SDL2 rendering does not go beyond 30 FPS on rockchip-based board - c

Question

I am trying to implement animations on a Rock64 ARM board, which has a Rockchip RK3328 with a Mali GPU. I am using SDL2 and I am experiencing a low framerate. So I wrote some testing code:
#include <SDL2/SDL.h>
#include <stdbool.h>
// generates a texture with the given color and the output's size
SDL_Texture *colorTexture(SDL_Renderer *renderer,
unsigned char r, unsigned char g, unsigned char b) {
int w, h, pitch;
SDL_GetRendererOutputSize(renderer, &w, &h);
SDL_Texture *ret = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888,
SDL_TEXTUREACCESS_STREAMING, w, h);
void *pixels;
SDL_LockTexture(ret, NULL, &pixels, &pitch);
for (int y = 0; y < h; ++y) {
unsigned char *cur = pixels + y * pitch;
for (int x = 0; x < w; ++x) {
*cur++ = 255; *cur++ = b; *cur++ = g; *cur++ = r;
}
}
SDL_UnlockTexture(ret);
return ret;
}
int main(int argc, char *argv[]) {
SDL_Init(SDL_INIT_VIDEO | SDL_INIT_EVENTS);
SDL_Window *window;
SDL_Renderer *renderer;
SDL_CreateWindowAndRenderer(640, 480, SDL_WINDOW_OPENGL, &window, &renderer);
SDL_Texture *blue = colorTexture(renderer, 0, 0, 255);
SDL_Texture *red = colorTexture(renderer, 255, 0, 0);
int w, h;
SDL_GetWindowSize(window, &w, &h);
bool running = true;
SDL_Texture *first = blue, *second = red;
uint32_t start = SDL_GetTicks();
int frameCount = 0;
while (running) {
SDL_Event evt;
for (int x = 0; x < w; ++x) {
while (SDL_PollEvent(&evt)) {
if (evt.type == SDL_QUIT) {
running = false;
goto after_animation;
}
}
SDL_Rect firstRect = {.x = 0, .y = 0, .w = x, .h = h},
secondRect = {.x = x, .y = 0, .w = w - x, .h = h};
SDL_RenderCopy(renderer, first, &firstRect, &firstRect);
SDL_RenderCopy(renderer, second, &secondRect, &secondRect);
SDL_RenderPresent(renderer);
frameCount++;
uint32_t cur = SDL_GetTicks();
if (cur - start >= 1000) {
printf("%d FPS\n", frameCount);
frameCount = 0;
start = cur;
}
}
after_animation:;
SDL_Texture *tmp = first; first = second; second = tmp;
}
SDL_Quit();
}
According to the post here, the board is able to reach framerates higher than 60 FPS even for 4k output (my monitor is a 4k TV). However, my testing code reports only 30 FPS. It only renders two textures that have exactly the screen's size, and scrolls horizontally showing one texture on the left and one on the right. I believe I should be able to reach framerates beyond 30 FPS. How can I speed up the rendering?