linux-kernel - Re: [PATCH/RFC v10 15/19] media: Add registration helpers for V4L2 flash sub-devices

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Date:	Mon, 12 Jan 2015 14:27:26 +0100
From:	Pavel Machek <pavel@....cz>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
Cc:	linux-leds@...r.kernel.org, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	kyungmin.park@...sung.com, b.zolnierkie@...sung.com,
	cooloney@...il.com, rpurdie@...ys.net, sakari.ailus@....fi,
	s.nawrocki@...sung.com, Hans Verkuil <hans.verkuil@...co.com>
Subject: Re: [PATCH/RFC v10 15/19] media: Add registration helpers for V4L2
 flash sub-devices

Hi!

> >>+	 * the state of V4L2_CID_FLASH_INDICATOR_INTENSITY control only.
> >>+	 * Therefore it must be possible to set it to 0 level which in
> >>+	 * the LED subsystem reflects LED_OFF state.
> >>+	 */
> >>+	if (cdata_id != INDICATOR_INTENSITY)
> >>+		++__intensity;
> >
> >And normally we'd do i++ instead of ++i, and avoid __ for local
> >variables...?
> 
> Pre-incrementation operator is favourable over the post-incrementation
> one if we don't want to have an access to the value of a variable before
> incrementation, which is the case here.

That may be some old C++ convention, but I'm pretty sure gcc does not
care.

> Maybe gcc detects the cases when the value of a variable is not assigned
> and doesn't copy it before incrementing, however I haven't found any
> reference. I see that often in the for loops the i++ version
> is used, but I am not sure if this is done because developers are
> aware that gcc will optimize it anyway or it is just an omission.

The code is equivalent, and normally the n++ version is used. gcc will
get it right.

> >>+struct v4l2_flash_ctrl_config {
> >>+	struct v4l2_ctrl_config intensity;
> >>+	struct v4l2_ctrl_config flash_intensity;
> >>+	struct v4l2_ctrl_config flash_timeout;
> >>+	u32 flash_faults;
> >>+	bool has_external_strobe:1;
> >>+	bool indicator_led:1;
> >>+};
> >
> >I don't think you are supposed to do boolean bit arrays.
> 
> These bit fields allow to reduce memory usage. If they were not bit
> fields, the address of the next variable would be aligned to the
> multiply of the CPU word size.
> Please look e.g. at struct dev_pm_info in the file include/linux/pm.h.
> It also contains boolean bit fields.

Looks like you are right. I guess I confused bool foo:1 with int
foo:1.

Thanks,
									Pavel

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