[psplash,RFC,10/14] Import drm-howto modeset.c as psplash-drm.c

diff --git a/psplash-drm.c b/psplash-drm.c
new file mode 100644
index 0000000..c9a9f5c
--- /dev/null
+++ b/psplash-drm.c
@@ -0,0 +1,735 @@ 
+/*
+ * modeset - DRM Modesetting Example
+ *
+ * Written 2012 by David Rheinsberg <david.rheinsberg@gmail.com>
+ * Dedicated to the Public Domain.
+ */
+
+/*
+ * DRM Modesetting Howto
+ * This document describes the DRM modesetting API. Before we can use the DRM
+ * API, we have to include xf86drm.h and xf86drmMode.h. Both are provided by
+ * libdrm which every major distribution ships by default. It has no other
+ * dependencies and is pretty small.
+ *
+ * Please ignore all forward-declarations of functions which are used later. I
+ * reordered the functions so you can read this document from top to bottom. If
+ * you reimplement it, you would probably reorder the functions to avoid all the
+ * nasty forward declarations.
+ *
+ * For easier reading, we ignore all memory-allocation errors of malloc() and
+ * friends here. However, we try to correctly handle all other kinds of errors
+ * that may occur.
+ *
+ * All functions and global variables are prefixed with "modeset_*" in this
+ * file. So it should be clear whether a function is a local helper or if it is
+ * provided by some external library.
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <time.h>
+#include <unistd.h>
+#include <xf86drm.h>
+#include <xf86drmMode.h>
+
+struct modeset_dev;
+static int modeset_find_crtc(int fd, drmModeRes *res, drmModeConnector *conn,
+			     struct modeset_dev *dev);
+static int modeset_create_fb(int fd, struct modeset_dev *dev);
+static int modeset_setup_dev(int fd, drmModeRes *res, drmModeConnector *conn,
+			     struct modeset_dev *dev);
+static int modeset_open(int *out, const char *node);
+static int modeset_prepare(int fd);
+static void modeset_draw(void);
+static void modeset_cleanup(int fd);
+
+/*
+ * When the linux kernel detects a graphics-card on your machine, it loads the
+ * correct device driver (located in kernel-tree at ./drivers/gpu/drm/<xy>) and
+ * provides two character-devices to control it. Udev (or whatever hotplugging
+ * application you use) will create them as:
+ *     /dev/dri/card0
+ *     /dev/dri/controlID64
+ * We only need the first one. You can hard-code this path into your application
+ * like we do here, but it is recommended to use libudev with real hotplugging
+ * and multi-seat support. However, this is beyond the scope of this document.
+ * Also note that if you have multiple graphics-cards, there may also be
+ * /dev/dri/card1, /dev/dri/card2, ...
+ *
+ * We simply use /dev/dri/card0 here but the user can specify another path on
+ * the command line.
+ *
+ * modeset_open(out, node): This small helper function opens the DRM device
+ * which is given as @node. The new fd is stored in @out on success. On failure,
+ * a negative error code is returned.
+ * After opening the file, we also check for the DRM_CAP_DUMB_BUFFER capability.
+ * If the driver supports this capability, we can create simple memory-mapped
+ * buffers without any driver-dependent code. As we want to avoid any radeon,
+ * nvidia, intel, etc. specific code, we depend on DUMB_BUFFERs here.
+ */
+
+static int modeset_open(int *out, const char *node)
+{
+	int fd, ret;
+	uint64_t has_dumb;
+
+	fd = open(node, O_RDWR | O_CLOEXEC);
+	if (fd < 0) {
+		ret = -errno;
+		fprintf(stderr, "cannot open '%s': %m\n", node);
+		return ret;
+	}
+
+	if (drmGetCap(fd, DRM_CAP_DUMB_BUFFER, &has_dumb) < 0 ||
+	    !has_dumb) {
+		fprintf(stderr, "drm device '%s' does not support dumb buffers\n",
+			node);
+		close(fd);
+		return -EOPNOTSUPP;
+	}
+
+	*out = fd;
+	return 0;
+}
+
+/*
+ * As a next step we need to find our available display devices. libdrm provides
+ * a drmModeRes structure that contains all the needed information. We can
+ * retrieve it via drmModeGetResources(fd) and free it via
+ * drmModeFreeResources(res) again.
