linux-kernel - Re: [PATCH v2 2/3] media: open.rst: document devnode-centric and mc-centric types

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Date:   Fri, 25 Aug 2017 15:42:21 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Cc:     Mauro Carvalho Chehab <mchehab@...radead.org>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v2 2/3] media: open.rst: document devnode-centric and
 mc-centric types

On 25/08/17 14:52, Mauro Carvalho Chehab wrote:
> From: "mchehab@...pensource.com" <mchehab@...pensource.com>
> 
> When we added support for omap3, back in 2010, we added a new
> type of V4L2 devices that aren't fully controlled via the V4L2
> device node. Yet, we never made it clear, at the V4L2 spec,
> about the differences between both types.
> 
> Let's document them with the current implementation.
> 
> Signed-off-by: Mauro Carvalho Chehab <mchehab@...pensource.com>
> ---
>  Documentation/media/uapi/v4l/open.rst | 53 +++++++++++++++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
> 
> diff --git a/Documentation/media/uapi/v4l/open.rst b/Documentation/media/uapi/v4l/open.rst
> index 9b98d10d5153..bbd1887f83a0 100644
> --- a/Documentation/media/uapi/v4l/open.rst
> +++ b/Documentation/media/uapi/v4l/open.rst
> @@ -6,6 +6,59 @@
>  Opening and Closing Devices
>  ***************************
>  
> +Types of V4L2 hardware control
> +==============================
> +
> +V4L2 hardware is usually complex: support for the hardware is implemented
> +via a main driver (also known as bridge driver) and often several
> +additional drivers. The main driver always exposes one or
> +more **V4L2 device nodes** (see :ref:`v4l2_device_naming`).
> +
> +The other drivers are called **V4L2 sub-devices** and provide control to
> +other parts of the hardware usually connected via a serial bus (like
> +I²C, SMBus or SPI). Depending on the main driver, they can be implicitly
> +controlled directly by the main driver or explicitly via
> +the **V4L2 sub-device API** (see :ref:`subdev`).
> +
> +When V4L2 was originally designed, there was only one type of hardware
> +control. The entire V4L2 hardware is controlled via the
> +**V4L2 device nodes**. We refer to this kind of control as
> +**V4L2 device node centric** (or, simply, **vdev-centric**).
> +
> +Since the end of 2010, a new type of V4L2 hardware control was added, in

Just drop 'the end of'.

s/, in/ in/

> +order to support complex devices that are common for embedded systems.
> +Those hardware are controlled mainly via the media controller and

Such hardware is

> +sub-devices. So, they are called: **Media controller centric**
> +(or, simply, "**MC-centric**").
> +
> +For **vdev-centric** hardware control, the hardware is controlled via
> +the **V4L2 device nodes**. They may optionally support the
> +:ref:`media controller API <media_controller>` as well, in order to let
> +the application to know with device nodes are available.

to know with -> know which

> +
> +.. note::
> +
> +   A **vdev-centric** may optionally expose V4L2 sub-devices via

I propose adding 'also' before 'expose' to indicate that it is in
addition to the V4L2 device nodes that were mentioned in the previous
paragraph.

> +   :ref:`sub-device API <subdev>`. In that case, it has to implement
> +   the :ref:`media controller API <media_controller>` as well.
> +
> +For **MC-centric** hardware control, before using the V4L2 hardware,
> +it is required to set the pipelines via the

I'd reword this a bit:

For **MC-centric** hardware control it is required to configure the pipelines
via the :ref:`media controller API <media_controller>` before the hardware can be used.

> +:ref:`media controller API <media_controller>`. For those devices, the

s/those/such/

> +sub-devices' configuration can be controlled via the
> +:ref:`sub-device API <subdev>`, whith creates one device node

s/whith/which/

> +per sub-device.
> +
> +In summary, for **MC-centric** hardware control:
> +
> +- The **V4L2 device** node is responsible for controlling the streaming
> +  features;
> +- The **media controller device** is responsible to setup the pipelines;
> +- The **V4L2 sub-devices** are responsible for sub-device
> +  specific settings.
> +
> +
> +.. _v4l2_device_naming:
>  
>  V4L2 Device Node Naming
>  =======================
> 

The only thing I am not sure about is vdev-centric vs V4L2-centric. 'Laziness while
typing' is not a convincing argument :-)

Regards,

	Hans