exposure vs. exposure_absolute was 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: mark.rutland@arm.com, andrew-ct.chen@mediatek.com,
	minghsiu.tsai@mediatek.com, sakari.ailus@linux.intel.com,
	nick@shmanahar.org, songjun.wu@microchip.com, hverkuil@xs4all.nl,
	Steve Longerbeam <steve_longerbeam@mentor.com>,
	robert.jarzmik@free.fr, devel@driverdev.osuosl.org,
	markus.heiser@darmarIT.de,
	laurent.pinchart+renesas@ideasonboard.com, shuah@kernel.org,
	linux@armlinux.org.uk, geert@linux-m68k.org,
	Steve Longerbeam <slongerbeam@gmail.com>,
	linux-media@vger.kernel.org, devicetree@vger.kernel.org,
	kernel@pengutronix.de, arnd@arndb.de, mchehab@kernel.org,
	bparrot@ti.com, robh+dt@kernel.org, horms+renesas@verge.net.au,
	tiffany.lin@mediatek.com, linux-arm-kernel@lists.infradead.org,
	niklas.soderlund+renesas@ragnatech.se,
	gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
	jean-christophe.trotin@st.com, p.zabel@pengutronix.de,
	fabio.estevam@nxp.com, shawnguo@kerne
Subject: exposure vs. exposure_absolute was Re: [PATCH v7 16/34] [media] add Omnivision OV5640 sensor driver
Date: Thu, 1 Jun 2017 10:43:20 +0200	[thread overview]
Message-ID: <20170601084320.GA16038@amd> (raw)
In-Reply-To: <20170601082659.GJ1019@valkosipuli.retiisi.org.uk>

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Hi!

> > > +	/* Auto/manual exposure */
> > > +	ctrls->auto_exp = v4l2_ctrl_new_std_menu(hdl, ops,
> > > +						 V4L2_CID_EXPOSURE_AUTO,
> > > +						 V4L2_EXPOSURE_MANUAL, 0,
> > > +						 V4L2_EXPOSURE_AUTO);
> > > +	ctrls->exposure = v4l2_ctrl_new_std(hdl, ops,
> > > +					    V4L2_CID_EXPOSURE_ABSOLUTE,
> > > +					    0, 65535, 1, 0);
> > 
> > Is exposure_absolute supposed to be in microseconds...?
> 
> Yes. 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.
> 
> Ideally we should have only one control for exposure.

No. N-o. No no no. NO! No. N-o. NONONO. No. NooooooooooOOO!!!!!!!!!!!!!

Sorry, no.

Userspace needs to know exposure times. It is not so important for a
webcam, but it is mandatory for digital camera. As it gets darker,
autogain wants to scale exposure to cca 1/100 sec, then it wants to
scale gain up to maximum, and only then it wants to continue scaling
exposure. (Threshold will be shorter in "sports" mode, perhaps
1/300sec?)

Plus, we want user to be able to manually set exposure parameters.

So... _this_ driver probably should use V4L2_CID_EXPOSURE. (If the
units are not known). But in general we'd prefer drivers using
V4L2_CID_EXPOSURE_ABSOLUTE. Your car has speedometer calibrated in
km/h or mph, not in "% of max", right?

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

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