Re: [PATCH v2] drm/framebuffer: Acquire internal references on GEM handles

["Christian König" <christian.koenig@amd.com>]

On 07.07.25 11:31, Thomas Zimmermann wrote:
> Acquire GEM handles in drm_framebuffer_init() and release them in
> the corresponding drm_framebuffer_cleanup(). Ties the handle's
> lifetime to the framebuffer. Not all GEM buffer objects have GEM
> handles. If not set, no refcounting takes place. This is the case
> for some fbdev emulation. This is not a problem as these GEM objects
> do not use dma-bufs and drivers will not release them while fbdev
> emulation is running. Framebuffer flags keep a bit per color plane
> of which the framebuffer holds a GEM handle reference.
> 
> As all drivers use drm_framebuffer_init(), they will now all hold
> dma-buf references as fixed in commit 5307dce878d4 ("drm/gem: Acquire
> references on GEM handles for framebuffers").
> 
> In the GEM framebuffer helpers, restore the original ref counting
> on buffer objects. As the helpers for handle refcounting are now
> no longer called from outside the DRM core, unexport the symbols.
> 
> v2:
> - track framebuffer handle refs by flag
> - drop gma500 cleanup (Christian)
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Fixes: 5307dce878d4 ("drm/gem: Acquire references on GEM handles for framebuffers")
> Reported-by: Bert Karwatzki <spasswolf@web.de>
> Closes: https://lore.kernel.org/dri-devel/20250703115915.3096-1-spasswolf@web.de/
> Tested-by: Bert Karwatzki <spasswolf@web.de>
> Tested-by: Mario Limonciello <superm1@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Anusha Srivatsa <asrivats@redhat.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: linux-media@vger.kernel.org
> Cc: dri-devel@lists.freedesktop.org
> Cc: linaro-mm-sig@lists.linaro.org
> Cc: <stable@vger.kernel.org>
> ---
>  drivers/gpu/drm/drm_framebuffer.c            | 31 ++++++++++++++--
>  drivers/gpu/drm/drm_gem.c                    | 38 ++++++++++++--------
>  drivers/gpu/drm/drm_gem_framebuffer_helper.c | 16 ++++-----
>  drivers/gpu/drm/drm_internal.h               |  2 +-
>  drivers/gpu/drm/drm_modeset_helper.c         |  2 +-
>  include/drm/drm_framebuffer.h                |  9 +++++
>  6 files changed, 71 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> index b781601946db..23b56cde21d7 100644
> --- a/drivers/gpu/drm/drm_framebuffer.c
> +++ b/drivers/gpu/drm/drm_framebuffer.c
> @@ -862,11 +862,23 @@ EXPORT_SYMBOL_FOR_TESTS_ONLY(drm_framebuffer_free);
>  int drm_framebuffer_init(struct drm_device *dev, struct drm_framebuffer *fb,
>  			 const struct drm_framebuffer_funcs *funcs)
>  {
> +	unsigned int i;
>  	int ret;
> +	bool exists;
>  
>  	if (WARN_ON_ONCE(fb->dev != dev || !fb->format))
>  		return -EINVAL;
>  
> +	for (i = 0; i < fb->format->num_planes; i++) {
> +		if (drm_WARN_ON_ONCE(dev, fb->flags & DRM_FRAMEBUFFER_HAS_HANDLE_REF(i)))

Looks mostly good, but fb->flags are the userspace mode flags IIRC.

Overloading that sounds like a bad idea. I would just create another bool fb->obj_has_handle_ref[DRM_FORMAT_MAX_PLANES] array for that state.

Regards,
Christian.


