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Subject[PATCH] vgacon: unconfuse vc_origin when using soft scrollback
When CONFIG_VGACON_SOFT_SCROLLBACK is selected, the VGA display memory
index and vc_visible_origin don't change when scrollback is activated.
The actual screen content is saved away and the scrollbackdata is copied
over it. However the vt code, and /dev/vcs devices in particular, still
expect vc_origin to always point at the actual screen content not the
displayed scrollback content.

So adjust vc_origin to point at the saved screen content when scrollback
is active and set it back to vc_visible_origin when restoring the screen.

This fixes /dev/vcsa<n> that return scrollback content when they
shouldn't (onli /dev/vcsa without a number should), and also fixes
/dev/vcsu that should return scrollback content when scrollback is
active but currently doesn't.

An unnecessary call to vga_set_mem_top() is also removed.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Cc: stable@vger.kernel.org # v4.19+

---

I tagged it for stable starting with v4.19 as this is the kernel that
introduced /dev/vcsu* which is directly affected. Users of earlier kernels
most likely won't care.

diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index 09731b2f68..c6b3bdbbdb 100644
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c
@@ -271,6 +271,7 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)

static void vgacon_restore_screen(struct vc_data *c)
{
+ c->vc_origin = c->vc_visible_origin;
vgacon_scrollback_cur->save = 0;

if (!vga_is_gfx && !vgacon_scrollback_cur->restore) {
@@ -287,8 +288,7 @@ static void vgacon_scrolldelta(struct vc_data *c, int lines)
int start, end, count, soff;

if (!lines) {
- c->vc_visible_origin = c->vc_origin;
- vga_set_mem_top(c);
+ vgacon_restore_screen(c);
return;
}

@@ -298,6 +298,7 @@ static void vgacon_scrolldelta(struct vc_data *c, int lines)
if (!vgacon_scrollback_cur->save) {
vgacon_cursor(c, CM_ERASE);
vgacon_save_screen(c);
+ c->vc_origin = (unsigned long)c->vc_screenbuf;
vgacon_scrollback_cur->save = 1;
}

@@ -335,7 +336,7 @@ static void vgacon_scrolldelta(struct vc_data *c, int lines)
int copysize;

int diff = c->vc_rows - count;
- void *d = (void *) c->vc_origin;
+ void *d = (void *) c->vc_visible_origin;
void *s = (void *) c->vc_screenbuf;

count *= c->vc_size_row;
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