+ *
+ * A physical connector on your graphics card is called a "connector". You can
+ * plug a monitor into it and control what is displayed. We are definitely
+ * interested in what connectors are currently used, so we simply iterate
+ * through the list of connectors and try to display a test-picture on each
+ * available monitor.
+ * However, this isn't as easy as it sounds. First, we need to check whether the
+ * connector is actually used (a monitor is plugged in and turned on). Then we
+ * need to find a CRTC that can control this connector. CRTCs are described
+ * later on. After that we create a framebuffer object. If we have all this, we
+ * can mmap() the framebuffer and draw a test-picture into it. Then we can tell
+ * the DRM device to show the framebuffer on the given CRTC with the selected
+ * connector.
+ *
+ * As we want to draw moving pictures on the framebuffer, we actually have to
+ * remember all these settings. Therefore, we create one "struct modeset_dev"
+ * object for each connector+crtc+framebuffer pair that we successfully
+ * initialized and push it into the global device-list.
+ *
+ * Each field of this structure is described when it is first used. But as a
+ * summary:
+ * "struct modeset_dev" contains: {
+ *  - @next: points to the next device in the single-linked list
+ *
+ *  - @width: width of our buffer object
+ *  - @height: height of our buffer object
+ *  - @stride: stride value of our buffer object
+ *  - @size: size of the memory mapped buffer
+ *  - @handle: a DRM handle to the buffer object that we can draw into
+ *  - @map: pointer to the memory mapped buffer
+ *
+ *  - @mode: the display mode that we want to use
+ *  - @fb: a framebuffer handle with our buffer object as scanout buffer
+ *  - @conn: the connector ID that we want to use with this buffer
+ *  - @crtc: the crtc ID that we want to use with this connector
+ *  - @saved_crtc: the configuration of the crtc before we changed it. We use it
+ *                 so we can restore the same mode when we exit.
+ * }
+ */
+
+struct modeset_dev {
+	struct modeset_dev *next;
+
+	uint32_t width;
+	uint32_t height;
+	uint32_t stride;
+	uint32_t size;
+	uint32_t handle;
+	uint8_t *map;
+
+	drmModeModeInfo mode;
+	uint32_t fb;
+	uint32_t conn;
+	uint32_t crtc;
+	drmModeCrtc *saved_crtc;
+};
+
+static struct modeset_dev *modeset_list = NULL;
+
+/*
+ * So as next step we need to actually prepare all connectors that we find. We
+ * do this in this little helper function:
+ *
+ * modeset_prepare(fd): This helper function takes the DRM fd as argument and
+ * then simply retrieves the resource-info from the device. It then iterates
+ * through all connectors and calls other helper functions to initialize this
+ * connector (described later on).
+ * If the initialization was successful, we simply add this object as new device
+ * into the global modeset device list.
+ *
+ * The resource-structure contains a list of all connector-IDs. We use the
+ * helper function drmModeGetConnector() to retrieve more information on each
+ * connector. After we are done with it, we free it again with
+ * drmModeFreeConnector().
+ * Our helper modeset_setup_dev() returns -ENOENT if the connector is currently
+ * unused and no monitor is plugged in. So we can ignore this connector.