> +			fb->flags &= ~DRM_FRAMEBUFFER_HAS_HANDLE_REF(i);
> +		if (fb->obj[i]) {
> +			exists = drm_gem_object_handle_get_if_exists_unlocked(fb->obj[i]);
> +			if (exists)
> +				fb->flags |= DRM_FRAMEBUFFER_HAS_HANDLE_REF(i);
> +		}
> +	}
> +
>  	INIT_LIST_HEAD(&fb->filp_head);
>  
>  	fb->funcs = funcs;
> @@ -875,7 +887,7 @@ int drm_framebuffer_init(struct drm_device *dev, struct drm_framebuffer *fb,
>  	ret = __drm_mode_object_add(dev, &fb->base, DRM_MODE_OBJECT_FB,
>  				    false, drm_framebuffer_free);
>  	if (ret)
> -		goto out;
> +		goto err;
>  
>  	mutex_lock(&dev->mode_config.fb_lock);
>  	dev->mode_config.num_fb++;
> @@ -883,7 +895,16 @@ int drm_framebuffer_init(struct drm_device *dev, struct drm_framebuffer *fb,
>  	mutex_unlock(&dev->mode_config.fb_lock);
>  
>  	drm_mode_object_register(dev, &fb->base);
> -out:
> +
> +	return 0;
> +
> +err:
> +	for (i = 0; i < fb->format->num_planes; i++) {
> +		if (fb->flags & DRM_FRAMEBUFFER_HAS_HANDLE_REF(i)) {
> +			drm_gem_object_handle_put_unlocked(fb->obj[i]);
> +			fb->flags &= ~DRM_FRAMEBUFFER_HAS_HANDLE_REF(i);
> +		}
> +	}
>  	return ret;
>  }
>  EXPORT_SYMBOL(drm_framebuffer_init);
> @@ -960,6 +981,12 @@ EXPORT_SYMBOL(drm_framebuffer_unregister_private);
>  void drm_framebuffer_cleanup(struct drm_framebuffer *fb)
>  {
>  	struct drm_device *dev = fb->dev;
> +	unsigned int i;
> +
> +	for (i = 0; i < fb->format->num_planes; i++) {
> +		if (fb->flags & DRM_FRAMEBUFFER_HAS_HANDLE_REF(i))
> +			drm_gem_object_handle_put_unlocked(fb->obj[i]);
> +	}
>  
>  	mutex_lock(&dev->mode_config.fb_lock);
>  	list_del(&fb->head);
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index bc505d938b3e..41cdab6088ae 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -224,23 +224,34 @@ static void drm_gem_object_handle_get(struct drm_gem_object *obj)
>  }
>  
>  /**
> - * drm_gem_object_handle_get_unlocked - acquire reference on user-space handles
> + * drm_gem_object_handle_get_if_exists_unlocked - acquire reference on user-space handle, if any
>   * @obj: GEM object
>   *
> - * Acquires a reference on the GEM buffer object's handle. Required
> - * to keep the GEM object alive. Call drm_gem_object_handle_put_unlocked()
> - * to release the reference.
> + * Acquires a reference on the GEM buffer object's handle. Required to keep
> + * the GEM object alive. Call drm_gem_object_handle_put_if_exists_unlocked()
> + * to release the reference. Does nothing if the buffer object has no handle.
> + *
> + * Returns:
> + * True if a handle exists, or false otherwise
>   */
> -void drm_gem_object_handle_get_unlocked(struct drm_gem_object *obj)
> +bool drm_gem_object_handle_get_if_exists_unlocked(struct drm_gem_object *obj)
>  {
>  	struct drm_device *dev = obj->dev;
>  
>  	guard(mutex)(&dev->object_name_lock);
>  
> -	drm_WARN_ON(dev, !obj->handle_count); /* first ref taken in create-tail helper */
> +	/*
> +	 * First ref taken during GEM object creation, if any. Some
> +	 * drivers set up internal framebuffers with GEM objects that
> +	 * do not have a GEM handle. Hence, this counter can be zero.
> +	 */
> +	if (!obj->handle_count)
> +		return false;
> +
>  	drm_gem_object_handle_get(obj);
> +
> +	return true;
>  }
> -EXPORT_SYMBOL(drm_gem_object_handle_get_unlocked);
>  
>  /**
>   * drm_gem_object_handle_free - release resources bound to userspace handles
> @@ -273,7 +284,7 @@ static void drm_gem_object_exported_dma_buf_free(struct drm_gem_object *obj)
>  }
>  
>  /**
> - * drm_gem_object_handle_put_unlocked - releases reference on user-space handles
> + * drm_gem_object_handle_put_unlocked - releases reference on user-space handle
>   * @obj: GEM object
>   *
>   * Releases a reference on the GEM buffer object's handle. Possibly releases
> @@ -284,14 +295,14 @@ void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
>  	struct drm_device *dev = obj->dev;
>  	bool final = false;
>  
> -	if (WARN_ON(READ_ONCE(obj->handle_count) == 0))
> +	if (drm_WARN_ON(dev, READ_ONCE(obj->handle_count) == 0))
>  		return;
>  
>  	/*
> -	* Must bump handle count first as this may be the last
> -	* ref, in which case the object would disappear before we
> -	* checked for a name
> -	*/
> +	 * Must bump handle count first as this may be the last
