Re: [PATCH v3 0/3] drm: atmel-hlcdc: clut support

[Boris Brezillon]

+Alexandre and Nicolas

Hi Peter,

Can you please Cc at91 maintainers next time?

On Mon, 19 Jun 2017 09:44:23 +0200
Peter Rosin <peda@xxxxxxxxxx> wrote:

> Hi!
> 
> This series adds support for an 8-bit clut mode in the atmel-hlcdc
> driver.
> 
> I have now tested patch 1 with the below program (modeset.c
> adapted from https://github.com/dvdhrm/docs/tree/master/drm-howto
> to use an 8-bit mode).
> 
> Since v2 I have also cleared up why the first 16 entries of the clut
> was not working right. It was of course my own damn fault, and the
> fix was in atmel_hlcdc_layer_write_clut function which called the
> ...write_reg function which in turn added an extra offset of 16
> registers...
> 
> Changes since v2:
> 
> - Fix mapping to the clut registers.
> 
> Changes since v1:
> 
> - Move the clut update from atmel_hlcdc_crtc_mode_valid to
>   atmel_hlcdc_plane_atomic_update.
> - Add default .gamma_set helper (drm_atomic_helper_legacy_gamma_set).
> - Don't keep a spare copy of the clut, reuse gamma_store instead.
> - Don't try to synchronize the legacy fb clut with the drm clut.
> 
> As I said in v2, I have not added any .clut_offset to the overlay2
> layer of sama5d4, since the chip does not appear to have that layer.
> I didn't do that to make it easier to work with the patch previously
> sent to remove that layer, but I suspect bad things may happen to
> sama5d4 users if they do not have that layer removed.
> 
> Cheers,
> peda
> 
> modeset-pal.c (didn't update any comments, sorry)
> ----------------8<---------------
> /*
>  * modeset - DRM Modesetting Example
>  *
>  * Written 2012 by David Herrmann <dh.herrmann@xxxxxxxxxxxxxx>
>  * 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(int fd);
> 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 = 8;
> 	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, 8, 8, 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(fd);
> 
> 	/* 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;
> }
> 
> static void crtc_lut(int fd, struct modeset_dev *dev, int p)
> {
> 	struct drm_mode_crtc_lut clut;
> 	uint16_t r[256];
> 	uint16_t g[256];
> 	uint16_t b[256];
> 	int ret;
> 	int i;
> 
> 	/* prepare buffer for memory mapping */
> 	memset(&clut, 0, sizeof(clut));
> 	clut.crtc_id = dev->crtc;
> 	clut.gamma_size = 256;
> 	clut.red = (uint64_t)r;
> 	clut.green = (uint64_t)g;
> 	clut.blue = (uint64_t)b;
> 
> 	for (i = 0; i < 256; ++i) {
> 		r[i] = ((p + 2 * i) & 255) * 257;
> 		g[i] = ((p + 3 * i) & 255) * 257;
> 		b[i] = ((p + 5 * i) & 255) * 257;
> 	}
> 	ret = drmIoctl(fd, DRM_IOCTL_MODE_SETGAMMA, &clut);
> 	if (ret)
> 		fprintf(stderr, "cannot set gamma lut (%d): %m\n",
> 			errno);
> }
> 
> /*
>  * 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(int fd)
> {
> 	uint8_t p = 0;
> 	unsigned int i, j, k;
> 	struct modeset_dev *iter;
> 
> 	for (iter = modeset_list; iter; iter = iter->next) {
> 		for (k = 0; k < iter->width; ++k) {
> 			for (j = 0; j < iter->height / 3; ++j) {
> 				iter->map[iter->stride * j + k] =
> 					k * 256 / iter->width;
> 			}
> 			for (; j < iter->height; ++j)
> 				iter->map[iter->stride * j + k] = 26;
> 		}
> 	}
> 
> 	for (i = 0; i < 50; ++i, ++p) {
> 		for (iter = modeset_list; iter; iter = iter->next)
> 			crtc_lut(fd, iter, p);
> 
> 		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 Herrmann <dh.herrmann@xxxxxxxxxxxxxx>
>  */
> ----------------8<---------------
> 
> Peter Rosin (3):
>   drm: atmel-hlcdc: add support for 8-bit color lookup table mode
>   drm/fb-cma-helper: expose more of fb cma guts
>   drm: atmel-hlcdc: add clut support for legacy fbdev
> 
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c  | 58 +++++++++++++++++++++++++
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c    | 25 ++++++++++-
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.h    | 20 +++++++++
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c | 29 +++++++++++++
>  drivers/gpu/drm/drm_fb_cma_helper.c             | 55 ++++++++++++++++++-----
>  include/drm/drm_fb_cma_helper.h                 |  8 +++-
>  6 files changed, 182 insertions(+), 13 deletions(-)
>