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SubjectRe: [RFC PATCH v3 4/6] media: tegra: Add Tegra210 Video input driver
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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;
> +}

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