Hi Kevin, On Tuesday 06 Dec 2016 08:49:38 Kevin Hilman wrote: > Laurent Pinchart writes: > > On Tuesday 29 Nov 2016 15:57:09 Kevin Hilman wrote: > >> 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@xxxxxxxxxxxx> > >> --- > >> 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. > > > 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. -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
