linux-kernel - Re: [PATCH v3 0/5] console: Miscellaneous clean-ups, do not use FNTCHARCNT() in fbcon.c

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Message-ID: <20201114124716.GA12895@PWN>
Date:   Sat, 14 Nov 2020 07:47:16 -0500
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, Jiri Slaby <jirislaby@...nel.org>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v3 0/5] console: Miscellaneous clean-ups, do not use
 FNTCHARCNT() in fbcon.c

On Sat, Nov 14, 2020 at 01:22:22PM +0100, Greg Kroah-Hartman wrote:
> On Sat, Nov 14, 2020 at 01:18:06PM +0100, Greg Kroah-Hartman wrote:
> > On Sat, Nov 14, 2020 at 03:10:21AM -0500, Peilin Ye wrote:
> > > Thanks for reviewing!  Questions about the last patch [5/5] though, it
> > > depends on the following assumption:
> > > 
> > > """
> > > For each console `idx`, `vc_cons[idx].d->vc_font.data` and
> > > `fb_display[idx].fontdata` always point to the same buffer.
> > > """
> > > 
> > > Is this true?  I think it is true by grepping for `fontdata`.  I also
> > > noticed that fbcon.c is using `vc->vc_font.data` and `p->fontdata`
> > > interchangeably, see fbcon_get_requirement():
> > > 
> > > 		vc = vc_cons[fg_console].d;
> > > 		[...]
> > > 			p = &fb_display[fg_console];
> > > 			caps->x = 1 << (vc->vc_font.width - 1);
> > > 					^^^^^^^^^^^
> > > 			caps->y = 1 << (vc->vc_font.height - 1);
> > > 					^^^^^^^^^^^
> > > 			caps->len = (p->userfont) ?
> > > 				FNTCHARCNT(p->fontdata) : 256;
> > > 					   ^^^^^^^^^^^
> > > 
> > > If it is true, then what is the point of using `fontdata` in `struct
> > > fbcon_display`?  Just for the `userfont` flag?  Should we delete
> > > `fontdata`, when we no longer need the `userfont` flag?
> > 
> > Yes, after a quick look, I think your analysis here is correct.  So if
> > you can delete that, it would be nice if possible.

I see, at the moment we still need `userfont` for refcount handling, I
will try to delete both `fontdata` and `userfont` after refcount is
taken care of.

> > > In this sense I think [5/5] needs more testing.  Do we have test files
> > > for fbcon, or should I try to write some tests from scratch?
> > 
> > I don't know of any direct tests, I usually just booted into that mode
> > and saw if everything looked like it did before.  There must be some
> > tool that you can use to change the current font, as it seems to happen
> > at boot time on some distros.  I can't remember what it's called at the
> > moment...
> 
> Ah, here's a hint:
> 	https://wiki.archlinux.org/index.php/Linux_console#Fonts
> 
> The setfont tool should help you out here.

Oh, I didn't know about this one.  I'll go experimenting with it,
thank you!

Peilin Ye