[PATCH v4 1/4] [media] davinci: vpif_capture: don't lock over s_stream

Laurent Pinchart [off-list ref] writes:

 > Hi Kevin,

On Tuesday 06 Dec 2016 08:49:38 Kevin Hilman wrote:

quoted

Laurent Pinchart writes:

quoted

On Tuesday 29 Nov 2016 15:57:09 Kevin Hilman wrote:

quoted

Video capture subdevs may be over I2C and may sleep during xfer, so we
cannot do IRQ-disabled locking when calling the subdev.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
---
 drivers/media/platform/davinci/vpif_capture.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/media/platform/davinci/vpif_capture.c

b/drivers/media/platform/davinci/vpif_capture.c index
5104cc0ee40e..9f8f41c0f251 100644

--- a/drivers/media/platform/davinci/vpif_capture.c
+++ b/drivers/media/platform/davinci/vpif_capture.c

@@ -193,7 +193,10 @@ static int vpif_start_streaming(struct vb2_queue

*vq, unsigned int count)
 		}
 	}

+	spin_unlock_irqrestore(&common->irqlock, flags);
 	ret = v4l2_subdev_call(ch->sd, video, s_stream, 1);
+	spin_lock_irqsave(&common->irqlock, flags);

I always get anxious when I see a spinlock being released randomly with an
operation in the middle of a protected section. Looking at the code it
looks like the spinlock is abused here. irqlock should only protect the
dma_queue and should thus only be taken around the following code:

spin_lock_irqsave(&common->irqlock, flags);
/* Get the next frame from the buffer queue */
common->cur_frm = common->next_frm = list_entry(common->dma_queue.next,
                            struct vpif_cap_buffer, list);

/* Remove buffer from the buffer queue */
list_del(&common->cur_frm->list);
spin_unlock_irqrestore(&common->irqlock, flags);

Yes, that looks correct.  Will update.

quoted

The code that is currently protected by the lock in the start and stop
streaming functions should be protected by a mutex instead.

I tried taking the mutex here, but lockdep pointed out a deadlock.  I
may not be fully understanding the V4L2 internals here, but it seems
that the ioctl is already taking a mutex, so taking it again in
start/stop streaming is a deadlock.  Unless you think the locking should
be nested here, it seems to me that the mutex isn't needed.

The V4L2 core can lock all ioctls using struct video_device::lock. For buffer-
related ioctls, it can optionally use a separate lock from struct 
vb2_queue::lock. See v4l2_ioctl_get_lock() in drivers/media/v4l2-core/v4l2-
ioctl.c.

The vpif-capture driver sets both the video_device and vb2_queue locks to the 
same lock (which would have the same effect as leaving the vb2_queue lock 
NULL). All ioctls are thus serialized. You would only need to handle locking 
in start_streaming and stop_streaming manually if you didn't rely on the core 
serializing the ioctls.

OK, thanks for clarifying how that works.

Kevin