Re: [RFC PATCH v3 4/6] media: tegra: Add Tegra210 Video input driver

From: Sowjanya Komatineni
Date: Sun Feb 16 2020 - 15:12:07 EST

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

On 2/16/20 3:03 AM, Hans Verkuil wrote:
External email: Use caution opening links or attachments

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@xxxxxxxxxx>
 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


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



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.

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.

+ÂÂÂÂ return 0;