linux-kernel - Re: [PATCH] m68k: Fix lost column on framebuffer debug console

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Message-ID: <b504b718-0ffb-ff28-0c91-badf92282b32@linux-m68k.org>
Date: Tue, 18 Mar 2025 09:48:40 +1100 (AEDT)
From: Finn Thain <fthain@...ux-m68k.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
cc: stable@...r.kernel.org, linux-m68k@...ts.linux-m68k.org, 
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH] m68k: Fix lost column on framebuffer debug console


Please disregard the patch below. Further testing shows that it does not 
completely solve the problem. In particular, when line-wrap also produces 
vertical scrolling. I think the recursive console_putc call may have to 
go.

On Fri, 7 Mar 2025, Finn Thain wrote:

> When long lines are sent to the debug console on the framebuffer, the
> right-most column is lost. Fix this by subtracting 1 from the column
> count before comparing it with console_struct_cur_column, as the latter
> counts from zero.
> 
> Linewrap is handled with a recursive call to console_putc, but this
> alters the console_struct_cur_row global. Store the old value before
> calling console_putc, so the right-most character gets rendered on the
> correct line.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@...r.kernel.org
> Signed-off-by: Finn Thain <fthain@...ux-m68k.org>
> ---
>  arch/m68k/kernel/head.S | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/m68k/kernel/head.S b/arch/m68k/kernel/head.S
> index 852255cf60de..9c60047764d0 100644
> --- a/arch/m68k/kernel/head.S
> +++ b/arch/m68k/kernel/head.S
> @@ -3583,11 +3583,16 @@ L(console_not_home):
>  	movel	%a0@(Lconsole_struct_cur_column),%d0
>  	addql	#1,%a0@(Lconsole_struct_cur_column)
>  	movel	%a0@(Lconsole_struct_num_columns),%d1
> +	subil	#1,%d1
>  	cmpl	%d1,%d0
>  	jcs	1f
> -	console_putc	#'\n'	/* recursion is OK! */
> +	/*	recursion will alter console_struct so load d1 register first */
> +	movel	%a0@(Lconsole_struct_cur_row),%d1
> +	console_putc	#'\n'
> +	jmp	2f
>  1:
>  	movel	%a0@(Lconsole_struct_cur_row),%d1
> +2:
>  
>  	/*
>  	 *	At this point we make a shift in register usage
>