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

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>

+/*
+ * 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

+
+	return 0;
+}