linux-kernel - Re: drm/msm/dpu: Correct dpu encoder spinlock initialization

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Date:   Mon, 01 Jul 2019 15:59:13 +0530
From:   dhar@...eaurora.org
To:     Jeykumar Sankaran <jsanka@...eaurora.org>
Cc:     dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        freedreno@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, robdclark@...il.com,
        seanpaul@...omium.org, hoegsberg@...omium.org,
        abhinavk@...eaurora.org, chandanu@...eaurora.org,
        nganji@...eaurora.org, jshekhar@...eaurora.org
Subject: Re: drm/msm/dpu: Correct dpu encoder spinlock initialization

On 2019-06-26 03:10, Jeykumar Sankaran wrote:
> On 2019-06-24 22:44, dhar@...eaurora.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@...eaurora.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;

In dpu_crtc_vblank(), we are looping through all the encoders in the 
present mode_config: 
https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c#L1082
and hence calling dpu_encoder_toggle_vblank_for_crtc() for all the 
encoders. But in dpu_encoder_toggle_vblank_for_crtc(), after acquiring 
the spinlock, we will do a early return for
the encoders which are not currently assigned to our crtc: 
https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c#L1318.
Since the encoder_setup for the secondary encoder(dp encoder in this 
case) is not called until dp hotplug, we are hitting kernel panic while 
acquiring the lock.