Re: [PATCH] media: nxp: imx8-isi: add m2m usage_count check in streamoff

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Mon, Sep 23, 2024 at 07:50:18PM +0200, Jacopo Mondi wrote:
> On Fri, Sep 20, 2024 at 05:27:15PM GMT, Laurentiu Palcu wrote:
> > Currently, if streamon/streamoff calls are imbalanced we can end up
> > with a negative ISI m2m usage_count. When that happens, the next
> > streamon call will not enable the ISI m2m channel.
> >
> > So, instead of throwing a warning in streamoff when usage_count drops
> > below 0, just make sure we don't get there.
> 
> Isn't the whole purpose of the WARN() to highlight something's wrong
> with userspace ? I think it's expected to have the same number of
> streamon and streamoff calls, do you have any idea why it might not be
> happening ?

WARN() is much too strong to report userspace problems.

> > Fixes: cf21f328fcafac ("media: nxp: Add i.MX8 ISI driver")
> > Signed-off-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
> > ---
> >  drivers/media/platform/nxp/imx8-isi/imx8-isi-m2m.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/media/platform/nxp/imx8-isi/imx8-isi-m2m.c b/drivers/media/platform/nxp/imx8-isi/imx8-isi-m2m.c
> > index 9745d6219a166..b71195a3ba256 100644
> > --- a/drivers/media/platform/nxp/imx8-isi/imx8-isi-m2m.c
> > +++ b/drivers/media/platform/nxp/imx8-isi/imx8-isi-m2m.c
> > @@ -575,6 +575,9 @@ static int mxc_isi_m2m_streamoff(struct file *file, void *fh,
> >
> >  	mutex_lock(&m2m->lock);
> >
> > +	if (m2m->usage_count == 0)
> > +		goto unlock;

This isn't right. Userspace can still call VIDIOC_STREAMOFF while not
streaming, which will decrement the usage count when it shouldn't. The
right fix is to return from mxc_isi_m2m_streamoff() without decrementing
the usage count if the particular context the function is called for is
not streaming. You will possibly need to also modify
mxc_isi_m2m_streamon() to make sure the two functions won't race each
other.

Could you work on an improved patch ? Please let me know if you need
help, it's important to fix this issue.

> > +
> >  	/*
> >  	 * If the last context is this one, reset it to make sure the device
> >  	 * will be reconfigured when streaming is restarted.
> > @@ -594,8 +597,7 @@ static int mxc_isi_m2m_streamoff(struct file *file, void *fh,
> >  		mxc_isi_channel_release(m2m->pipe);
> >  	}
> >
> > -	WARN_ON(m2m->usage_count < 0);
> > -
> > +unlock:
> >  	mutex_unlock(&m2m->lock);
> >
> >  	return 0;

-- 
Regards,

Laurent Pinchart