Re: [PATCH v2 3/3] media: amphion: Remove lock in s_ctrl callback

[ming qian <ming.qian@xxxxxxxxxxx>]

Hi Andrzej,

> Hi Ming,
>
> W dniu 30.04.2024 o 04:32, ming qian pisze:
> > Hi Andrzej,
> >
> > > Hi Ming Qian,
> > >
> > > W dniu 25.04.2024 o 08:50, Ming Qian pisze:
> > > > There is no need to add lock in s_ctrl callback, it has been
> > > > synchronized by the ctrl_handler's lock, otherwise it may led to
> > > > deadlock if driver call v4l2_ctrl_s_ctrl().
> > > >
> > > > Signed-off-by: Ming Qian <ming.qian@xxxxxxx>
> > > > ---
> > > >   drivers/media/platform/amphion/vdec.c | 2 --
> > > >   drivers/media/platform/amphion/venc.c | 2 --
> > > >   2 files changed, 4 deletions(-)
> > > >
> > > > diff --git a/drivers/media/platform/amphion/vdec.c 
> > > > b/drivers/media/platform/amphion/vdec.c
> > > > index a57f9f4f3b87..6a38a0fa0e2d 100644
> > > > --- a/drivers/media/platform/amphion/vdec.c
> > > > +++ b/drivers/media/platform/amphion/vdec.c
> > > > @@ -195,7 +195,6 @@ static int vdec_op_s_ctrl(struct v4l2_ctrl *ctrl)
> > > >       struct vdec_t *vdec = inst->priv;
> > > >       int ret = 0;
> > > > -    vpu_inst_lock(inst);
> > >
> > > I assume that PATCH v2 2/3 might cause the said deadlock to happen?
> > > If so, maybe it would make more sense to make the current patch preceed
> > >   PATCH v2 2/3? Otherwise the kernel at PATCH v2 2/3 introduces a 
> > > potential
> > > deadlock.
> > >
> > > Regards,
> > >
> > > Andrzej
> >
> > I actually discovered this problem when I was preparing the v2 2/3 patch.
> >
> > But in the v2 2/3 patch, it tried to add a read-only ctrl, then I just
> > unset the s_ctrl callback for the new added ctrl, the potential deadlock
> > is caused by call the s_ctrl back in a locked environment, so after unset
> > the s_ctrl callback, the 2/3 patch won't trigger the deadlock even if
> > this patch is missing.
> >
> > In order to avoid encountering similar problems in the future, that
> > driver may set or get some ctrl, I added this patch.
>
> Do I understand you correctly that patch 2/3 is written in such a way that
> it does not introduce a deadlock, and you add patch 3/3 only to prevent 
> future
> problems? If so, it seems to me that patch 3/3 could/should be separate 
> from
> this series, as it does not quite match "Add average qp control".
>
> Regards,
>
> Andrzej

You're right, when I prepare the 2/3 patch, I add the new ctrl just like
the previous ctrl, but I got a deadlock. then I tried to fix a deadlock
in 2/3 patch first, and make this patch again to avoid similar errors.

I put this patch to this series, as I think this better explains why I
made this change.

Regards,
Ming

> > Best regards,
> > Ming
> >
> > > >       switch (ctrl->id) {
> > > >       case V4L2_CID_MPEG_VIDEO_DEC_DISPLAY_DELAY_ENABLE:
> > > >           vdec->params.display_delay_enable = ctrl->val;
> > > > @@ -207,7 +206,6 @@ static int vdec_op_s_ctrl(struct v4l2_ctrl *ctrl)
> > > >           ret = -EINVAL;
> > > >           break;
> > > >       }
> > > > -    vpu_inst_unlock(inst);
> > > >       return ret;
> > > >   }
> > > > diff --git a/drivers/media/platform/amphion/venc.c 
> > > > b/drivers/media/platform/amphion/venc.c
> > > > index cdfaba9d107b..351b4edc8742 100644
> > > > --- a/drivers/media/platform/amphion/venc.c
> > > > +++ b/drivers/media/platform/amphion/venc.c
> > > > @@ -518,7 +518,6 @@ static int venc_op_s_ctrl(struct v4l2_ctrl *ctrl)
> > > >       struct venc_t *venc = inst->priv;
> > > >       int ret = 0;
> > > > -    vpu_inst_lock(inst);
> > > >       switch (ctrl->id) {
> > > >       case V4L2_CID_MPEG_VIDEO_H264_PROFILE:
> > > >           venc->params.profile = ctrl->val;
> > > > @@ -579,7 +578,6 @@ static int venc_op_s_ctrl(struct v4l2_ctrl *ctrl)
> > > >           ret = -EINVAL;
> > > >           break;
> > > >       }
> > > > -    vpu_inst_unlock(inst);
> > > >       return ret;
> > > >   }