DRM example tutorial

DRM example tutorial

DRM is a display driver framework, which encapsulates functions into standard interfaces such as open/close/ioctl, and applications call these interfaces to drive devices and display data. We will look at how to verify and use the DRM driver from the perspective of usage.

DRM device node

The DRM driver will create 3 device nodes under /dev/dri :

 

card0
controlD64
renderD128

libdrm library

DRM driver provides a special calling library libdrm.so for user space, and user space can indirectly call and use the driver through this library.

Turn on the device

 

    int fd = open("/dev/dri/card0", O_RDWR | O_CLOEXEC);
    if (fd < 0) {
        ret = -errno;
        fprintf(stderr, "cannot open '%s': %m\n", node);
        return ret;
    }

There is a dedicated interface for opening the device: drmOpen

Check DRM capabilities

The capability of DRM is obtained through the drmGetCap interface, which is described by the drm_get_cap structure:

 

/** DRM_IOCTL_GET_CAP ioctl argument type */
struct drm_get_cap {
    __u64 capability;
    __u64 value;
};

int drmGetCap(int fd, uint64_t capability, uint64_t *value)
{
    struct drm_get_cap cap;
    int ret;

    memclear(cap);
    cap.capability = capability;

    ret = drmIoctl(fd, DRM_IOCTL_GET_CAP, &cap);
    if (ret)
        return ret;

    *value = cap.value;
    return 0;
}

Usage example:

 

        uint64_t has_dumb;
    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;
    }

Retrieve Resource

It takes two times to obtain the Resource. The first time, the quantity is obtained, and the second time, the specific Resource is actually obtained. Look at this function specifically:

 

drmModeResPtr drmModeGetResources(int fd)

Resource structure package:

 

struct drm_mode_card_res {
    __u64 fb_id_ptr;
    __u64 crtc_id_ptr;
    __u64 connector_id_ptr;
    __u64 encoder_id_ptr;
    __u32 count_fbs;
    __u32 count_crtcs;
    __u32 count_connectors;
    __u32 count_encoders;
    __u32 min_width, max_width;
    __u32 min_height, max_height;
};

 

typedef struct _drmModeRes {

    int count_fbs;
    uint32_t *fbs;

    int count_crtcs;
    uint32_t *crtcs;

    int count_connectors;
    uint32_t *connectors;

    int count_encoders;
    uint32_t *encoders;

    uint32_t min_width, max_width;
    uint32_t min_height, max_height;
} drmModeRes, *drmModeResPtr;

Instance

 

    /* retrieve resources */
    int ret = drmModeGetResources(fd);
    if (!res) {
        fprintf(stderr, "cannot retrieve DRM resources (%d): %m\n",
            errno);
        return -errno;
    }

Get Connector

_drmModeConnector description structure:

 

typedef struct _drmModeConnector {
    uint32_t connector_id;
    uint32_t encoder_id; /**< Encoder currently connected to */
    uint32_t connector_type;
    uint32_t connector_type_id;
    drmModeConnection connection;
    uint32_t mmWidth, mmHeight; /**< HxW in millimeters */
    drmModeSubPixel subpixel;

    int count_modes;
    drmModeModeInfoPtr modes;

    int count_props;
    uint32_t *props; /**< List of property ids */
    uint64_t *prop_values; /**< List of property values */

    int count_encoders;
    uint32_t *encoders; /**< List of encoder ids */
} drmModeConnector, *drmModeConnectorPtr;

Example:

 

        drmModeConnector *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;
        }

Encoder

Encoder structure description:

 

typedef struct _drmModeEncoder {
    uint32_t encoder_id;
    uint32_t encoder_type;
    uint32_t crtc_id;
    uint32_t possible_crtcs;
    uint32_t possible_clones;
} drmModeEncoder, *drmModeEncoderPtr;

Example:

 

    if (conn->encoder_id)
        drmModeEncoder *enc = drmModeGetEncoder(fd, conn->encoder_id);
    }