+ */
+
+static int modeset_prepare(int fd)
+{
+	drmModeRes *res;
+	drmModeConnector *conn;
+	unsigned int i;
+	struct modeset_dev *dev;
+	int ret;
+
+	/* retrieve resources */
+	res = drmModeGetResources(fd);
+	if (!res) {
+		fprintf(stderr, "cannot retrieve DRM resources (%d): %m\n",
+			errno);
+		return -errno;
+	}
+
+	/* iterate all connectors */
+	for (i = 0; i < res->count_connectors; ++i) {
+		/* get information for each connector */
+		conn = drmModeGetConnector(fd, res->connectors[i]);
+		if (!conn) {
+			fprintf(stderr, "cannot retrieve DRM connector %u:%u (%d): %m\n",
+				i, res->connectors[i], errno);
+			continue;
+		}
+
+		/* create a device structure */
+		dev = malloc(sizeof(*dev));
+		memset(dev, 0, sizeof(*dev));
+		dev->conn = conn->connector_id;
+
+		/* call helper function to prepare this connector */
+		ret = modeset_setup_dev(fd, res, conn, dev);
+		if (ret) {
+			if (ret != -ENOENT) {
+				errno = -ret;
+				fprintf(stderr, "cannot setup device for connector %u:%u (%d): %m\n",
+					i, res->connectors[i], errno);
+			}
+			free(dev);
+			drmModeFreeConnector(conn);
+			continue;
+		}
+
+		/* free connector data and link device into global list */
+		drmModeFreeConnector(conn);
+		dev->next = modeset_list;
+		modeset_list = dev;
+	}
+
+	/* free resources again */
+	drmModeFreeResources(res);
+	return 0;
+}
+
+/*
+ * Now we dig deeper into setting up a single connector. As described earlier,
+ * we need to check several things first:
+ *   * If the connector is currently unused, that is, no monitor is plugged in,
+ *     then we can ignore it.
+ *   * We have to find a suitable resolution and refresh-rate. All this is
+ *     available in drmModeModeInfo structures saved for each crtc. We simply
+ *     use the first mode that is available. This is always the mode with the
+ *     highest resolution.
+ *     A more sophisticated mode-selection should be done in real applications,
+ *     though.
+ *   * Then we need to find an CRTC that can drive this connector. A CRTC is an
+ *     internal resource of each graphics-card. The number of CRTCs controls how
+ *     many connectors can be controlled indepedently. That is, a graphics-cards
+ *     may have more connectors than CRTCs, which means, not all monitors can be
+ *     controlled independently.
+ *     There is actually the possibility to control multiple connectors via a
+ *     single CRTC if the monitors should display the same content. However, we
+ *     do not make use of this here.
+ *     So think of connectors as pipelines to the connected monitors and the
+ *     CRTCs are the controllers that manage which data goes to which pipeline.
+ *     If there are more pipelines than CRTCs, then we cannot control all of
+ *     them at the same time.
+ *   * We need to create a framebuffer for this connector. A framebuffer is a
+ *     memory buffer that we can write XRGB32 data into. So we use this to
+ *     render our graphics and then the CRTC can scan-out this data from the
+ *     framebuffer onto the monitor.
+ */
+
+static int modeset_setup_dev(int fd, drmModeRes *res, drmModeConnector *conn,
+			     struct modeset_dev *dev)
+{
+	int ret;
+
+	/* check if a monitor is connected */
+	if (conn->connection != DRM_MODE_CONNECTED) {
+		fprintf(stderr, "ignoring unused connector %u\n",
+			conn->connector_id);
+		return -ENOENT;
+	}
+
+	/* check if there is at least one valid mode */
+	if (conn->count_modes == 0) {
+		fprintf(stderr, "no valid mode for connector %u\n",
+			conn->connector_id);
+		return -EFAULT;
+	}
+
+	/* copy the mode information into our device structure */
+	memcpy(&dev->mode, &conn->modes[0], sizeof(dev->mode));
+	dev->width = conn->modes[0].hdisplay;
+	dev->height = conn->modes[0].vdisplay;
+	fprintf(stderr, "mode for connector %u is %ux%u\n",
+		conn->connector_id, dev->width, dev->height);
+
+	/* find a crtc for this connector */
+	ret = modeset_find_crtc(fd, res, conn, dev);
+	if (ret) {
+		fprintf(stderr, "no valid crtc for connector %u\n",
+			conn->connector_id);
+		return ret;
+	}
+
+	/* create a framebuffer for this CRTC */
+	ret = modeset_create_fb(fd, dev);
+	if (ret) {
+		fprintf(stderr, "cannot create framebuffer for connector %u\n",
+			conn->connector_id);
+		return ret;
+	}
+
+	return 0;
+}
+
+/*
+ * modeset_find_crtc(fd, res, conn, dev): This small helper tries to find a
+ * suitable CRTC for the given connector. We have actually have to introduce one
+ * more DRM object to make this more clear: Encoders.
