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

[dhar]

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