linux-kernel - Re: [PATCH] amba-clcd: add RGB444 support

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Message-ID: <20100315135945.GA4584@pengutronix.de>
Date:	Mon, 15 Mar 2010 14:59:45 +0100
From:	Luotao Fu <l.fu@...gutronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Luotao Fu <l.fu@...gutronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andres Salomon <dilinger@...ian.org>,
	Alessandro Rubini <rubini@...pv.it>,
	Krzysztof Helt <krzysztof.h1@...pl>,
	Kevin Wells <wellsk40@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] amba-clcd: add RGB444 support

Hi Russell,

On Mon, Mar 15, 2010 at 01:38:10PM +0000, Russell King - ARM Linux wrote:
> On Mon, Mar 15, 2010 at 02:30:09PM +0100, Luotao Fu wrote:
> > This one adds RGB444 (bpp=12) support to amba clcd drivers. Tested on a lpc3250
> > based platform.
> 
> No - this is wrong.  You have a 16bpp display.  bpp is the number of bits
> in the framebuffer between two successive pixels.  It is not the number of
> bits used for the pixel information.
> 

> The only case where you have a 12bpp display is if you have two pixels
> packed into three bytes.
>
For 12bpp actually 16bits/pixel is used. I'm aware of that. That's the
reason why the fb.var.bits_per_pixel is set to 16 in clcdfb_decode if
it is originally set to 12 from the platform file to make usage of 16bpp
display. omapfb driver does quite the same.

On my framebuffer controller (here on a lpc3250) there is hardware RGB444
support. Switching to this mode will spare some pins. What I intend to
do with this patch is to make use of this feature. Similar RGB444
Support can be found in omapfb.

cheers
Luotao Fu
-- 
Pengutronix e.K.                           | Dipl.-Ing. Luotao Fu        |
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