用户空间内存操作

为了便于说明, 额外定义一个外部内存结构:

// drm相关操作需要引用该头文件
#include <drm.h>
struct bo {
   int fd;
   void *ptr;
   size_t size;
   size_t offset;
   size_t pitch;
   unsigned handle;
};

获取设备节点:

bo->fd = open("/dev/dri/card0", O_RDWR, 0);

alloc

struct drm_mode_create_dumb arg;
int handle, size, pitch;
int ret;
memset(&arg, 0, sizeof(arg));
arg.bpp = bpp;
arg.width = width;
arg.height = height;
ret = drmIoctl(bo->fd, DRM_IOCTL_MODE_CREATE_DUMB, &arg);
if (ret) {
    fprintf(stderr, "failed to create dumb buffer: %s\n", strerror(errno));
    return ret;
}
bo->handle = arg.handle;
bo->size = arg.size;
bo->pitch = arg.pitch;

mmap

struct drm_mode_map_dumb arg;
void *map;
int ret;

memset(&arg, 0, sizeof(arg));
arg.handle = bo->handle;
ret = drmIoctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &arg);
if (ret)
    return ret;
map = mmap(0, bo->size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, arg.offset);
//64位
map = mmap64(0, bo->size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, arg.offset);
if (map == MAP_FAILED)
   return -EINVAL;
bo->ptr = map

unmap

drm_munmap(bo->ptr, bo->size);
bo->ptr = NULL;

free

struct drm_mode_destroy_dumb arg;
int ret;

memset(&arg, 0, sizeof(arg));
arg.handle = bo->handle;
ret = drmIoctl(bo->fd, DRM_IOCTL_MODE_DESTROY_DUMB, &arg);
if (ret)
    fprintf(stderr, "failed to destroy dumb buffer: %s\n", strerror(errno));

free gem handle:

// gem handle减引用操作
struct drm_gem_close args;
memset(&args, 0, sizeof(args));
args.handle = bo->handle;
drmIoctl(bo->fd, DRM_IOCTL_GEM_CLOSE, &args);

export dmafd

int export_dmafd;
ret = drmPrimeHandleToFD(bo->fd, bo->handle, 0, &export_dmafd);
// drmPrimeHandleToFD是会给dma_buf加引用计数的
// 使用完export_dmafd后, 需要使用close(export_dmafd)来减掉引用计数

import dmafd

ret = drmPrimeFDToHandle(bo->fd, import_dmafd, &bo->handle);
// drmPrimeHandleToFD是会给dma_buf加引用计数的
// 使用完bo->handle后, 需要对handle减引用, 参看free gem handle部分.