+ * Encoders help the CRTC to convert data from a framebuffer into the right
+ * format that can be used for the chosen connector. We do not have to
+ * understand any more of these conversions to make use of it. However, you must
+ * know that each connector has a limited list of encoders that it can use. And
+ * each encoder can only work with a limited list of CRTCs. So what we do is
+ * trying each encoder that is available and looking for a CRTC that this
+ * encoder can work with. If we find the first working combination, we are happy
+ * and write it into the @dev structure.
+ * But before iterating all available encoders, we first try the currently
+ * active encoder+crtc on a connector to avoid a full modeset.
+ *
+ * However, before we can use a CRTC we must make sure that no other device,
+ * that we setup previously, is already using this CRTC. Remember, we can only
+ * drive one connector per CRTC! So we simply iterate through the "modeset_list"
+ * of previously setup devices and check that this CRTC wasn't used before.
+ * Otherwise, we continue with the next CRTC/Encoder combination.
+ */
+
+static int modeset_find_crtc(int fd, drmModeRes *res, drmModeConnector *conn,
+			     struct modeset_dev *dev)
+{
+	drmModeEncoder *enc;
+	unsigned int i, j;
+	int32_t crtc;
+	struct modeset_dev *iter;
+
+	/* first try the currently conected encoder+crtc */
+	if (conn->encoder_id)
+		enc = drmModeGetEncoder(fd, conn->encoder_id);
+	else
+		enc = NULL;
+
+	if (enc) {
+		if (enc->crtc_id) {
+			crtc = enc->crtc_id;
+			for (iter = modeset_list; iter; iter = iter->next) {
+				if (iter->crtc == crtc) {
+					crtc = -1;
+					break;
+				}
+			}
+
+			if (crtc >= 0) {
+				drmModeFreeEncoder(enc);
+				dev->crtc = crtc;
+				return 0;
+			}
+		}
+
+		drmModeFreeEncoder(enc);
+	}
+
+	/* If the connector is not currently bound to an encoder or if the
+	 * encoder+crtc is already used by another connector (actually unlikely
+	 * but lets be safe), iterate all other available encoders to find a
+	 * matching CRTC. */
+	for (i = 0; i < conn->count_encoders; ++i) {
+		enc = drmModeGetEncoder(fd, conn->encoders[i]);
+		if (!enc) {
+			fprintf(stderr, "cannot retrieve encoder %u:%u (%d): %m\n",
+				i, conn->encoders[i], errno);
+			continue;
+		}
+
+		/* iterate all global CRTCs */
+		for (j = 0; j < res->count_crtcs; ++j) {
+			/* check whether this CRTC works with the encoder */
+			if (!(enc->possible_crtcs & (1 << j)))
+				continue;
+
+			/* check that no other device already uses this CRTC */
+			crtc = res->crtcs[j];
+			for (iter = modeset_list; iter; iter = iter->next) {
+				if (iter->crtc == crtc) {
+					crtc = -1;
+					break;
+				}
+			}
+
+			/* we have found a CRTC, so save it and return */
+			if (crtc >= 0) {
+				drmModeFreeEncoder(enc);
+				dev->crtc = crtc;
+				return 0;
+			}
+		}
+
+		drmModeFreeEncoder(enc);
+	}
+
+	fprintf(stderr, "cannot find suitable CRTC for connector %u\n",
+		conn->connector_id);
+	return -ENOENT;
+}
+
+/*
+ * modeset_create_fb(fd, dev): After we have found a crtc+connector+mode
+ * combination, we need to actually create a suitable framebuffer that we can
+ * use with it. There are actually two ways to do that:
+ *   * We can create a so called "dumb buffer". This is a buffer that we can
+ *     memory-map via mmap() and every driver supports this. We can use it for
+ *     unaccelerated software rendering on the CPU.
