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

[Inki Dae]

On 2014ë 09ì 12ì 17:57, Daniel Vetter wrote:
> On Fri, Sep 12, 2014 at 05:34:50PM +0900, Inki Dae wrote:
>> Hi Andrzej,
>>
>> On 2014ë 09ì 09ì 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@xxxxxxxxxxx>
>>> ---
>>>  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.

Actually, drm_flip_work_commit function will be called at irq context so
the reference will be dropped by work queue handler so mutex lock would
be required before dropping the reference. My concern was that gem
memory may be freed before the memory region is scanned out completely
so I thought we need to make sure to scan out completely somehow.

> 
> 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 ;-)

Thanks for comments. Yes, there may be refcounting bug somewhere else if
drm core makes sure to take care of this. It seems to need more checking.

Thanks,
Inki Dae

> -Daniel
> 

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