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

[Andrzej Hajda]

On 09/12/2014 10:57 AM, Daniel Vetter wrote:
> 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 at 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.

I agree with that.

>
> 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 ;-)

Hmm, maybe I miss something but I do not see the core grabbing fb reference
on plane->fb update. On the other side drm_framebuffer_remove calls
drm_plane_force_disable which drops plane->fb reference.
I am not yet familiar with this code so maybe there is better solution.

If not I guess it would be better to move this code to
exynos_plane_mode_set.
At least it is done this way in omap and msm, in fact it seems better place
for such things. What do you think?

Regards
Andrzej

> -Daniel