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 [thread overview]
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;
> +}
next prev parent reply other threads:[~2020-02-16 11:03 UTC|newest]
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
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