> +	 * ref, in which case the object would disappear before
> +	 * we checked for a name.
> +	 */
>  
>  	mutex_lock(&dev->object_name_lock);
>  	if (--obj->handle_count == 0) {
> @@ -304,7 +315,6 @@ void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
>  	if (final)
>  		drm_gem_object_put(obj);
>  }
> -EXPORT_SYMBOL(drm_gem_object_handle_put_unlocked);
>  
>  /*
>   * Called at device or object close to release the file's
> diff --git a/drivers/gpu/drm/drm_gem_framebuffer_helper.c b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> index c60d0044d036..618ce725cd75 100644
> --- a/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> +++ b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> @@ -100,7 +100,7 @@ void drm_gem_fb_destroy(struct drm_framebuffer *fb)
>  	unsigned int i;
>  
>  	for (i = 0; i < fb->format->num_planes; i++)
> -		drm_gem_object_handle_put_unlocked(fb->obj[i]);
> +		drm_gem_object_put(fb->obj[i]);
>  
>  	drm_framebuffer_cleanup(fb);
>  	kfree(fb);
> @@ -183,10 +183,8 @@ int drm_gem_fb_init_with_funcs(struct drm_device *dev,
>  		if (!objs[i]) {
>  			drm_dbg_kms(dev, "Failed to lookup GEM object\n");
>  			ret = -ENOENT;
> -			goto err_gem_object_handle_put_unlocked;
> +			goto err_gem_object_put;
>  		}
> -		drm_gem_object_handle_get_unlocked(objs[i]);
> -		drm_gem_object_put(objs[i]);
>  
>  		min_size = (height - 1) * mode_cmd->pitches[i]
>  			 + drm_format_info_min_pitch(info, i, width)
> @@ -196,22 +194,22 @@ int drm_gem_fb_init_with_funcs(struct drm_device *dev,
>  			drm_dbg_kms(dev,
>  				    "GEM object size (%zu) smaller than minimum size (%u) for plane %d\n",
>  				    objs[i]->size, min_size, i);
> -			drm_gem_object_handle_put_unlocked(objs[i]);
> +			drm_gem_object_put(objs[i]);
>  			ret = -EINVAL;
> -			goto err_gem_object_handle_put_unlocked;
> +			goto err_gem_object_put;
>  		}
>  	}
>  
>  	ret = drm_gem_fb_init(dev, fb, mode_cmd, objs, i, funcs);
>  	if (ret)
> -		goto err_gem_object_handle_put_unlocked;
> +		goto err_gem_object_put;
>  
>  	return 0;
>  
> -err_gem_object_handle_put_unlocked:
> +err_gem_object_put:
>  	while (i > 0) {
>  		--i;
> -		drm_gem_object_handle_put_unlocked(objs[i]);
> +		drm_gem_object_put(objs[i]);
>  	}
>  	return ret;
>  }
> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> index f921cc73f8b8..e79c3c623c9a 100644
> --- a/drivers/gpu/drm/drm_internal.h
> +++ b/drivers/gpu/drm/drm_internal.h
> @@ -161,7 +161,7 @@ void drm_sysfs_lease_event(struct drm_device *dev);
>  
>  /* drm_gem.c */
>  int drm_gem_init(struct drm_device *dev);
> -void drm_gem_object_handle_get_unlocked(struct drm_gem_object *obj);
> +bool drm_gem_object_handle_get_if_exists_unlocked(struct drm_gem_object *obj);
>  void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj);
>  int drm_gem_handle_create_tail(struct drm_file *file_priv,
>  			       struct drm_gem_object *obj,
> diff --git a/drivers/gpu/drm/drm_modeset_helper.c b/drivers/gpu/drm/drm_modeset_helper.c
> index ef32f6af10d4..1e8822c4b370 100644
> --- a/drivers/gpu/drm/drm_modeset_helper.c
> +++ b/drivers/gpu/drm/drm_modeset_helper.c
> @@ -94,7 +94,7 @@ void drm_helper_mode_fill_fb_struct(struct drm_device *dev,
>  		fb->offsets[i] = mode_cmd->offsets[i];
>  	}
>  	fb->modifier = mode_cmd->modifier[0];
> -	fb->flags = mode_cmd->flags;
> +	fb->flags = mode_cmd->flags & DRM_FRAMEBUFFER_FLAGS_UAPI_MASK;
>  }
>  EXPORT_SYMBOL(drm_helper_mode_fill_fb_struct);
>  
> diff --git a/include/drm/drm_framebuffer.h b/include/drm/drm_framebuffer.h
> index 668077009fce..11fa20d21c58 100644
> --- a/include/drm/drm_framebuffer.h
> +++ b/include/drm/drm_framebuffer.h
> @@ -23,6 +23,7 @@
>  #ifndef __DRM_FRAMEBUFFER_H__
>  #define __DRM_FRAMEBUFFER_H__
>  
> +#include <linux/bits.h>
>  #include <linux/ctype.h>
>  #include <linux/list.h>
>  #include <linux/sched.h>
> @@ -100,6 +101,14 @@ struct drm_framebuffer_funcs {
>  		     unsigned num_clips);
>  };
>  
> +#define __DRM_FRAMEBUFFER_FLAGS_BIT_OFFSET 16
> +
> +#define DRM_FRAMEBUFFER_FLAGS_UAPI_MASK \
> +	GENMASK(__DRM_FRAMEBUFFER_FLAGS_BIT_OFFSET - 1, 0)
> +
> +#define DRM_FRAMEBUFFER_HAS_HANDLE_REF(_i) \
> +	BIT((__DRM_FRAMEBUFFER_FLAGS_BIT_OFFSET + (_i)))
> +
>  /**
>   * struct drm_framebuffer - frame buffer object
>   *