+ *   * We can use libgbm to create buffers available for hardware-acceleration.
+ *     libgbm is an abstraction layer that creates these buffers for each
+ *     available DRM driver. As there is no generic API for this, each driver
+ *     provides its own way to create these buffers.
+ *     We can then use such buffers to create OpenGL contexts with the mesa3D
+ *     library.
+ * We use the first solution here as it is much simpler and doesn't require any
+ * external libraries. However, if you want to use hardware-acceleration via
+ * OpenGL, it is actually pretty easy to create such buffers with libgbm and
+ * libEGL. But this is beyond the scope of this document.
+ *
+ * So what we do is requesting a new dumb-buffer from the driver. We specify the
+ * same size as the current mode that we selected for the connector.
+ * Then we request the driver to prepare this buffer for memory mapping. After
+ * that we perform the actual mmap() call. So we can now access the framebuffer
+ * memory directly via the dev->map memory map.
+ */
+
+static int modeset_create_fb(int fd, struct modeset_dev *dev)
+{
+	struct drm_mode_create_dumb creq;
+	struct drm_mode_destroy_dumb dreq;
+	struct drm_mode_map_dumb mreq;
+	int ret;
+
+	/* create dumb buffer */
+	memset(&creq, 0, sizeof(creq));
+	creq.width = dev->width;
+	creq.height = dev->height;
+	creq.bpp = 32;
+	ret = drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &creq);
+	if (ret < 0) {
+		fprintf(stderr, "cannot create dumb buffer (%d): %m\n",
+			errno);
+		return -errno;
+	}
+	dev->stride = creq.pitch;
+	dev->size = creq.size;
+	dev->handle = creq.handle;
+
+	/* create framebuffer object for the dumb-buffer */
+	ret = drmModeAddFB(fd, dev->width, dev->height, 24, 32, dev->stride,
+			   dev->handle, &dev->fb);
+	if (ret) {
+		fprintf(stderr, "cannot create framebuffer (%d): %m\n",
+			errno);
+		ret = -errno;
+		goto err_destroy;
+	}
+
+	/* prepare buffer for memory mapping */
+	memset(&mreq, 0, sizeof(mreq));
+	mreq.handle = dev->handle;
+	ret = drmIoctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &mreq);
+	if (ret) {
+		fprintf(stderr, "cannot map dumb buffer (%d): %m\n",
+			errno);
+		ret = -errno;
+		goto err_fb;
+	}
+
+	/* perform actual memory mapping */
+	dev->map = mmap(0, dev->size, PROT_READ | PROT_WRITE, MAP_SHARED,
+		        fd, mreq.offset);
+	if (dev->map == MAP_FAILED) {
+		fprintf(stderr, "cannot mmap dumb buffer (%d): %m\n",
+			errno);
+		ret = -errno;
+		goto err_fb;
+	}
+
+	/* clear the framebuffer to 0 */
+	memset(dev->map, 0, dev->size);
+
+	return 0;
+
+err_fb:
+	drmModeRmFB(fd, dev->fb);
+err_destroy:
+	memset(&dreq, 0, sizeof(dreq));
+	dreq.handle = dev->handle;
+	drmIoctl(fd, DRM_IOCTL_MODE_DESTROY_DUMB, &dreq);
+	return ret;
+}
+
+/*
+ * Finally! We have a connector with a suitable CRTC. We know which mode we want
+ * to use and we have a framebuffer of the correct size that we can write to.
+ * There is nothing special left to do. We only have to program the CRTC to
+ * connect each new framebuffer to each selected connector for each combination
+ * that we saved in the global modeset_list.
+ * This is done with a call to drmModeSetCrtc().
