On Wed, Feb 13, 2013 at 05:05:00PM +0100, Thierry Reding wrote:
> 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@xxxxxxxx>
> Signed-off-by: Thierry Reding <thierry.reding@xxxxxxxxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> Documentation/DocBook/drm.tmpl | 6 ++++++
> drivers/gpu/drm/drm_crtc.c | 7 +++++++
> 2 files changed, 13 insertions(+)
>
> diff --git a/Documentation/DocBook/drm.tmpl b/Documentation/DocBook/drm.tmpl
> index fae8018..8dfaeb0 100644
> --- a/Documentation/DocBook/drm.tmpl
> +++ b/Documentation/DocBook/drm.tmpl
> @@ -1161,6 +1161,12 @@ int max_width, max_height;</synopsis>
> 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-<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
> -<errorname>EBUSY</errorname>.
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index 838c9b6..d86edc1 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -3766,6 +3766,13 @@ 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;
> }
> --
> 1.8.1.2
>
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--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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