Re: drm/msm/dpu: Correct dpu encoder spinlock initialization

[Jeykumar Sankaran <jsanka@codeaurora.org>]

On 2019-06-24 22:44, dhar@codeaurora.org wrote:
> On 2019-06-25 03:56, Jeykumar Sankaran wrote:
>> On 2019-06-23 23:27, Shubhashree Dhar wrote:
>>> dpu encoder spinlock should be initialized during dpu encoder
>>> init instead of dpu encoder setup which is part of commit.
>>> There are chances that vblank control uses the uninitialized
>>> spinlock if not initialized during encoder init.
>> Not much can be done if someone is performing a vblank operation
>> before encoder_setup is done.
>> Can you point to the path where this lock is acquired before
>> the encoder_setup?
>> 
>> Thanks
>> Jeykumar S.
>>> 
> 
> When running some dp usecase, we are hitting this callstack.
> 
> Process kworker/u16:8 (pid: 215, stack limit = 0x00000000df9dd930)
> Call trace:
>  spin_dump+0x84/0x8c
>  spin_dump+0x0/0x8c
>  do_raw_spin_lock+0x80/0xb0
>  _raw_spin_lock_irqsave+0x34/0x44
>  dpu_encoder_toggle_vblank_for_crtc+0x8c/0xe8
>  dpu_crtc_vblank+0x168/0x1a0
>  dpu_kms_enable_vblank+0[   11.648998]  vblank_ctrl_worker+0x3c/0x60
>  process_one_work+0x16c/0x2d8
>  worker_thread+0x1d8/0x2b0
>  kthread+0x124/0x134
> 
> Looks like vblank is getting enabled earlier causing this issue and we
> are using the spinlock without initializing it.
> 
> Thanks,
> Shubhashree
> 
DP calls into set_encoder_mode during hotplug before even notifying the
u/s. Can you trace out the original caller of this stack?

Even though the patch is harmless, I am not entirely convinced to move 
this
initialization. Any call which acquires the lock before encoder_setup
will be a no-op since there will not be any physical encoder to work 
with.

Thanks and Regards,
Jeykumar S.

>>> Change-Id: I5a18b95fa47397c834a266b22abf33a517b03a4e
>>> Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>
>>> ---
>>>  drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c | 3 +--
>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>> 
>>> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>> b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>> index 5f085b5..22938c7 100644
>>> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>> @@ -2195,8 +2195,6 @@ int dpu_encoder_setup(struct drm_device *dev, 
>>> struct
>>> drm_encoder *enc,
>>>  	if (ret)
>>>  		goto fail;
>>> 
>>> -	spin_lock_init(&dpu_enc->enc_spinlock);
>>> -
>>>  	atomic_set(&dpu_enc->frame_done_timeout, 0);
>>>  	timer_setup(&dpu_enc->frame_done_timer,
>>>  			dpu_encoder_frame_done_timeout, 0);
>>> @@ -2250,6 +2248,7 @@ struct drm_encoder *dpu_encoder_init(struct
>>> drm_device *dev,
>>> 
>>>  	drm_encoder_helper_add(&dpu_enc->base, &dpu_encoder_helper_funcs);
>>> 
>>> +	spin_lock_init(&dpu_enc->enc_spinlock);
>>>  	dpu_enc->enabled = false;
>>> 
>>>  	return &dpu_enc->base;

-- 
Jeykumar S