Linux-Devicetree Archive on lore.kernel.org
 help / color / Atom feed
From: Hans Verkuil <hverkuil@xs4all.nl>
To: Sowjanya Komatineni <skomatineni@nvidia.com>,
	thierry.reding@gmail.com, jonathanh@nvidia.com,
	frankc@nvidia.com, helen.koike@collabora.com, sboyd@kernel.org
Cc: linux-media@vger.kernel.org, devicetree@vger.kernel.org,
	linux-clk@vger.kernel.org, linux-tegra@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v3 4/6] media: tegra: Add Tegra210 Video input driver
Date: Sun, 16 Feb 2020 12:03:42 +0100
Message-ID: <30e417ba-84e1-63d2-de74-22cfe859bddb@xs4all.nl> (raw)
In-Reply-To: <1581704608-31219-5-git-send-email-skomatineni@nvidia.com>

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;
> +}

  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-14 18:23 [RFC PATCH v3 0/6] Add Tegra driver for video capture Sowjanya Komatineni
2020-02-14 18:23 ` [RFC PATCH v3 1/6] dt-bindings: clock: tegra: Add clk id for CSI TPG clock Sowjanya Komatineni
2020-02-14 18:23 ` [RFC PATCH v3 2/6] clk: tegra: Add Tegra210 CSI TPG clock gate Sowjanya Komatineni
2020-02-14 18:23 ` [RFC PATCH v3 3/6] dt-binding: tegra: Add VI and CSI bindings Sowjanya Komatineni
2020-02-18 23:15   ` Rob Herring
2020-02-19  3:28     ` Sowjanya Komatineni
2020-02-20 19:45       ` Rob Herring
2020-02-20 20:39         ` Sowjanya Komatineni
2020-02-14 18:23 ` [RFC PATCH v3 4/6] media: tegra: Add Tegra210 Video input driver Sowjanya Komatineni
2020-02-16 11:03   ` Hans Verkuil [this message]
2020-02-16 19:54     ` Sowjanya Komatineni
2020-02-16 20:11       ` Sowjanya Komatineni
2020-02-16 20:22         ` Sowjanya Komatineni
2020-02-17  8:04           ` Hans Verkuil
2020-02-18  0:59             ` Sowjanya Komatineni
2020-02-18  1:04               ` Sowjanya Komatineni
2020-02-18  3:19                 ` Sowjanya Komatineni
2020-02-19 15:08                   ` Hans Verkuil
2020-02-19 16:22                     ` Sowjanya Komatineni
2020-02-20  0:09                       ` Sowjanya Komatineni
2020-02-20  9:29                         ` Hans Verkuil
2020-02-20 16:21                           ` Sowjanya Komatineni
2020-02-20 12:44   ` Hans Verkuil
2020-02-20 13:33     ` Hans Verkuil
2020-02-20 17:29       ` Sowjanya Komatineni
2020-02-20 19:11       ` Sowjanya Komatineni
2020-02-24  4:45         ` Sowjanya Komatineni
2020-03-18 11:48           ` Hans Verkuil
2020-03-18 16:14             ` Sowjanya Komatineni
2020-03-18 16:25               ` Sowjanya Komatineni
2020-03-18 17:17                 ` Sowjanya Komatineni
2020-03-19 14:29                   ` Hans Verkuil
2020-03-19 18:49                     ` Sowjanya Komatineni
2020-02-26  4:49     ` Sowjanya Komatineni
2020-02-26  5:50       ` Sowjanya Komatineni
2020-02-14 18:23 ` [RFC PATCH v3 5/6] MAINTAINERS: Add Tegra Video driver section Sowjanya Komatineni
2020-02-14 18:23 ` [RFC PATCH v3 6/6] arm64: tegra: Add Tegra VI CSI support in device tree Sowjanya Komatineni
2020-02-17 14:26 ` [RFC PATCH v3 0/6] Add Tegra driver for video capture Hans Verkuil

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=30e417ba-84e1-63d2-de74-22cfe859bddb@xs4all.nl \
    --to=hverkuil@xs4all.nl \
    --cc=devicetree@vger.kernel.org \
    --cc=frankc@nvidia.com \
    --cc=helen.koike@collabora.com \
    --cc=jonathanh@nvidia.com \
    --cc=linux-clk@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-media@vger.kernel.org \
    --cc=linux-tegra@vger.kernel.org \
    --cc=sboyd@kernel.org \
    --cc=skomatineni@nvidia.com \
    --cc=thierry.reding@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Devicetree Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-devicetree/0 linux-devicetree/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-devicetree linux-devicetree/ https://lore.kernel.org/linux-devicetree \
		devicetree@vger.kernel.org
	public-inbox-index linux-devicetree

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-devicetree


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git