Re: [PATCH] drm/fbdev: Update legacy plane->fb refcounting for atomic restore

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Hey,

Op 22-09-15 om 11:55 schreef Daniel Vetter:
> From: Matt Roper <matthew.d.roper@intel.com>
>
> Starting with commit
>
>         commit 28cc504e8d52248962f5b485bdc65f539e3fe21d
>         Author: Rob Clark <robdclark@gmail.com>
>         Date:   Tue Aug 25 15:36:00 2015 -0400
>
>             drm/i915: enable atomic fb-helper
>
> I've been seeing some panics on i915 when the DRM master shuts down that appear
> to be caused by using an already-freed framebuffer (i.e., we're unexpectedly
> dropping our initial FB's reference count to 0 and freeing it, which causes a
> crash when we try to restore it later).  Digging deeper, the state FB
> refcounting is working as expected, but we seem to be missing proper
> refcounting on the legacy plane->fb pointers in the new atomic fbdev code.
>
> Tracking plane->old_fb and then doing a ref/unref at the end of the
> fbdev restore like we do in the legacy ioctl's ensures we don't miscount
> references on plane->fb and avoids the panics.
>
> v2 from Daniel:
>
> Really do what the atomic ioctl does:
> - Also update plane->fb and plane->crtc.
> - Clear out plane->old_fb on failures too.
>
> Cc: Rob Clark <robdclark@gmail.com>
> Cc: intel-gfx@lists.freedesktop.org
> Signed-off-by: Matt Roper <matthew.d.roper@intel.com> (v1)
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)

Looks sane, but I see a lot of this boilerplate for the plane->fb updates, and we often get it wrong. Like for example in drm_mode_atomic_ioctl.

Could all the plane->fb and old_fb updates be done by drm_atomic_commit or async_commit instead?

> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 64fc5ca8fda2..1b564a69f561 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -352,6 +352,8 @@ retry:
>  	drm_for_each_plane(plane, dev) {
>  		struct drm_plane_state *plane_state;
>  
> +		plane->old_fb = plane->fb;
> +
>  		plane_state = drm_atomic_get_plane_state(state, plane);
>  		if (IS_ERR(plane_state)) {
>  			ret = PTR_ERR(plane_state);
> @@ -382,6 +384,20 @@ retry:
>  	}
>  
>  	ret = drm_atomic_commit(state);
> +
> +	drm_for_each_plane(plane, dev) {
> +		if (ret == 0) {
> +			struct drm_framebuffer *new_fb = plane->state->fb;
> +			if (new_fb)
> +				drm_framebuffer_reference(new_fb);
> +			plane->fb = new_fb;
> +			plane->crtc = plane->state->crtc;
> +
> +			if (plane->old_fb)
> +				drm_framebuffer_unreference(plane->old_fb);
> +		}
> +	}
> +
>  	if (ret != 0)
>  		goto fail;
>  

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