On 2/16/20 9:22 PM, Sowjanya Komatineni wrote:

On 2/16/20 12:11 PM, Sowjanya Komatineni wrote:

quoted

On 2/16/20 11:54 AM, Sowjanya Komatineni wrote:

quoted

On 2/16/20 3:03 AM, Hans Verkuil wrote:

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On 2/14/20 7:23 PM, Sowjanya Komatineni wrote:

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Tegra210 contains a powerful Video Input (VI) hardware controller
which can support up to 6 MIPI CSI camera sensors.

Each Tegra CSI port can be one-to-one mapped to VI channel and can
capture from an external camera sensor connected to CSI or from
built-in test pattern generator.

Tegra210 supports built-in test pattern generator from CSI to VI.

This patch adds a V4L2 media controller and capture driver support
for Tegra210 built-in CSI to VI test pattern generator.

Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
---
  drivers/staging/media/Kconfig              |    2 +
  drivers/staging/media/Makefile             |    1 +
  drivers/staging/media/tegra/Kconfig        |   10 +
  drivers/staging/media/tegra/Makefile       |    8 +
  drivers/staging/media/tegra/TODO           |   10 +
  drivers/staging/media/tegra/tegra-common.h |  239 +++++++
  drivers/staging/media/tegra/tegra-csi.c    |  374 ++++++++++
  drivers/staging/media/tegra/tegra-csi.h    |  115 ++++
  drivers/staging/media/tegra/tegra-vi.c     | 1019 
++++++++++++++++++++++++++++
  drivers/staging/media/tegra/tegra-vi.h     |   79 +++
  drivers/staging/media/tegra/tegra-video.c  |  118 ++++
  drivers/staging/media/tegra/tegra-video.h  |   32 +
  drivers/staging/media/tegra/tegra210.c     |  767 
+++++++++++++++++++++
  drivers/staging/media/tegra/tegra210.h     |  190 ++++++
  14 files changed, 2964 insertions(+)
  create mode 100644 drivers/staging/media/tegra/Kconfig
  create mode 100644 drivers/staging/media/tegra/Makefile
  create mode 100644 drivers/staging/media/tegra/TODO
  create mode 100644 drivers/staging/media/tegra/tegra-common.h
  create mode 100644 drivers/staging/media/tegra/tegra-csi.c
  create mode 100644 drivers/staging/media/tegra/tegra-csi.h
  create mode 100644 drivers/staging/media/tegra/tegra-vi.c
  create mode 100644 drivers/staging/media/tegra/tegra-vi.h
  create mode 100644 drivers/staging/media/tegra/tegra-video.c
  create mode 100644 drivers/staging/media/tegra/tegra-video.h
  create mode 100644 drivers/staging/media/tegra/tegra210.c
  create mode 100644 drivers/staging/media/tegra/tegra210.h

<snip>

quoted

+/*
+ * videobuf2 queue operations
+ */
+static int tegra_channel_queue_setup(struct vb2_queue *vq,
+                                  unsigned int *nbuffers,
+                                  unsigned int *nplanes,
+                                  unsigned int sizes[],
+                                  struct device *alloc_devs[])
+{
+     struct tegra_vi_channel *chan = vb2_get_drv_priv(vq);
+
+     if (*nplanes)
+             return sizes[0] < chan->format.sizeimage ? -EINVAL : 0;
+
+     *nplanes = 1;
+     sizes[0] = chan->format.sizeimage;
+     alloc_devs[0] = chan->vi->dev;
+
+     /*
+      * allocate min 3 buffers in queue to avoid race between DMA
+      * writes and userspace reads.
+      */
+     if (*nbuffers < 3)
+             *nbuffers = 3;

First of all, don't check this here, instead set the struct 
vb2_queue field
'min_buffers_needed' to 3 instead.

But the reason given for this check is peculiar: there should not be 
any
race at all. Usually the reason for requiring a specific minimum 
number of
buffers is that the DMA engine needs at least 2 buffers before it 
can start
streaming: it can't give back a buffer to userspace (vb2_buffer_done())
unless there is a second buffer it can start to capture to next. So 
for many
DMA implementations you need a minimum of 2 buffers: two buffers for 
the
DMA engine, one buffer being processed by userspace.

If the driver is starved of buffers it will typically keep capturing to
the last buffer until a new buffer is queued.

In any case, once the driver releases a buffer via vb2_buffer_done() 
the
buffer memory is no longer owned by the driver.

To be precise, buffer ownership is as follows:

userspace -> VIDIOC_QBUF -> vb2 -> buf_queue -> driver -> 
vb2_buffer_done() -> vb2 -> VIDIOC_DQBUF -> userspace

(vb2 == videobuf2 framework)

Note that vb2 never touches the buffer memory.

So if you get a race condition in this driver, then there is something
strange going on. It looks like vb2_buffer_done() is called while 
DMA is
still ongoing, or because the driver really needs to keep one buffer
available at all times.

Regards,

         Hans

Thanks Hans.

On running v4l2-compliance streaming tests for longer run, I noticed 
kernel reporting unable to write to read-only memory and with debugs 
I observed when this error was reported, I see 2 buffers queued and 
both using same address.

for first buffer capture start thread initiates capture and wakes 
done thread to wait for memory write ack and once its done buffer is 
released to user space but I see upon buffer released to user space 
immediate next buffer capture single shot gets issued (as soon as 
single shot is issued frame capture data is written to memory by DMA) 
and I see this kernel error of unable to write to read-only memory.

This error happens rare and happens on long run and all the times of 
repro's, I see when other thread releases buffer immediate I see 
single shot gets issued as 2 buffers are queued up at the same time 
with same DMA address.

Just to be clear, I meant all the times when kernel reports error 
unable to write to read-only memory, I see 2 buffers gets queued and 
as the capture start thread and done thread are parallel and when 
capture thread wakes done thread on receiving FS event, done thread 
for waiting for memory write happens parallel to next frame capture 
and I see while vb2_buffer_done happens in done thread next frame 
single shot has been issues by capture start thread in parallel when 
it hits this error.

For low latency, we use 2 threads one thread for capture and wait for FS 
and on receiving FS even wakes other done thread to wait for memory 
write to finish.

While other done thread waits for memory write to finish, capture thread 
can start capture for next frame and as soon as single shot is issued 
capture frame is written to memory and as this thread runs in parallel 
to done thread

there is a possibility vb2_buffer_done being called by 
kthread_capture_done while DMA is ongoing by kthread_capture_start and I 
observed same DMA address being used got both buffers that got queued at 
same time when it hits this error.

"buffers that got queued": you mean that tegra_channel_buffer_queue() is
called twice with different buffers (i.e. with different buffer index values)
but with the same DMA address?

That should not happen (unless the first buffer was returned with
vb2_buffer_done() before the second buffer was queued).

Can you provide more details? E.g. buffer index, memory model used when
streaming, total number of buffers allocated by REQBUFS.

I would like to fully understand this. Just increasing the minimum number
of buffers, while reasonable by itself, *does* feel like it is just
hiding the symptoms.

Regards,

	Hans

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With using minimum 3 buffers, this issue doesnt happen at all from 
almost 72 hours of testing.


Will try with setting vb2 queue field min_buffers_needed as 3 instead 
of adding check in queue setup.

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+
+     return 0;
+}