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Commit b62584e3 authored by Daniel Vetter's avatar Daniel Vetter
Browse files

drm: optimize drm_framebuffer_remove 



Now that all framebuffer usage is properly refcounted, we are no
longer required to hold the modeset locks while dropping the last
reference. Hence implemented a fastpath which avoids the potential
stalls associated with grabbing mode_config.lock for the case where
there's no other reference around.

Explain in a big comment why it is safe. Also update kerneldocs with
the new locking rules around drm_framebuffer_remove.

Reviewed-by: default avatarRob Clark <rob@ti.com>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
parent 2fd5eaba

drivers/gpu/drm/drm_crtc.c

+45 −27
Original line number Diff line number Diff line
@@ -517,7 +517,11 @@ EXPORT_SYMBOL(drm_framebuffer_cleanup); 
 *

 * Scans all the CRTCs and planes in @dev's mode_config.  If they're

 * using @fb, removes it, setting it to NULL. Then drops the reference to the

 * passed-in framebuffer.

 * passed-in framebuffer. Might take the modeset locks.

 *

 * Note that this function optimizes the cleanup away if the caller holds the

 * last reference to the framebuffer. It is also guaranteed to not take the

 * modeset locks in this case.

 */

void drm_framebuffer_remove(struct drm_framebuffer *fb)

{
@@ -527,9 +531,25 @@ void drm_framebuffer_remove(struct drm_framebuffer *fb) 
	struct drm_mode_set set;

	int ret;



	WARN_ON(!drm_modeset_is_locked(dev));

	WARN_ON(!list_empty(&fb->filp_head));



	/*

	 * drm ABI mandates that we remove any deleted framebuffers from active

	 * useage. But since most sane clients only remove framebuffers they no

	 * longer need, try to optimize this away.

	 *

	 * Since we're holding a reference ourselves, observing a refcount of 1

	 * means that we're the last holder and can skip it. Also, the refcount

	 * can never increase from 1 again, so we don't need any barriers or

	 * locks.

	 *

	 * Note that userspace could try to race with use and instate a new

	 * usage _after_ we've cleared all current ones. End result will be an

	 * in-use fb with fb-id == 0. Userspace is allowed to shoot its own foot

	 * in this manner.

	 */

	if (atomic_read(&fb->refcount.refcount) > 1) {

		drm_modeset_lock_all(dev);

		/* remove from any CRTC */

		list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {

			if (crtc->fb == fb) {
@@ -555,6 +575,8 @@ void drm_framebuffer_remove(struct drm_framebuffer *fb) 
				plane->crtc = NULL;

			}

		}

		drm_modeset_unlock_all(dev);

	}



	drm_framebuffer_unreference(fb);

}
@@ -2538,9 +2560,7 @@ int drm_mode_rmfb(struct drm_device *dev, 
	mutex_unlock(&dev->mode_config.fb_lock);

	mutex_unlock(&file_priv->fbs_lock);



	drm_modeset_lock_all(dev);

	drm_framebuffer_remove(fb);

	drm_modeset_unlock_all(dev);



	return 0;
@@ -2691,9 +2711,7 @@ void drm_fb_release(struct drm_file *priv) 
		list_del_init(&fb->filp_head);



		/* This will also drop the fpriv->fbs reference. */

		drm_modeset_lock_all(dev);

		drm_framebuffer_remove(fb);

		drm_modeset_unlock_all(dev);

	}

	mutex_unlock(&priv->fbs_lock);

}