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drm: Add consistency check for page-flipping

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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: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
This commit is contained in:
Thierry Reding 2013-02-13 16:08:33 +01:00
parent c976cb37a9
commit 8cf1e98114
2 changed files with 13 additions and 0 deletions
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6
Documentation/DocBook/drm.tmpl
View file

@ -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

7
drivers/gpu/drm/drm_crtc.c
View file

@ -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;
}