Re: [PATCH v7 16/34] [media] add Omnivision OV5640 sensor driver

From: Pavel Machek <pavel@ucw.cz>
To: Sakari Ailus <sakari.ailus@iki.fi>
Cc: Steve Longerbeam <slongerbeam@gmail.com>,
	robh+dt@kernel.org, mark.rutland@arm.com, shawnguo@kernel.org,
	kernel@pengutronix.de, fabio.estevam@nxp.com,
	linux@armlinux.org.uk, mchehab@kernel.org, hverkuil@xs4all.nl,
	nick@shmanahar.org, markus.heiser@darmarIT.de,
	p.zabel@pengutronix.de,
	laurent.pinchart+renesas@ideasonboard.com, bparrot@ti.com,
	geert@linux-m68k.org, arnd@arndb.de, sudipm.mukherjee@gmail.com,
	minghsiu.tsai@mediatek.com, tiffany.lin@mediatek.com,
	jean-christophe.trotin@st.com, horms+renesas@verge.net.au,
	niklas.soderlund+renesas@ragnatech.se, robert.jarzmik@free.fr,
	songjun.wu@microchip.com, andrew-ct.chen@mediatek.com,
	gregkh@linuxfoundation.org, shuah@kernel.org,
	sakari.ailus@linux.intel.com, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org, devel@driverdev.osuosl.org,
	Steve Longerbeam <steve_longerbeam@mentor.com>
Subject: Re: [PATCH v7 16/34] [media] add Omnivision OV5640 sensor driver
Date: Sun, 4 Jun 2017 00:17:41 +0200	[thread overview]
Message-ID: <20170603221741.GA2379@amd> (raw)
In-Reply-To: <20170603215709.GU1019@valkosipuli.retiisi.org.uk>

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Hi!

> > > According to the docs V4L2_CID_EXPOSURE_ABSOLUTE is in 100 usec units.
> > > 
> > >  OTOH V4L2_CID_EXPOSURE has no defined unit, so it's a better fit IMO.
> > > >Way more drivers appear to be using EXPOSURE than EXPOSURE_ABSOLUTE, too.
> > > 
> > > Done, switched to V4L2_CID_EXPOSURE. It's true, this control is not
> > > taking 100 usec units, so unit-less is better.
> > 
> > Thanks. If you know the units, it would be of course better to use
> > right units...
> 
> Steve: what's the unit in this case? Is it lines or something else?
> 
> Pavel: we do need to make sure the user space will be able to know the unit,
> too. It's rather a case with a number of controls: the unit is known but
> there's no API to convey it to the user.
> 
> The exposure is a bit special, too: granularity matters a lot on small
> values. On most other controls it does not.

Yeah. Basically problem with exposure is that the control is
logarithmic; by using linear scale we got too much resolution at long
times and too little resolution at short times.

(Plus, 100 usec ... n900 can do times _way_ shorter than that.)

Anyway, even u32 gives us enough range, but I so the linear/log
confusion does not matter. But it would be nicer if values were in 10
usec or usec, not 100 usec... 

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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