DRM实例教程

DRM是一个显示驱动框架，也就是把功能封装成 open/close/ioctl 等标准接口，应用程序调用这些接口来驱动设备，显示数据。我们这里将从使用的角度来看看，怎么验证和使用DRM驱动。

DRM设备节点

DRM驱动会在/dev/dri下创建3个设备节点：

card0
controlD64
renderD128

libdrm库

DRM驱动，对用户空间，提供了专门的的调用库libdrm.so，用户空间通过该库可以间接的调用和使用驱动。

打开设备

    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;
    }

打开设备有专门的接口：drmOpen

检查DRM的能力

DRM的能力通过drmGetCap接口获取，用drm_get_cap结构描述：

/** 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;
}

使用示例：

        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;
    }

检索Resource

Resource的获取需要两次，第一次，获取数量大小，第二次才真正获取具体的Resource。具体看这个函数：

drmModeResPtr drmModeGetResources(int fd)

Resource结构封装：

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;

实例

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

获取Connector

_drmModeConnector描述结构：

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;

示例：

        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的结构描述：

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;

示例：

    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结构描述：

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

创建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;
    }

添加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;
    }

准备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;
    }

做map操作：

    /* 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;
    }

CRTC的准备

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);
}

绘制

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);
    }
}

具体的代码，可以参考how-to实例：
how-to代码实例