 

drmModeEncoderPtr drmModeGetEncoder(int fd, uint32_t encoder_id)
{
    struct drm_mode_get_encoder enc;
    drmModeEncoderPtr r = NULL;

    memclear(enc);
    enc.encoder_id = encoder_id;

    if (drmIoctl(fd, DRM_IOCTL_MODE_GETENCODER, &enc))
        return 0;

    if (!(r = drmMalloc(sizeof(*r))))
        return 0;

    r->encoder_id = enc.encoder_id;
    r->crtc_id = enc.crtc_id;
    r->encoder_type = enc.encoder_type;
    r->possible_crtcs = enc.possible_crtcs;
    r->possible_clones = enc.possible_clones;

    return r;
}

crtc

CRTC structure description:

 

struct crtc {
    drmModeCrtc *crtc;
    drmModeObjectProperties *props;
    drmModePropertyRes **props_info;
    drmModeModeInfo *mode;
};

 

typedef struct _drmModeCrtc {
    uint32_t crtc_id;
    uint32_t buffer_id; /**< FB id to connect to 0 = disconnect */

    uint32_t x, y; /**< Position on the framebuffer */
    uint32_t width, height;
    int mode_valid;
    drmModeModeInfo mode;

    int gamma_size; /**< Number of gamma stops */

} drmModeCrtc, *drmModeCrtcPtr;

FrameBuffer

Create DUMB Buffer

 

    ret = drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &creq);
    if (ret < 0) {
        fprintf(stderr, "cannot create dumb buffer (%d): %m\n",
            errno);
        return -errno;
    }

Add FB

 

    /* 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 map

 

    /* 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;
    }

Do map operations:

 

    /* 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;
    }

Preparation for CRTC

drmModeGetCrtc
drmModeSetCrtc

 

drmModeCrtcPtr drmModeGetCrtc(int fd, uint32_t crtcId)
{
    struct drm_mode_crtc crtc;
    drmModeCrtcPtr r;

    memclear(crtc);
    crtc.crtc_id = crtcId;

    if (drmIoctl(fd, DRM_IOCTL_MODE_GETCRTC, &crtc))
        return 0;

    /*
     * return
     */

    if (!(r = drmMalloc(sizeof(*r))))
        return 0;

    r->crtc_id         = crtc.crtc_id;
    r->x               = crtc.x;
    r->y               = crtc.y;
    r->mode_valid      = crtc.mode_valid;
    if (r->mode_valid) {
        memcpy(&r->mode, &crtc.mode, sizeof(struct drm_mode_modeinfo));
        r->width = crtc.mode.hdisplay;
        r->height = crtc.mode.vdisplay;
    }
    r->buffer_id       = crtc.fb_id;
    r->gamma_size      = crtc.gamma_size;
    return r;
}

int drmModeSetCrtc(int fd, uint32_t crtcId, uint32_t bufferId,
           uint32_t x, uint32_t y, uint32_t *connectors, int count,
           drmModeModeInfoPtr mode)
{
    struct drm_mode_crtc crtc;

    memclear(crtc);
    crtc.x             = x;
    crtc.y             = y;
    crtc.crtc_id       = crtcId;
    crtc.fb_id         = bufferId;
    crtc.set_connectors_ptr = VOID2U64(connectors);
    crtc.count_connectors = count;
    if (mode) {
      memcpy(&crtc.mode, mode, sizeof(struct drm_mode_modeinfo));
      crtc.mode_valid = 1;
    }

    return DRM_IOCTL(fd, DRM_IOCTL_MODE_SETCRTC, &crtc);
}

draw

 

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);
    }
}

For specific code, you can refer to how-to examples:
how-to code examples

Author: wind eve
link: https: //www.jianshu.com/p/f41f98a40455
Source: Jane books
are copyrighted by the author. For commercial reprints, please contact the author for authorization. For non-commercial reprints, please indicate the source.