linux-kernel - Re: [PATCH RESEND] video: mxsfb: Fix colors display on lower color depth

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Message-Id: <201304221108.17339.jbe@pengutronix.de>
Date:	Mon, 22 Apr 2013 11:08:16 +0200
From:	Juergen Beisert <jbe@...gutronix.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Shawn Guo <shawn.guo@...aro.org>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	Marek Vasut <marex@...x.de>, linux-fbdev@...r.kernel.org,
	Brian Lilly <brian@...stalfontz.com>,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	Thomas Petazzoni <thomas@...e-electrons.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
Subject: Re: [PATCH RESEND] video: mxsfb: Fix colors display on lower color depth

Hi Maxime,

Maxime Ripard wrote:
> The current code always registers as a 32 bits display, and uses the
> hardware to drop the MSB of each color to abjust to the interface width
> used by the panel.
>
> This results on 18 bits (and probably 16 bits display as well) in colors
> being displayed poorly, because the MSB are obviously the most important
> bits for each color definition.

But the "def_rgb666" bitfield description reports correctly to use bit 0..5 
for each colour component in this mode. Maybe your userland program has a bug 
and forgets to honor the bitfield description?

> The default controller behaviour when using an interface width smaller
> than the color depth is to drop the LSBs of each color, which makes more
> sense because you lose the least important part of the color definition.
>
> So, to fix the colors display, just get back to the default controller
> behaviour.

If you do this change, you also must adapt the "def_rgb666" bitfield 
description.
This structure currently reports the lower 6 bits to be used for this mode:
  blue -> 0..5 (skip 6/7)
  green -> 8..13 (skip 14/15)
  red -> 16..21 (skip 22/23).

> [...]

jbe

-- 
Pengutronix e.K.                              | Juergen Beisert             |
Linux Solutions for Science and Industry      | http://www.pengutronix.de/  |
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