linux-kernel - Re: [PATCH] vt_ioctl: make VT_RESIZEX behave like VT_RESIZE

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Date:   Mon, 12 Apr 2021 09:01:32 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        syzbot <syzbot+b308f5fd049fbbc6e74f@...kaller.appspotmail.com>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Greg KH <gregkh@...uxfoundation.org>,
        Helge Deller <deller@....de>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Martin Hostettler <textshell@...uujin.de>,
        George Kennedy <george.kennedy@...cle.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Peilin Ye <yepeilin.cs@...il.com>
Subject: Re: [PATCH] vt_ioctl: make VT_RESIZEX behave like VT_RESIZE

On Mon, Apr 12, 2021 at 12:15 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, Apr 11, 2021 at 2:43 PM Maciej W. Rozycki <macro@...am.me.uk> wrote:
> >
> >  So it does trigger with vgacon and my old server, which I have started
> > experimenting with and for a start I have switched to a new kernel for an
> > unrelated purpose (now that I have relieved it from all its usual tasks
> > except for the last remaining one for which I haven't got the required
> > user software ported to the new system yet):
> >
> > "struct vt_consize"->v_vlin is ignored. Please report if you need this.
> > "struct vt_consize"->v_clin is ignored. Please report if you need this.
>
> Note that it's entirely possible that things continue to work well
> despite this warning. It's unclear to me from your email if you
> actually see any difference (and apparently you're not able to see it
> right now due to not being close to the machine).

Original search didn't turn up any users of VT_RESIZEX, this is the
first. And looking at the source code I think we could outright remove
support for VT_RESIZEX (but make it silent) and everything should keep
working:

        /*
         * ALWAYS do a VT_RESIZE, even if we already did a VT_RESIZEX
on a 1.3.3 or higher kernel,
         * until those kernel programmers make this unambiguous
         */

       if (do_VT_RESIZE(curr_textmode->cols, curr_textmode->rows,
resize1x1)) sresize=TRUE;

       if (check_kernel_version(1,3,3, "VT_RESIZEX"))
         {
           /*
            * VDisplay must de divided by 2 for DoubleScan modes,
            * or VT_RESIZEX will fail -- until someone fixes the kernel
            * so it understands about doublescan modes.
            */
           if (do_VT_RESIZEX(curr_textmode->cols,
                             curr_textmode->rows,
                             curr_textmode->VDisplay /
(MOFLG_ISSET(curr_textmode, ATTR_DOUBLESCAN) ? 2 : 1),
                             curr_textmode->FontHeight,
                             curr_textmode->HDisplay/8*curr_textmode->FontWidth,
                             curr_textmode->FontWidth, resize1x1)) sresize=TRUE;
         }

The functions are just straightforward wrappers. There's also no cvs
repo, changelog or old releases before 2000 that would shed some light
on why this code even exists.

I think we can just tune down the pr_info_once to pr_debug and done.
Maybe a comment about where the single user we're aware of is.
-Daniel

>
> The fact that v_vlin/v_clin are ignored doesn't necessarily mean that
> they are different from what they were before, so the warning may be a
> non-issue.
>
> > It continues using svgatextmode with its glass (CRT) VT to set my usual
> > 80x37 text mode (720x576 pixel resolution) by manipulating the VGA clock
> > chip and the CRT controller appropriately for a nice refresh rate of 85Hz:
> >
> > Chipset = `TVGA8900', Textmode clock = 44.90 MHz, 80x37 chars, CharCell = 9x16. Refresh = 52.51kHz/84.7Hz.
>
> That doesn't seem necessarily wrong to me.
>
> >  So what's the supposed impact of this change that prompted the inclusion
> > of the messages?
>
> There _may_ be no impact at all apart from the messages.
>
> The code _used_ to set the scan lines (v_vlin) and font height
> (v_clin) from those numbers if they were non-zero, and now it just
> ignores them and warns instead.
>
> The code now just sets the font height from the actual font size when
> the font is set. Which is honestly the only thing that ever made
> sense. Trying to set it with v_clin is ignored, but it's entirely
> possible - maybe even likely - that your user of VT_RESIZEX sets it to
> the same values it already has.
>
> Exactly because setting a font line number to anything else than the
> font size isn't exactly sensible.
>
> But if your screen looks different than it used to, that is obviously
> interesting and says something actually changed (outside of the
> message itself).
>
>            Linus
> _______________________________________________
> dri-devel mailing list
> dri-devel@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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