linux-kernel - Re: [PATCH] [efifb] Fix 16 color palette entry calculation

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Date:	Tue, 7 Jun 2016 13:23:05 -0400
From:	Peter Jones <pjones@...hat.com>
To:	Max Staudt <mstaudt@...e.de>
Cc:	plagnioj@...osoft.com, tomi.valkeinen@...com, eich@...e.de,
	tiwai@...e.de, agraf@...e.de, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH] [efifb] Fix 16 color palette entry calculation

On Tue, Jun 07, 2016 at 03:45:43PM +0200, Max Staudt wrote:
> When using efifb with a 16-bit (5:6:5) visual, fbcon's text is rendered
> in the wrong colors - e.g. text gray (#aaaaaa) is rendered as green
> (#50bc50) and neighboring pixels have slightly different values
> (such as #50bc78).
> 
> The reason is that fbcon loads its 16 color palette through
> efifb_setcolreg(), which in turn calculates a 32-bit value to write
> into memory for each palette index.
> Until now, this code could only handle 8-bit visuals and didn't mask
> overlapping values when ORing them.
> 
> With this patch, fbcon displays the correct colors when a qemu VM is
> booted in 16-bit mode (in GRUB: "set gfxpayload=800x600x16").
> 
> Signed-off-by: Max Staudt <mstaudt@...e.de>
> ---
>  drivers/video/fbdev/efifb.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/video/fbdev/efifb.c b/drivers/video/fbdev/efifb.c
> index 924bad4..37a37c4 100644
> --- a/drivers/video/fbdev/efifb.c
> +++ b/drivers/video/fbdev/efifb.c
> @@ -50,9 +50,9 @@ static int efifb_setcolreg(unsigned regno, unsigned red, unsigned green,
>  		return 1;
>  
>  	if (regno < 16) {
> -		red   >>= 8;
> -		green >>= 8;
> -		blue  >>= 8;
> +		red   >>= 16 - info->var.red.length;
> +		green >>= 16 - info->var.green.length;
> +		blue  >>= 16 - info->var.blue.length;
>  		((u32 *)(info->pseudo_palette))[regno] =
>  			(red   << info->var.red.offset)   |
>  			(green << info->var.green.offset) |
> -- 
> 2.6.6

Looks right to me.

Acked-By: Peter Jones <pjones@...hat.com>

-- 
  Peter