linux-kernel - Re: [PATCH] media: nxp: imx8-isi: add m2m usage

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Message-ID: <o5pvycxgjqop4636tf6u4g64uhqyro4lkmspv5f6rxlnvsg24t@zr22tgsh62yw>
Date: Tue, 24 Sep 2024 10:41:07 +0300
From: Laurentiu Palcu <laurentiu.palcu@....nxp.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Jacopo Mondi <jacopo.mondi@...asonboard.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Shawn Guo <shawnguo@...nel.org>, 
	Sascha Hauer <s.hauer@...gutronix.de>, Pengutronix Kernel Team <kernel@...gutronix.de>, 
	Fabio Estevam <festevam@...il.com>, Guoniu Zhou <guoniu.zhou@....com>, 
	Jacopo Mondi <jacopo@...ndi.org>, Christian Hemp <c.hemp@...tec.de>, 
	Stefan Riedmueller <s.riedmueller@...tec.de>, linux-media@...r.kernel.org, imx@...ts.linux.dev, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: nxp: imx8-isi: add m2m usage_count check in
 streamoff

Hi Laurent,

On Tue, Sep 24, 2024 at 12:52:46AM +0300, Laurent Pinchart wrote:
> 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@....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.

Hmm, I didn't consider that... :/

> 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 ?

I'll give it a shot.

> Please let me know if you need help, it's important to fix this issue.

Thanks, I'll ping you on IRC if I get stuck.

Laurentiu

> 
> > > +
> > >  	/*
> > >  	 * 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