+ *
+ * So we are ready for our main() function. First we check whether the user
+ * specified a DRM device on the command line, otherwise we use the default
+ * /dev/dri/card0. Then we open the device via modeset_open(). modeset_prepare()
+ * prepares all connectors and we can loop over "modeset_list" and call
+ * drmModeSetCrtc() on every CRTC/connector combination.
+ *
+ * But printing empty black pages is boring so we have another helper function
+ * modeset_draw() that draws some colors into the framebuffer for 5 seconds and
+ * then returns. And then we have all the cleanup functions which correctly free
+ * all devices again after we used them. All these functions are described below
+ * the main() function.
+ *
+ * As a side note: drmModeSetCrtc() actually takes a list of connectors that we
+ * want to control with this CRTC. We pass only one connector, though. As
+ * explained earlier, if we used multiple connectors, then all connectors would
+ * have the same controlling framebuffer so the output would be cloned. This is
+ * most often not what you want so we avoid explaining this feature here.
+ * Furthermore, all connectors will have to run with the same mode, which is
+ * also often not guaranteed. So instead, we only use one connector per CRTC.
+ *
+ * Before calling drmModeSetCrtc() we also save the current CRTC configuration.
+ * This is used in modeset_cleanup() to restore the CRTC to the same mode as was
+ * before we changed it.
+ * If we don't do this, the screen will stay blank after we exit until another
+ * application performs modesetting itself.
+ */
+
+int main(int argc, char **argv)
+{
+	int ret, fd;
+	const char *card;
+	struct modeset_dev *iter;
+
+	/* check which DRM device to open */
+	if (argc > 1)
+		card = argv[1];
+	else
+		card = "/dev/dri/card0";
+
+	fprintf(stderr, "using card '%s'\n", card);
+
+	/* open the DRM device */
+	ret = modeset_open(&fd, card);
+	if (ret)
+		goto out_return;
+
+	/* prepare all connectors and CRTCs */
+	ret = modeset_prepare(fd);
+	if (ret)
+		goto out_close;
+
+	/* perform actual modesetting on each found connector+CRTC */
+	for (iter = modeset_list; iter; iter = iter->next) {
+		iter->saved_crtc = drmModeGetCrtc(fd, iter->crtc);
+		ret = drmModeSetCrtc(fd, iter->crtc, iter->fb, 0, 0,
+				     &iter->conn, 1, &iter->mode);
+		if (ret)
+			fprintf(stderr, "cannot set CRTC for connector %u (%d): %m\n",
+				iter->conn, errno);
+	}
+
+	/* draw some colors for 5seconds */
+	modeset_draw();
+
+	/* cleanup everything */
+	modeset_cleanup(fd);
+
+	ret = 0;
+
+out_close:
+	close(fd);
+out_return:
+	if (ret) {
+		errno = -ret;
+		fprintf(stderr, "modeset failed with error %d: %m\n", errno);
+	} else {
+		fprintf(stderr, "exiting\n");
+	}
+	return ret;
+}
+
+/*
+ * A short helper function to compute a changing color value. No need to
+ * understand it.
+ */
+
+static uint8_t next_color(bool *up, uint8_t cur, unsigned int mod)
+{
+	uint8_t next;
+
+	next = cur + (*up ? 1 : -1) * (rand() % mod);
+	if ((*up && next < cur) || (!*up && next > cur)) {
+		*up = !*up;
+		next = cur;
+	}
+
+	return next;
+}
+
+/*
+ * modeset_draw(): This draws a solid color into all configured framebuffers.
+ * Every 100ms the color changes to a slightly different color so we get some
+ * kind of smoothly changing color-gradient.
+ *
+ * The color calculation can be ignored as it is pretty boring. So the
+ * interesting stuff is iterating over "modeset_list" and then through all lines
+ * and width. We then set each pixel individually to the current color.
+ *
+ * We do this 50 times as we sleep 100ms after each redraw round. This makes
+ * 50*100ms = 5000ms = 5s so it takes about 5seconds to finish this loop.
+ *
+ * Please note that we draw directly into the framebuffer. This means that you
+ * will see flickering as the monitor might refresh while we redraw the screen.
