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:

quoted

On Fri, Sep 20, 2024 at 05:27:15PM GMT, Laurentiu Palcu wrote:

quoted

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.

quoted

quoted

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.

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

quoted

quoted

+
 	/*
 	 * 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