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d01b7c6 Jan 17, 2013
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/*
* modeset - DRM Modesetting Example
*
* Written 2012 by David Herrmann <dh.herrmann@googlemail.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 Herrmann <dh.herrmann@googlemail.com>
*/