Re: [kbd] [patchv2 3/3] VT: Bump font size limitation to 64x128 pixels

Samuel Thibault
2022-12-18T14:55:40Z

Hello,

Alexey Gladkov, le dim. 18 déc. 2022 15:39:38 +0100, a ecrit:
> On Sun, Dec 18, 2022 at 01:32:12AM +0100, Samuel Thibault wrote:
> > This moves 32x32 font size limitation checking down to drivers, so that
> > fbcon can allow large fonts.
> >
> > We still keep a limitation to 64x128 pixels so as to have a simple bounded
> > allocation for con_font_get and in the userland kbd tool. That glyph size
> > will however be enough to have 128x36 characters on a "16/9 8K display".
> >
> > Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> >
> > ---
> > V1 -> V2: Switch con_font_get to kvmalloc/kvfree instead of kmalloc/kfree
> >
> > Index: linux-6.0/drivers/tty/vt/vt.c
> > ===================================================================
> > --- linux-6.0.orig/drivers/tty/vt/vt.c
> > +++ linux-6.0/drivers/tty/vt/vt.c
> > @@ -4575,17 +4575,20 @@ void reset_palette(struct vc_data *vc)
> > /*
> > * Font switching
> > *
> > - * Currently we only support fonts up to 32 pixels wide, at a maximum height
> > - * of 32 pixels. Userspace fontdata is stored with 32 bytes (shorts/ints,
> > - * depending on width) reserved for each character which is kinda wasty, but
> > - * this is done in order to maintain compatibility with the EGA/VGA fonts. It
> > - * is up to the actual low-level console-driver convert data into its favorite
> > - * format (maybe we should add a `fontoffset' field to the `display'
> > - * structure so we won't have to convert the fontdata all the time.
> > + * Currently we only support fonts up to 128 pixels wide, at a maximum height
> > + * of 128 pixels. Userspace fontdata may have to be stored with 32 bytes
> > + * (shorts/ints, depending on width) reserved for each character which is
> > + * kinda wasty, but this is done in order to maintain compatibility with the
> > + * EGA/VGA fonts. It is up to the actual low-level console-driver convert data
> > + * into its favorite format (maybe we should add a `fontoffset' field to the
> > + * `display' structure so we won't have to convert the fontdata all the time.
> > * /Jes
> > */
> >
> > -#define max_font_size 65536
> > +#define max_font_width 64
> > +#define max_font_height 128
> > +#define max_font_glyphs 512
> > +#define max_font_size (max_font_glyphs*max_font_width*max_font_height)
>
> As a suggestion that you can safely ignore. Maybe make max_font_glyphs a
> sysctl parameter to be able to use larger fonts ?
>
> I get requests from time to time in kbd that it is not possible to load a
> larger font.

64KiB was really low, while the theoretically possible max was
32*32*512 = 512KB, so I understand there used to be such requests :)

Here, by setting the max to 64x128*512, I don't think we'll need more.
Even with a 8K screen (will such really exist and want a text console?)
the max 64x128 gets 128 characters on a single line, for soemthing like
36 lines. So I guess it's not worth adding yet another sysctl :)

Samuel