Re: [PATCH 5/9] drm/exynos/crtc: fix framebuffer reference sequence

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Sep 12, 2014 at 05:34:50PM +0900, Inki Dae wrote:
> Hi Andrzej,
> 
> On 2014년 09월 09일 22:16, Andrzej Hajda wrote:
> > Adding reference to framebuffer should be accompanied with removing
> > reference to old framebuffer assigned to the plane.
> > This patch removes following warning:
> > 
> > [   95.038017] WARNING: CPU: 1 PID: 3067 at drivers/gpu/drm/drm_crtc.c:5115 drm_mode_config_cleanup+0x258/0x268()
> > [   95.048086] Modules linked in:
> > [   95.051430] CPU: 1 PID: 3067 Comm: bash Tainted: G        W      3.16.0-11355-g7a6eca5-dirty #3015
> > [   95.060058] [<c0013e90>] (unwind_backtrace) from [<c0011128>] (show_stack+0x10/0x14)
> > [   95.067766] [<c0011128>] (show_stack) from [<c04a5dc4>] (dump_stack+0x70/0xbc)
> > [   95.074953] [<c04a5dc4>] (dump_stack) from [<c0021784>] (warn_slowpath_common+0x64/0x88)
> > [   95.083005] [<c0021784>] (warn_slowpath_common) from [<c00217c4>] (warn_slowpath_null+0x1c/0x24)
> > [   95.091780] [<c00217c4>] (warn_slowpath_null) from [<c0275fa0>] (drm_mode_config_cleanup+0x258/0x268)
> > [   95.100983] [<c0275fa0>] (drm_mode_config_cleanup) from [<c0281724>] (exynos_drm_unload+0x38/0x50)
> > [   95.109915] [<c0281724>] (exynos_drm_unload) from [<c026e248>] (drm_dev_unregister+0x24/0x98)
> > [   95.118414] [<c026e248>] (drm_dev_unregister) from [<c026ecb0>] (drm_put_dev+0x28/0x64)
> > [   95.126412] [<c026ecb0>] (drm_put_dev) from [<c02947c4>] (take_down_master+0x24/0x44)
> > [   95.134218] [<c02947c4>] (take_down_master) from [<c0294870>] (component_del+0x8c/0xc8)
> > [   95.142201] [<c0294870>] (component_del) from [<c0286c10>] (exynos_dsi_remove+0x18/0x2c)
> > [   95.150294] [<c0286c10>] (exynos_dsi_remove) from [<c0299e04>] (platform_drv_remove+0x18/0x1c)
> > [   95.158872] [<c0299e04>] (platform_drv_remove) from [<c02987a4>] (__device_release_driver+0x70/0xc4)
> > [   95.167981] [<c02987a4>] (__device_release_driver) from [<c0298818>] (device_release_driver+0x20/0x2c)
> > [   95.177268] [<c0298818>] (device_release_driver) from [<c02979d0>] (unbind_store+0x5c/0x94)
> > [   95.185597] [<c02979d0>] (unbind_store) from [<c0297278>] (drv_attr_store+0x20/0x2c)
> > [   95.193323] [<c0297278>] (drv_attr_store) from [<c012aa90>] (sysfs_kf_write+0x4c/0x50)
> > [   95.201224] [<c012aa90>] (sysfs_kf_write) from [<c0129e94>] (kernfs_fop_write+0xc4/0x184)
> > [   95.209393] [<c0129e94>] (kernfs_fop_write) from [<c00d03ec>] (vfs_write+0xa0/0x1a8)
> > [   95.217111] [<c00d03ec>] (vfs_write) from [<c00d07f8>] (SyS_write+0x40/0x8c)
> > [   95.224146] [<c00d07f8>] (SyS_write) from [<c000e660>] (ret_fast_syscall+0x0/0x48)
> > 
> > Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
> > ---
> >  drivers/gpu/drm/exynos/exynos_drm_crtc.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/exynos/exynos_drm_crtc.c b/drivers/gpu/drm/exynos/exynos_drm_crtc.c
> > index b68e58f..bde19f4 100644
> > --- a/drivers/gpu/drm/exynos/exynos_drm_crtc.c
> > +++ b/drivers/gpu/drm/exynos/exynos_drm_crtc.c
> > @@ -145,10 +145,16 @@ exynos_drm_crtc_mode_set(struct drm_crtc *crtc, struct drm_display_mode *mode,
> >  	if (ret)
> >  		return ret;
> >  
> > +	/* we need to unreference current fb after replacing it with new one */
> > +	old_fb = plane->fb;
> > +
> >  	plane->crtc = crtc;
> >  	plane->fb = crtc->primary->fb;
> >  	drm_framebuffer_reference(plane->fb);
> >  
> > +	if (old_fb)
> > +		drm_framebuffer_unreference(old_fb);
> 
> This time would be a good chance that we can consider drm flip queue to
> make sure that whole memory region to old_fb is scanned out completely
> before dropping a reference of old_fb. the reference of old_fb should be
> dropped at irq handler of each crtc devices, fimd and mixer.

Generally it's not a good idea to drop fb references from irq context,
since if you actually drop the last reference it'll blow up: fb cleanup
needs a bunch of mutexes.

Also the drm core really should be taking care of this for you, you only
need to grab references yourself for async flips if you want the buffer to
survive a bit. crtc_mode_set has not need for this. I expect that the
refcounting bug is somewhere else, at least from my experience chasing
such issues in i915 ;-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch