IMX Scaler / CSC m2m driver.

[Nicolas Dufresne]

Le mercredi 25 mars 2015 à 14:39 +0000, Ian Molton a écrit :
> On 25/03/15 11:36, Nicolas Dufresne wrote:
> 
> > The thread name does not mean this element is doing the copy. The thread
> > name simply say which element is running a streaming thread.
> 
> I'm not sure I follow that.

44.53%  v4l2video3video  libc-2.19.so         [.] __GI___memcpy_neon

This in no ways means v4l2video3video element is calling memcpy. It only
means the memcpy is being called on the thread created by
v4l2video3video element.

> 
> > The remaining copy happens in ximagesink, because we need to copy to
> > shared X11 server memory. Unless your converter driver can do
> > capture-io-mode=userptr,
> 
> Hm. have I got this the wrong way around? does this mean "capture-to-userspace".
> 
> ie. this would refer to data coming /from/ v4l2videoconvert to userspace?

V4L2_TYPE_CAPTURE matches GST source pad
V4L2_TYPE_OUTPUT matches GST sink pad

> 
> > there is nothing GStreamer can do about this
> > copy. It's X11 design that you get X11 to allocate the memory (even
> > though it's often normal vmalloc memory, and that the server often need
> > to copy again).
> 
> Attempting to use capture-io-mode=userptr on the v4l2videoconvert element
> resulted in a kernel oops due to circular locking.
> 
> commenting out
> 
> //      dst_vq->lock = &ctx->ipu_scaler->dev_mutex;
> 
> in drivers/media/platform/imx/imx-ipu-scaler.c
> 
> prevents the oops, but doesn't help much - gstreamer sits there with a blank
> output window. The problem seems to be:
> 
> On one CPU:
> mm->mmap_sem is locked in vm_mmap_pgoff() followed by [what ammounts to]
> dst_vq->lock in v4l2_m2m_fop_mmap().
> 
> On the other:
> 
> v4l2_ioctl() takes dst_vq->lock, and later, __buf_prepare() takes mm->mm_sem.
> 
> But I'm unclear on how to solve this. My gut feeling is that __buf_prepare()
> is wrong somehow.

In anyways, wrong ML for that part. I believe this driver does not
support that, and I do remember seeing patch that removes the offending
flag that would lead to this kind of weirdness.