[PATCH v3 0/3] m68k: Bug fix and cleanup for framebuffer debug console

[PATCH v3 0/3] m68k: Bug fix and cleanup for framebuffer debug console

[Finn Thain]

This series has a bug fix for the early bootconsole as well as some
related efficiency improvements and cleanup.

The relevant code is subject to CONSOLE_DEBUG, which is presently only
used with CONFIG_MAC. To test this series (in qemu-system-m68k, for example)
it's helpful to enable CONFIG_EARLY_PRINTK and
CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER and boot with
kernel parameters 'console=ttyS0 earlyprintk keep_bootcon'.

---
Changed since v1: 
 - Solved problem with line wrap while scrolling.
 - Added two additional patches.

Changed since v2:
 - Adopted addq and subq as suggested by Andreas.


Finn Thain (3):
  m68k: Fix lost column on framebuffer debug console
  m68k: Avoid pointless recursion in debug console rendering
  m68k: Remove unused "cursor home" code from debug console

 arch/m68k/kernel/head.S | 73 +++++++++++++++++++++--------------------
 1 file changed, 37 insertions(+), 36 deletions(-)

-- 
2.45.3

[PATCH v3 1/3] m68k: Fix lost column on framebuffer debug console

[Finn Thain]

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@vger.kernel.org
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
Changed since v2:
 - addil and subil are now addql and subql resp.
---
 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..9bd8adaa756d 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 */
+	subql	#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:
+	addql	#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
+	addql	#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

Re: [PATCH v3 0/3] m68k: Bug fix and cleanup for framebuffer debug console

[Stan Johnson]

Tested-by: Stan Johnson <userm57@yahoo.com>

On a Mac IIci, the three patches fix the lost-column issue.

I didn't notice any regressions booting the new kernel to multiuser
and accessing serial, scsi and network.

-Stan Johnson


On 3/27/25 4:39 PM, Finn Thain wrote:
> This series has a bug fix for the early bootconsole as well as some
> related efficiency improvements and cleanup.
> 
> The relevant code is subject to CONSOLE_DEBUG, which is presently only
> used with CONFIG_MAC. To test this series (in qemu-system-m68k, for example)
> it's helpful to enable CONFIG_EARLY_PRINTK and
> CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER and boot with
> kernel parameters 'console=ttyS0 earlyprintk keep_bootcon'.
> 
> ---
> Changed since v1: 
>  - Solved problem with line wrap while scrolling.
>  - Added two additional patches.
> 
> Changed since v2:
>  - Adopted addq and subq as suggested by Andreas.
> 
> 
> Finn Thain (3):
>   m68k: Fix lost column on framebuffer debug console
>   m68k: Avoid pointless recursion in debug console rendering
>   m68k: Remove unused "cursor home" code from debug console
> 
>  arch/m68k/kernel/head.S | 73 +++++++++++++++++++++--------------------
>  1 file changed, 37 insertions(+), 36 deletions(-)
> 

Re: [PATCH v3 1/3] m68k: Fix lost column on framebuffer debug console

[Geert Uytterhoeven]

On Thu, 27 Mar 2025 at 23:44, Finn Thain <fthain@linux-m68k.org> wrote:
> 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@vger.kernel.org
> Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> ---
> Changed since v2:
>  - addil and subil are now addql and subql resp.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
i.e. will queue in the m68k tree for v6.17.

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds