lv_draw_buf_g2d.c: draw buffer callbacks
src/draw/nxp/g2d/lv_draw_g2d_fill.c: fill area
src/draw/nxp/g2d/lv_draw_g2d_img.c: blit image (w/ optional recolor or transformation)
src/draw/nxp/g2d/lv_draw_g2d.c: draw unit initialization
src/draw/nxp/g2d/lv_g2d_buf_map.c: hash map for G2D buffers
src/draw/nxp/g2d/lv_g2d_utils.c: function helpers