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

[ming qian]

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.

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;
> >   }