KWin、libdrm、DRM从上到下全过程 —— drmModeAddFBxxx（3）

接前一篇文章：KWin、libdrm、DRM从上到下全过程 —— drmModeAddFBxxx（2）

前文书说到drmModeAddFB、drmModeAddFB2和drmModeAddFB2WithModifiers函数最终“三分归一统”，在内核层统一调用到drm_mode_addfb2函数。上回书深入回顾了从上到下逐步合并统一的过程，本文正式开始对于drm_mode_addfb2函数进行详细解析。

实际上之前笔者进行过分析，参见DRM全解析 —— ADD_FB2（4）。

但是这里由于最近研究的深入，因此要比之前进行更细致一些的代码解析。

drm_mode_addfb2函数在Linux内核源码根目录下的drivers/gpu/drm/drm_framebuffer.c中，代码如下：

/**
 * drm_mode_addfb2 - add an FB to the graphics configuration
 * @dev: drm device for the ioctl
 * @data: data pointer for the ioctl
 * @file_priv: drm file for the ioctl call
 *
 * Add a new FB to the specified CRTC, given a user request with format. This is
 * the 2nd version of the addfb ioctl, which supports multi-planar framebuffers
 * and uses fourcc codes as pixel format specifiers.
 *
 * Called by the user via ioctl.
 *
 * Returns:
 * Zero on success, negative errno on failure.
 */
int drm_mode_addfb2(struct drm_device *dev,
		    void *data, struct drm_file *file_priv)
{
	struct drm_mode_fb_cmd2 *r = data;
	struct drm_framebuffer *fb;
 
	if (!drm_core_check_feature(dev, DRIVER_MODESET))
		return -EOPNOTSUPP;
 
	fb = drm_internal_framebuffer_create(dev, r, file_priv);
	if (IS_ERR(fb))
		return PTR_ERR(fb);
 
	drm_dbg_kms(dev, "[FB:%d]\n", fb->base.id);
	r->fb_id = fb->base.id;
 
	/* Transfer ownership to the filp for reaping on close */
	mutex_lock(&file_priv->fbs_lock);
	list_add(&fb->filp_head, &file_priv->fbs);
	mutex_unlock(&file_priv->fbs_lock);
 
	return 0;
}

先来看第一步暨第一个函数：drm_core_check_feature。对应的代码片段为：

    if (!drm_core_check_feature(dev, DRIVER_MODESET))
		return -EOPNOTSUPP;

drm_core_check_feature函数在include/drm/drm_drv.h中，代码如下：

/**
 * drm_core_check_feature - check driver feature flags
 * @dev: DRM device to check
 * @feature: feature flag
 *
 * This checks @dev for driver features, see &drm_driver.driver_features,
 * &drm_device.driver_features, and the various &enum drm_driver_feature flags.
 *
 * Returns true if the @feature is supported, false otherwise.
 */
static inline bool drm_core_check_feature(const struct drm_device *dev,
					  enum drm_driver_feature feature)
{
	return drm_core_check_all_features(dev, feature);
}

可以看到，实际的工作drm_core_check_all_features函数完成的。drm_core_check_all_features函数就在上边，代码如下：

/**
 * drm_core_check_all_features - check driver feature flags mask
 * @dev: DRM device to check
 * @features: feature flag(s) mask
 *
 * This checks @dev for driver features, see &drm_driver.driver_features,
 * &drm_device.driver_features, and the various &enum drm_driver_feature flags.
 *
 * Returns true if all features in the @features mask are supported, false
 * otherwise.
 */
static inline bool drm_core_check_all_features(const struct drm_device *dev,
					       u32 features)
{
	u32 supported = dev->driver->driver_features & dev->driver_features;
 
	return features && (supported & features) == features;
}

函数的功能是先将struct drm_device *dev（设备实例）对应的struct drm_driver *driver（驱动实例）中的driver_features与struct drm_device *dev（设备实例）自身的driver_features相与，得到仅限于设备自身特性空间的几项。然后看这几项中是否支持features特性。如果features本身不为0并且支持，则返回真（true）；否则返回假（false）。而一旦返回false，则drm_core_check_feature函数最终返回-EOPNOTSUPP（include/uapi/asm-generic/errno.h中定义），表示“Operation not supported on transport endpoint”即传输端点上不支持该操作。

深入跟进到具体调用细节。DRIVER_MODESET是一个宏，在include/drm/drm_drv.h中定义，如下：

    /**
	 * @DRIVER_MODESET:
	 *
	 * Driver supports mode setting interfaces (KMS).
	 */
	DRIVER_MODESET			= BIT(1),

