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

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Message-ID: <20100315141304.GF700@n2100.arm.linux.org.uk>
Date:	Mon, 15 Mar 2010 14:13:04 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Luotao Fu <l.fu@...gutronix.de>
Cc:	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

On Mon, Mar 15, 2010 at 02:59:45PM +0100, Luotao Fu wrote:
> 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.

Then omapfb is also buggy and needs fixing in a similar manner.

The de-facto way of determining the bitfield structure is as I highlighted,
which is used by all the x86 drivers supporting RGB555 and RGB565.

We don't use bits_per_pixel=15 for RGB555, and using bits_per_pixel=12 for
RGB444 is wrong for all the same reasons.
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