linux-kernel - exposure vs. exposure_absolute was Re: [PATCH v7 16/34] [media] add Omnivision OV5640 sensor driver

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Message-ID: <20170601084320.GA16038@amd>
Date:   Thu, 1 Jun 2017 10:43:20 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Steve Longerbeam <slongerbeam@...il.com>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, linux@...linux.org.uk, mchehab@...nel.org,
        hverkuil@...all.nl, 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,
        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: exposure vs. exposure_absolute was Re: [PATCH v7 16/34] [media] add
 Omnivision OV5640 sensor driver

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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