在此传入drm_core_check_all_features函数的features为DRIVER_MODESET（值为2），代表支持KMS（Kernel Mode Setting）接口（与否）。

重点看一下dev->driver->driver_features。dev->driver的意思是drm设备对应的drm驱动。实际上dev->driver对于不同的显卡驱动，是对应不同的struct drm_driver实例的。这里以笔者实际使用的电脑中的显卡Intel i915为例进行说明。

Intel i915显卡驱动对应的struct drm_driver初始化代码在drivers/gpu/drm/i915/i915_driver.c中，如下：

static const struct drm_driver i915_drm_driver = {
	/* Don't use MTRRs here; the Xserver or userspace app should
	 * deal with them for Intel hardware.
	 */
	.driver_features =
	    DRIVER_GEM |
	    DRIVER_RENDER | DRIVER_MODESET | DRIVER_ATOMIC | DRIVER_SYNCOBJ |
	    DRIVER_SYNCOBJ_TIMELINE,
	.release = i915_driver_release,
	.open = i915_driver_open,
	.lastclose = i915_driver_lastclose,
	.postclose = i915_driver_postclose,
 
	.prime_handle_to_fd = drm_gem_prime_handle_to_fd,
	.prime_fd_to_handle = drm_gem_prime_fd_to_handle,
	.gem_prime_import = i915_gem_prime_import,
 
	.dumb_create = i915_gem_dumb_create,
	.dumb_map_offset = i915_gem_dumb_mmap_offset,
 
	.ioctls = i915_ioctls,
	.num_ioctls = ARRAY_SIZE(i915_ioctls),
	.fops = &i915_driver_fops,
	.name = DRIVER_NAME,
	.desc = DRIVER_DESC,
	.date = DRIVER_DATE,
	.major = DRIVER_MAJOR,
	.minor = DRIVER_MINOR,
	.patchlevel = DRIVER_PATCHLEVEL,
};

由代码可见，对于i915显卡驱动来说，其支持的特性为：

    .driver_features =
	    DRIVER_GEM |
	    DRIVER_RENDER | DRIVER_MODESET | DRIVER_ATOMIC | DRIVER_SYNCOBJ |
	    DRIVER_SYNCOBJ_TIMELINE,

可见，以上3款显卡都是支持DRIVER_MODESET这一feature的。

而对于dev->driver_features即设备实例的driver_features，就不像dev->driver->driver_features即驱动实例的driver_features那样明显了。在DRM即内核源码路径下，共有几处相关，分别如下：

1）drivers/gpu/drm/drm_drv.c的drm_dev_init函数中：

static int drm_dev_init(struct drm_device *dev,
			const struct drm_driver *driver,
			struct device *parent)
{
	struct inode *inode;
	int ret;
 
	……
	/* no per-device feature limits by default */
	dev->driver_features = ~0u;
    ……
}

2）drivers/vdpa/vdpa_user/vduse_dev.c的vduse_dev_reset函数中：

static void vduse_dev_reset(struct vduse_dev *dev)
{
	int i;
	struct vduse_iova_domain *domain = dev->domain;
 
	/* The coherent mappings are handled in vduse_dev_free_coherent() */
	if (domain->bounce_map)
		vduse_domain_reset_bounce_map(domain);
 
	down_write(&dev->rwsem);
 
	dev->status = 0;
	dev->driver_features = 0;
	dev->generation++;
	spin_lock(&dev->irq_lock);
	dev->config_cb.callback = NULL;
	dev->config_cb.private = NULL;
	spin_unlock(&dev->irq_lock);
	flush_work(&dev->inject);
 
    ……
}

3）drivers/vdpa/vdpa_user/vduse_dev.c的vduse_vdpa_set_driver_features函数中：

static int vduse_vdpa_set_driver_features(struct vdpa_device *vdpa, u64 features)
{
	struct vduse_dev *dev = vdpa_to_vduse(vdpa);
 
	dev->driver_features = features;
	return 0;
}

可以想见，虽然目前尚不完全清楚设置dev->driver_features即设备实例的driver_features的具体过程和细节，但是由于代码可以正常执行而并未返回false，可以推断出一定在某处（很可能是vduse_vdpa_set_driver_features函数中）将相应位设置为了DRIVER_MODESET。

至此，drm_mode_addfb2函数中的第一步：drm_core_check_feature函数就分析完了。余下的步骤和函数在接下来的文章中继续进行解析。