+ * To avoid this you would need to use two framebuffers and a call to
+ * drmModeSetCrtc() to switch between both buffers.
+ * You can also use drmModePageFlip() to do a vsync'ed pageflip. But this is
+ * beyond the scope of this document.
+ */
+
+static void modeset_draw(void)
+{
+	uint8_t r, g, b;
+	bool r_up, g_up, b_up;
+	unsigned int i, j, k, off;
+	struct modeset_dev *iter;
+
+	srand(time(NULL));
+	r = rand() % 0xff;
+	g = rand() % 0xff;
+	b = rand() % 0xff;
+	r_up = g_up = b_up = true;
+
+	for (i = 0; i < 50; ++i) {
+		r = next_color(&r_up, r, 20);
+		g = next_color(&g_up, g, 10);
+		b = next_color(&b_up, b, 5);
+
+		for (iter = modeset_list; iter; iter = iter->next) {
+			for (j = 0; j < iter->height; ++j) {
+				for (k = 0; k < iter->width; ++k) {
+					off = iter->stride * j + k * 4;
+					*(uint32_t*)&iter->map[off] =
+						     (r << 16) | (g << 8) | b;
+				}
+			}
+		}
+
+		usleep(100000);
+	}
+}
+
+/*
+ * modeset_cleanup(fd): This cleans up all the devices we created during
+ * modeset_prepare(). It resets the CRTCs to their saved states and deallocates
+ * all memory.
+ * It should be pretty obvious how all of this works.
+ */
+
+static void modeset_cleanup(int fd)
+{
+	struct modeset_dev *iter;
+	struct drm_mode_destroy_dumb dreq;
+
+	while (modeset_list) {
+		/* remove from global list */
+		iter = modeset_list;
+		modeset_list = iter->next;
+
+		/* restore saved CRTC configuration */
+		drmModeSetCrtc(fd,
+			       iter->saved_crtc->crtc_id,
+			       iter->saved_crtc->buffer_id,
+			       iter->saved_crtc->x,
+			       iter->saved_crtc->y,
+			       &iter->conn,
+			       1,
+			       &iter->saved_crtc->mode);
+		drmModeFreeCrtc(iter->saved_crtc);
+
+		/* unmap buffer */
+		munmap(iter->map, iter->size);
+
+		/* delete framebuffer */
+		drmModeRmFB(fd, iter->fb);
+
+		/* delete dumb buffer */
+		memset(&dreq, 0, sizeof(dreq));
+		dreq.handle = iter->handle;
+		drmIoctl(fd, DRM_IOCTL_MODE_DESTROY_DUMB, &dreq);
+
+		/* free allocated memory */
+		free(iter);
+	}
+}
+
+/*
+ * I hope this was a short but easy overview of the DRM modesetting API. The DRM
+ * API offers much more capabilities including:
+ *  - double-buffering or tripple-buffering (or whatever you want)
+ *  - vsync'ed page-flips
+ *  - hardware-accelerated rendering (for example via OpenGL)
+ *  - output cloning
+ *  - graphics-clients plus authentication
+ *  - DRM planes/overlays/sprites
+ *  - ...
+ * If you are interested in these topics, I can currently only redirect you to
+ * existing implementations, including:
+ *  - plymouth (which uses dumb-buffers like this example; very easy to understand)
+ *  - kmscon (which uses libuterm to do this)
+ *  - wayland (very sophisticated DRM renderer; hard to understand fully as it
+ *             uses more complicated techniques like DRM planes)
+ *  - xserver (very hard to understand as it is split across many files/projects)
+ *
+ * But understanding how modesetting (as described in this document) works, is
+ * essential to understand all further DRM topics.
+ *
+ * Any feedback is welcome. Feel free to use this code freely for your own
+ * documentation or projects.
+ *
+ *  - Hosted on http://github.com/dvdhrm/docs
+ *  - Written by David Rheinsberg <david.rheinsberg@gmail.com>
+ */