Re: [RFCv2 PATCH 02/21] v4l2-ctrls: add unit string.

[Hans Verkuil <hansverk@cisco.com>]

On 01/24/2014 11:35 AM, Sakari Ailus wrote:
> Hi Hans,
> 
> Thanks for the patchset!
> 
> On Mon, Jan 20, 2014 at 01:45:55PM +0100, Hans Verkuil wrote:
>> diff --git a/include/media/v4l2-ctrls.h b/include/media/v4l2-ctrls.h
>> index 0b347e8..3998049 100644
>> --- a/include/media/v4l2-ctrls.h
>> +++ b/include/media/v4l2-ctrls.h
>> @@ -85,6 +85,7 @@ typedef void (*v4l2_ctrl_notify_fnc)(struct v4l2_ctrl *ctrl, void *priv);
>>    * @ops:	The control ops.
>>    * @id:	The control ID.
>>    * @name:	The control name.
>> +  * @unit:	The control's unit. May be NULL.
>>    * @type:	The control type.
>>    * @minimum:	The control's minimum value.
>>    * @maximum:	The control's maximum value.
>> @@ -130,6 +131,7 @@ struct v4l2_ctrl {
>>  	const struct v4l2_ctrl_ops *ops;
>>  	u32 id;
>>  	const char *name;
>> +	const char *unit;
> 
> What would you think of using a numeric value (with the standardised units
> #defined)? I think using a string begs for unmanaged unit usage. Code that
> deals with units might work with one driver but not with another since it
> uses a slightly different string for unit x.

First of all, you always need a string. You don't want GUIs like qv4l2 to have
to switch on a unit in order to generate the unit strings. That's impossible to
keep up to date.

In addition, private controls can have really strange custom units, so you want
to have a string there as well.

Standard controls can have their unit string set in v4l2-ctrls.c, just as their
name is set there these days, thus ensuring consistency.

What I had in mind is that videodev2.h defines a list of standardized unit strings,
e.g.:

#define V4L2_CTRL_UNIT_USECS "usecs"
#define V4L2_CTRL_UNIT_MSECS "msecs"

and apps can do strcmp(qc->unit, V4L2_CTRL_UNIT_USECS) to see what the unit is.
If a driver doesn't use one of those standardized unit strings, then it is a
driver bug.

> A prefix could be potentially nice, too, so ms and µs would still have the
> same unit but a different prefix.

Can you give an example of a prefix? I don't really follow what you want to
achieve.

Regards,

	Hans