Commit 8cf1e981 authored by Thierry Reding's avatar Thierry Reding
Browse files

drm: Add consistency check for page-flipping 



Driver implementations of the drm_crtc's .page_flip() function are
required to update the crtc->fb field on success to reflect that the new
framebuffer is now in use. This is important to keep reference counting
on the framebuffers balanced.

While at it, document this requirement to keep others from falling into
the same trap.

Suggested-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: default avatarThierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
parent c976cb37

Documentation/DocBook/drm.tmpl

+6 −0
Original line number Diff line number Diff line
@@ -1160,6 +1160,12 @@ int max_width, max_height;</synopsis> 
            without waiting for rendering or page flip to complete and must block

            any new rendering to the frame buffer until the page flip completes.

          </para>

          <para>

            If a page flip can be successfully scheduled the driver must set the

            <code>drm_crtc-&lt;fb</code> field to the new framebuffer pointed to

            by <code>fb</code>. This is important so that the reference counting

            on framebuffers stays balanced.

          </para>

          <para>

            If a page flip is already pending, the

            <methodname>page_flip</methodname> operation must return

drivers/gpu/drm/drm_crtc.c

+7 −0
Original line number Diff line number Diff line
@@ -3792,6 +3792,13 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev, 
		/* Keep the old fb, don't unref it. */

		old_fb = NULL;

	} else {

		/*

		 * Warn if the driver hasn't properly updated the crtc->fb

		 * field to reflect that the new framebuffer is now used.

		 * Failing to do so will screw with the reference counting

		 * on framebuffers.

		 */

		WARN_ON(crtc->fb != fb);

		/* Unref only the old framebuffer. */

		fb = NULL;

	}

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