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

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Date:   Sat, 3 Jun 2017 21:46:36 -0700
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Sakari Ailus <sakari.ailus@....fi>, Pavel Machek <pavel@....cz>
Cc:     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: Re: [PATCH v7 16/34] [media] add Omnivision OV5640 sensor driver



On 06/03/2017 02:57 PM, Sakari Ailus wrote:
> On Sat, Jun 03, 2017 at 09:51:39PM +0200, Pavel Machek wrote:
>> 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.
>>>
>>> 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?

Yes, the register interface for exposure takes lines*16.

Maybe converting from seconds to lines is as simple as
framerate * height * seconds. But I'm not sure about that.

Steve

> 
> 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.
>