linux-kernel - Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit controls from a pipeline

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Date:   Mon, 20 Mar 2017 12:16:20 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Steve Longerbeam <slongerbeam@...il.com>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, mchehab@...nel.org, nick@...anahar.org,
        markus.heiser@...marIT.de, p.zabel@...gutronix.de,
        laurent.pinchart+renesas@...asonboard.com, bparrot@...com,
        geert@...ux-m68k.org, arnd@...db.de, sudipm.mukherjee@...il.com,
        minghsiu.tsai@...iatek.com, tiffany.lin@...iatek.com,
        jean-christophe.trotin@...com, horms+renesas@...ge.net.au,
        niklas.soderlund+renesas@...natech.se, robert.jarzmik@...e.fr,
        songjun.wu@...rochip.com, andrew-ct.chen@...iatek.com,
        gregkh@...uxfoundation.org, shuah@...nel.org, pavel@....cz,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>,
        Jacek Anaszewski <j.anaszewski@...sung.com>
Subject: Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit
 controls from a pipeline

On 03/17/2017 12:55 PM, Sakari Ailus wrote:
> Hi Russell,
> 
> On 03/17/17 13:42, Russell King - ARM Linux wrote:
>> On Tue, Mar 14, 2017 at 08:55:36AM +0100, Hans Verkuil wrote:
>>> We're all very driver-development-driven, and userspace gets very little
>>> attention in general. So before just throwing in the towel we should take
>>> a good look at the reasons why there has been little or no development: is
>>> it because of fundamental design defects, or because nobody paid attention
>>> to it?
>>>
>>> I strongly suspect it is the latter.
>>>
>>> In addition, I suspect end-users of these complex devices don't really care
>>> about a plugin: they want full control and won't typically use generic
>>> applications. If they would need support for that, we'd have seen much more
>>> interest. The main reason for having a plugin is to simplify testing and
>>> if this is going to be used on cheap hobbyist devkits.
>>
>> I think you're looking at it with a programmers hat on, not a users hat.
>>
>> Are you really telling me that requiring users to 'su' to root, and then
>> use media-ctl to manually configure the capture device is what most
>> users "want" ?
> 
> It depends on who the user is. I don't think anyone is suggesting a
> regular end user is the user of all these APIs: it is either an
> application tailored for that given device, a skilled user with his test
> scripts or as suggested previously, a libv4l plugin knowing the device
> or a generic library geared towards providing best effort service. The
> last one of this list does not exist yet and the second last item
> requires help.
> 
> Typically this class of devices is simply not up to provide the level of
> service you're requesting without additional user space control library
> which is responsible for automatic white balance, exposure and focus.
> 
> Making use of the full potential of the hardware requires using a more
> expressive interface. That's what the kernel interface must provide. If
> we decide to limit ourselves to a small sub-set of that potential on the
> level of the kernel interface, we have made a wrong decision. It's as
> simple as that. This is why the functionality (and which requires taking
> a lot of policy decisions) belongs to the user space. We cannot have
> multiple drivers providing multiple kernel interfaces for the same hardware.

Right. With my Cisco hat on I can tell you that Cisco would want full low-level
control. If the driver would limit us we would not be able to use it.

Same with anyone who wants to put Android CameraHAL on top of a V4L2 driver:
they would need full control. Some simplified interface would be unacceptable.

> 
> That said, I'm not trying to provide an excuse for not having libraries
> available to help the user to configure and control the device more or
> less automatically even in terms of best effort. It's something that
> does require attention, a lot more of it than it has received in recent
> few years.

Right.

Regards,

	Hans