linux-kernel - Re: [PATCH] vt: Fix non-blinking cursor regression

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Date:   Tue, 28 Jan 2020 12:11:07 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Matthew Whitehead <tedheadster@...il.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vt: Fix non-blinking cursor regression

On Sun, Jan 26, 2020 at 12:32:10PM -0500, Nicolas Pitre wrote:
> On Sun, 26 Jan 2020, Lukas Wunner wrote:
> > On Wed, Jan 22, 2020 at 11:40:38AM -0500, Nicolas Pitre wrote:
> > > On Wed, 22 Jan 2020, Lukas Wunner wrote:
> > > > Since commit a6dbe4427559 ("vt: perform safe console erase in the right
> > > > order"), when userspace clears both the scrollback buffer and the screen
> > > > by writing "\e[3J" to an fbdev virtual console, the cursor stops blinking
> > > > if that virtual console is not in the foreground.  I'm witnessing this
> > > > on every boot of Raspbian since updating to v4.19.37+ because agetty
> > > > writes the sequence to /dev/tty6 while the console is still switched to
> > > > /dev/tty1.  Switching consoles once makes the cursor blink again.
> > > > 
> > > > The commit added an invocation of ->con_switch() to flush_scrollback().
> > > > Normally this is only invoked from switch_screen() to switch consoles.
> > > > switch_screen() updates *vc->vc_display_fg to the new console and
> > > > fbcon_switch() updates ops->currcon.  Because the commit only invokes
> > > > fbcon_switch() but doesn't update *vc->vc_display_fg, it performs an
> > > > incomplete console switch.
> > > > 
> > > > When fb_flashcursor() subsequently blinks the cursor, it retrieves the
> > > > foreground console from ops->currcon.  Because *vc->vc_display_fg wasn't
> > > > updated, con_is_visible() incorrectly returns false and as a result,
> > > > fb_flashcursor() bails out without blinking the cursor.
> > > > 
> > > > The invocation of ->con_switch() appears to have been erroneous.  After
> > > > all, why should a console switch be performed when clearing the screen?
> > > > The commit message doesn't provide a rationale either.  So delete it.
> > > 
> > > The problem here is that only vgacon provides a con_flush_scrollback 
> > > method. When not provided, the only way to flush the scrollback buffer 
> > > is to invoke the switch method. If you remove it the scrollback buffer 
> > > of the foreground console won't be flushed in the fb case and possibly 
> > > others.
[...]
> Still, I'd prefer to get back to the same functional state from before 
> commit a6dbe44275 with the switch method first. Can you confirm that the 
> patch I propose does fix it for you?

Yes, your patch is

Reported-and-tested-by: Lukas Wunner <lukas@...ner.de>

I'm withdrawing my own patch because further testing has shown that while it
fixes the non-blinking cursor issue, it doesn't flush scrollback if "\e[3J"
is written to the foreground console.

Would you prefer me to submit your patch with your Signed-off-by or rather
submit it yourself with my Tested-by?  If the latter, please include a code
comment explaining that ->con_switch() has the side effect of flushing
scrollback.  That seems quite non-obvious to me.


> > A dumb question perhaps, but why is it necessary to hide the cursor?
> 
> Many console implements the cursor by changing the background color of 
> the cursor position. If the switch occurs while the cursor is in its 
> visible period, the rest of the code will assume that the cursor is the 
> actual background color, effectively leaving the drawn cursor there 
> after it moved.

I see, thanks for the explanation.

Lukas