linux-kernel - [PATCH v2 1/3] m68k: Fix lost column on framebuffer debug console

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Message-ID: <0fa5e203bb2f811e36e9711dfd461a8f760a1ed6.1742376675.git.fthain@linux-m68k.org>
Date: Wed, 19 Mar 2025 20:31:15 +1100
From: Finn Thain <fthain@...ux-m68k.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: linux-m68k@...ts.linux-m68k.org,
    stable@...r.kernel.org,
    linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/3] m68k: Fix lost column on framebuffer debug console

Move the cursor position rightward after rendering the character,
not before. This avoids complications that arise when the recursive
console_putc call has to wrap the line and/or scroll the display.
This also fixes the linewrap bug that crops off the rightmost column.

When the cursor is at the bottom of the display, a linefeed will not
move the cursor position further downward. Instead, the display scrolls
upward. Avoid the repeated add/subtract sequence by way of a single
subtraction at the initialization of console_struct_num_rows.

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 | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/arch/m68k/kernel/head.S b/arch/m68k/kernel/head.S
index 852255cf60de..d0d77b1adbde 100644
--- a/arch/m68k/kernel/head.S
+++ b/arch/m68k/kernel/head.S
@@ -3400,6 +3400,7 @@ L(console_clear_loop):
 
 	movel	%d4,%d1				/* screen height in pixels */
 	divul	%a0@(FONT_DESC_HEIGHT),%d1	/* d1 = max num rows */
+	subil	#1,%d1				/* row range is 0 to num - 1 */
 
 	movel	%d0,%a2@(Lconsole_struct_num_columns)
 	movel	%d1,%a2@(Lconsole_struct_num_rows)
@@ -3546,15 +3547,14 @@ func_start	console_putc,%a0/%a1/%d0-%d7
 	cmpib	#10,%d7
 	jne	L(console_not_lf)
 	movel	%a0@(Lconsole_struct_cur_row),%d0
-	addil	#1,%d0
-	movel	%d0,%a0@(Lconsole_struct_cur_row)
 	movel	%a0@(Lconsole_struct_num_rows),%d1
 	cmpl	%d1,%d0
 	jcs	1f
-	subil	#1,%d0
-	movel	%d0,%a0@(Lconsole_struct_cur_row)
 	console_scroll
+	jra	L(console_exit)
 1:
+	addil	#1,%d0
+	movel	%d0,%a0@(Lconsole_struct_cur_row)
 	jra	L(console_exit)
 
 L(console_not_lf):
@@ -3581,12 +3581,6 @@ L(console_not_cr):
  */
 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
-	cmpl	%d1,%d0
-	jcs	1f
-	console_putc	#'\n'	/* recursion is OK! */
-1:
 	movel	%a0@(Lconsole_struct_cur_row),%d1
 
 	/*
@@ -3633,6 +3627,23 @@ L(console_do_font_scanline):
 	addq	#1,%d1
 	dbra	%d7,L(console_read_char_scanline)
 
+	/*
+	 *	Register usage in the code below:
+	 *	a0 = pointer to console globals
+	 *	d0 = cursor column
+	 *	d1 = cursor column limit
+	 */
+
+	lea	%pc@(L(console_globals)),%a0
+
+	movel	%a0@(Lconsole_struct_cur_column),%d0
+	addil	#1,%d0
+	movel	%d0,%a0@(Lconsole_struct_cur_column)	/* Update cursor pos */
+	movel	%a0@(Lconsole_struct_num_columns),%d1
+	cmpl	%d1,%d0
+	jcs	L(console_exit)
+	console_putc	#'\n'		/* Line wrap using tail recursion */
+
 L(console_exit):
 func_return	console_putc
 
-- 
2.45.3