Implement back-buffering

main.c

@@ -3,15 +3,70 @@
 #include <assert.h>
 #include <fcntl.h>
 #include <stdint.h>
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
+#include <sys/select.h>
 #include <unistd.h>
 #include <xf86drm.h>
 #include <xf86drmMode.h>
 
+typedef struct {
+	uint32_t *buf;
+	uint32_t handle;
+	uint32_t id;
+	uint32_t pitch;
+	uint32_t size;
+} fb_info_t;
+
+fb_info_t fb_init(int fd, drmModeModeInfo mode)
+{
+	int err;
+
+	struct drm_mode_create_dumb create = {
+		.width = mode.hdisplay,
+		.height = mode.vdisplay,
+		.bpp = 32,
+	};
+	err = ioctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &create);
+	assert(err != -1);
+
+	uint32_t id;
+	err = drmModeAddFB(
+	    fd, create.width, create.height, 24, 32, create.pitch, create.handle,
+	    &id);
+	assert(err == 0);
+
+	struct drm_mode_map_dumb map = { .handle = create.handle };
+	err = ioctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &map);
+	assert(err != -1);
+
+	uint32_t *buf = mmap(
+	    0, create.size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, map.offset);
+	assert(buf != MAP_FAILED);
+
+	return (fb_info_t) {
+		.buf = buf,
+		.handle = create.handle,
+		.id = id,
+		.pitch = create.pitch,
+		.size = create.size,
+	};
+}
+
+void fb_cleanup(int fd, fb_info_t fb)
+{
+	munmap(fb.buf, fb.size);
+	drmModeRmFB(fd, fb.id);
+	struct drm_mode_destroy_dumb delete = { .handle = fb.handle };
+	ioctl(fd, DRM_IOCTL_MODE_DESTROY_DUMB, &delete);
+}
+
+static void page_flip_handler(int, unsigned, unsigned, unsigned, void *)
+{
+}
+
 int main()
 {
 	int err;
@@ -26,53 +81,54 @@ int main()
 	drmModeModeInfo mode = conn->modes[0];
 	uint32_t conn_id = conn->connector_id;
 
-	struct drm_mode_create_dumb create_req = {
-		.width = mode.hdisplay,
-		.height = mode.vdisplay,
-		.bpp = 32,
+	fb_info_t fbs[2] = {
+		fb_init(fd, mode),
+		fb_init(fd, mode),
 	};
-	err = ioctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &create_req);
-	assert(err != -1);
+	int front = 0, back = 1;
 
-	uint32_t fb_id;
-	err = drmModeAddFB(
-	    fd, create_req.width, create_req.height, 24, 32, create_req.pitch,
-	    create_req.handle, &fb_id);
-	assert(err == 0);
-
-	err = drmModeSetCrtc(fd, res->crtcs[0], fb_id, 0, 0, &conn_id, 1, &mode);
+	err = drmModeSetCrtc(
+	    fd, res->crtcs[0], fbs[front].id, 0, 0, &conn_id, 1, &mode);
 	assert(err == 0);
 	drmModeCrtc *crtc = drmModeGetCrtc(fd, res->crtcs[0]);
 	assert(crtc != nullptr);
 
-	struct drm_mode_map_dumb map_req = { .handle = create_req.handle };
-	err = ioctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &map_req);
-	assert(err != -1);
+	fd_set set;
+	FD_ZERO(&set);
+	FD_SET(fd, &set);
 
-	uint32_t *const fb = mmap(
-	    0, create_req.size, PROT_READ | PROT_WRITE, MAP_SHARED, fd,
-	    map_req.offset);
-	assert(fb != MAP_FAILED);
+	drmEventContext ev = {
+		.version = DRM_EVENT_CONTEXT_VERSION,
+		.page_flip_handler = page_flip_handler,
+	};
 
-	for (int y = 0; y < mode.vdisplay; ++y) {
-		for (int x = 0; x < mode.hdisplay; ++x) {
-			const uint8_t r = (x * 255) / mode.hdisplay;
-			const uint8_t g = (y * 255) / mode.vdisplay;
-			const uint8_t b = 128;
-			fb[y * (create_req.pitch / 4) + x] = (r << 16) | (g << 8) | b;
+	for (uint64_t frame = 0; frame < (10 * 60); ++frame) {
+		for (int y = 0; y < mode.vdisplay; ++y) {
+			for (int x = 0; x < mode.hdisplay; ++x) {
+				const uint8_t r = (x + frame) % 256;
+				const uint8_t g = (y + frame) & 256;
+				const uint8_t b = (x + y + frame) % 256;
+				fbs[back].buf[y * (fbs[back].pitch / 4) + x]
+				    = (r << 16) | (g << 8) | b;
+			}
 		}
+
+		drmModePageFlip(
+		    fd, crtc->crtc_id, fbs[back].id, DRM_MODE_PAGE_FLIP_EVENT,
+		    nullptr);
+		select(fd + 1, &set, nullptr, nullptr, nullptr);
+		drmHandleEvent(fd, &ev);
+
+		front = (front + 1) & 1;
+		back = (back + 1) & 1;
 	}
 
-	getchar();
-
-	munmap(fb, create_req.size);
-	struct drm_mode_destroy_dumb delete_req
-	    = { .handle = create_req.handle };
-	ioctl(fd, DRM_IOCTL_MODE_DESTROY_DUMB, &delete_req);
-
 	drmModeSetCrtc(
 	    fd, crtc->crtc_id, crtc->buffer_id, crtc->x, crtc->y, &conn_id, 1,
 	    &crtc->mode);
+
+	fb_cleanup(fd, fbs[0]);
+	fb_cleanup(fd, fbs[1]);
 	drmModeFreeCrtc(crtc);
 	drmModeFreeConnector(conn);
 	drmModeFreeResources(res);