linux-kernel - Re: [PATCH v2 1/5] media/v4l2-ctrls: volatiles should not generate CH

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Date:	Tue, 21 Apr 2015 23:32:18 +0200
From:	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Hans Verkuil <hans.verkuil@...co.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Arun Kumar K <arun.kk@...sung.com>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Antti Palosaari <crope@....fi>,
	linux-media <linux-media@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: [PATCH v2 1/5] media/v4l2-ctrls: volatiles should not generate CH_VALUE

Hello Laurent

On Tue, Apr 21, 2015 at 7:44 PM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
> Hi Ricardo,
>
> Thank you for the patch, and sorry for the late review (so late that the patch
> has already been merged).

No worries.

>
> On Friday 20 March 2015 14:30:46 Ricardo Ribalda Delgado wrote:
>> Volatile controls should not generate CH_VALUE events.
>
> What's the rationale for that ? I would actually expect the value change
> events to be more useful for volatile controls than non-volatile controls.
> Volatile controls can have their value changed by the hardware without
> software intervention, and it makes sense to me to report that to userspace.

Imagine a temperature register on the sensor. It is changing
constantly, resolution 10 milidegrees:

Do you want to get an event for every change? Who will poll the
temperature? The driver? The hardware will irq the driver....?

So I guess the less wrong solution is not throwing the ch_value event.

This is just my two cents, probably Hans has a much better global view :)

Regards.
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