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

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Date:   Fri, 10 Mar 2017 12:53:42 -0300
From:   Mauro Carvalho Chehab <mchehab@...pensource.com>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        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,
        sakari.ailus@...ux.intel.com, 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>
Subject: Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit
 controls from a pipeline

Em Fri, 10 Mar 2017 15:20:48 +0100
Hans Verkuil <hverkuil@...all.nl> escreveu:

> 
> > As I've already mentioned, from talking about this with Mauro, it seems
> > Mauro is in agreement with permitting the control inheritence... I wish
> > Mauro would comment for himself, as I can't quote our private discussion
> > on the subject.  
> 
> I can't comment either, not having seen his mail and reasoning.

The rationale is that we should support the simplest use cases first.

In the case of the first MC-based driver (and several subsequent
ones), the simplest use case required MC, as it was meant to suport
a custom-made sophisticated application that required fine control
on each component of the pipeline and to allow their advanced
proprietary AAA userspace-based algorithms to work.

That's not true, for example, for the UVC driver. There, MC
is optional, as it should be.

> > Right now, my view is that v4l2 is currently being screwed up by people
> > with different opinions - there is no unified concensus on how any of
> > this stuff is supposed to work, everyone is pulling in different
> > directions.  That needs solving _really_ quickly, so I suggest that
> > v4l2 people urgently talk to each other and thrash out some of the
> > issues that Steve's patch set has brought up, and settle on a way
> > forward, rather than what is seemingly happening today - which is
> > everyone working in isolation of everyone else with their own bias on
> > how things should be done.  
> 
> The simple fact is that to my knowledge no other MC applications inherit
> controls from subdevs. Suddenly doing something different here seems very
> wrong to me and needs very good reasons.

That's because it was not needed before, as other subdev-based drivers
are meant to be used only on complex scenarios with custom-made apps.

Thanks,
Mauro