vgacon/vt: clear buffer attributes when we load a 512 character font

From: Dave Airlie <[email protected]>

When we switch from 256->512 byte font rendering mode, it means the current contents of the screen is being reinterpreted. The bit that holds
the high bit of the 9-bit font, may have been previously set, and thus the new font misrenders.

The problem case we see is grub2 writes spaces with the bit set, so it ends up with data like 0x820, which gets reinterpreted into 0x120 char which the font translates into G with a circumflex. This flashes up on screen at boot and is quite ugly.

A current side effect of this patch though is that any rendering on the screen changes color to a slightly darker color, but at least the screen
no longer corrupts.

Signed-off-by Dave Airlie <[email protected]>

---

drivers/tty/vt/vt.c            |  2 +-
drivers/video/console/vgacon.c | 19 ++++++++++++-------
include/linux/vt_kern.h        |  1 +
3 files changed, 14 insertions(+), 8 deletions(-)

drivers/tty/vt/vt.c +1 -1

index 8fd8968..c8067ae 100644
@@ -638,7 +638,7 @@ static inline void save_screen(struct vc_data *vc)
 *	Redrawing of screen
 */
 
-static void clear_buffer_attributes(struct vc_data *vc)
+void clear_buffer_attributes(struct vc_data *vc)
 {
 	unsigned short *p = (unsigned short *)vc->vc_origin;
 	int count = vc->vc_screenbuf_size / 2;

drivers/video/console/vgacon.c +1 -1

index d449a74..271b5d0 100644
@@ -1064,7 +1064,7 @@ static int vgacon_do_font_op(struct vgastate
 *state,char *arg,int set,int ch512)
 	unsigned short video_port_status = vga_video_port_reg + 6;
 	int font_select = 0x00, beg, i;
 	char *charmap;
-	
+	bool clear_attribs = false;
 	if (vga_video_type != VIDEO_TYPE_EGAM) {
 		charmap = (char *) VGA_MAP_MEM(colourmap, 0);

		beg = 0x0e;

drivers/video/console/vgacon.c +0 -6

@@ -1169,12 +1169,6 @@ static int vgacon_do_font_op(struct vgastate
 *state,char *arg,int set,int ch512)
 
 	/* if 512 char mode is already enabled don't re-enable it. */
 	if ((set) && (ch512 != vga_512_chars)) {
-		/* attribute controller */
-		for (i = 0; i < MAX_NR_CONSOLES; i++) {
-			struct vc_data *c = vc_cons[i].d;
-			if (c && c->vc_sw == &vga_con)
-				c->vc_hi_font_mask = ch512 ? 0x0800 : 0;
-		}
 		vga_512_chars = ch512;
 		/* 256-char: enable intensity bit

512-char: disable intensity bit */

drivers/video/console/vgacon.c +11 -0

@@ -1185,8 +1179,19 @@ static int vgacon_do_font_op(struct vgastate
 *state,char *arg,int set,int ch512)
 		   it means, but it works, and it appears necessary */
 		inb_p(video_port_status);
 		vga_wattr(state->vgabase, VGA_AR_ENABLE_DISPLAY, 0);	
+		clear_attribs = true;
 	}
 	raw_spin_unlock_irq(&vga_lock);
+
+	if (clear_attribs) {
+		for (i = 0; i < MAX_NR_CONSOLES; i++) {
+			struct vc_data *c = vc_cons[i].d;
+			if (c && c->vc_sw == &vga_con) {
+				clear_buffer_attributes(c);
+				c->vc_hi_font_mask = ch512 ? 0x0800 : 0;
+			}
+		}
+	}
 	return 0;
 }

include/linux/vt_kern.h +1 -0

index 50ae7d0..1f55665 100644
@@ -47,6 +47,7 @@ int con_set_cmap(unsigned char __user *cmap);
 int con_get_cmap(unsigned char __user *cmap);
 void scrollback(struct vc_data *vc, int lines);
 void scrollfront(struct vc_data *vc, int lines);
+void clear_buffer_attributes(struct vc_data *vc);
 void update_region(struct vc_data *vc, unsigned long start, int count);
 void redraw_screen(struct vc_data *vc, int is_switch);
 #define update_screen(x) redraw_screen(x, 0)

We should clear this bit presumably on switching either from or to 512-char mode, since the bit doesn't really make sense either way.

Quoted 1 line · H. Peter Anvin

We should clear this bit presumably on switching either from or to

512-char mode, since the bit doesn't really make sense either way.

Yeah the only problem going from 512-char is that chars above 256 will morph into garbage chars below 256, like ctrl chars, I don't really want to face into doing some sort of safe translation out of 512-char mode, from reading around the net, 512 char mode doesn't seem all that brilliantly useful.

Dave.

The characters will morph anyway, it is just a matter off having them randomly scream with the intensity bit.

512-character mode is definitely useful... we get much wider language coverage with 512 than with 256, which is why most distros use a 512 console font.

Quoted 1 line · H. Peter Anvin

The characters will morph anyway, it is just a matter off having them

randomly scream with the intensity bit.

Quoted 2 lines

512-character mode is definitely useful... we get much wider language

coverage with 512 than with 256, which is why most distros use a 512 console font.

Yeah really use a graphics console :-)

But I've sent a v2 patch that should clear the attribute space going both directions.

Dave.