Re: [PATCH v5 2/3] VT: Add KDFONTINFO ioctl

[Alexey Gladkov <legion@kernel.org>]

On Thu, Apr 18, 2024 at 08:18:33AM +0200, Greg Kroah-Hartman wrote:
> On Wed, Apr 17, 2024 at 07:37:36PM +0200, Alexey Gladkov wrote:
> > Each driver has its own restrictions on font size. There is currently no
> > way to understand what the requirements are. The new ioctl allows
> > userspace to get the minimum and maximum font size values.
> 
> Is there any userspace code that uses this yet that we can point to
> here?

Yes. I have a code that uses this. It waits for this ioctl to appear in
the kernel.

https://git.kernel.org/pub/scm/linux/kernel/git/legion/kbd.git/commit/?h=kdfontinfo-v1&id=e2ad0117ca8e46cedd8668934db7b04e9054d5d7

> I know tty ioctls are woefully undocumented, but could there be some
> documentation here?

Yes, this is a big problem for this interface. The ioctl_console(2)
describes PIO_FONT/PIO_FONTX, which is no longer supported, but does not
describe KDFONTOP at all, which is exactly used by userspace.

My TODO has a task to fix this.

But I would suggest creating documentation in the kernel because life
shows that man-page is far behind what is implemented.

> > 
> > Acked-by: Helge Deller <deller@gmx.de>
> > Signed-off-by: Alexey Gladkov <legion@kernel.org>
> > ---
> >  drivers/tty/vt/vt.c       | 24 ++++++++++++++++++++++++
> >  drivers/tty/vt/vt_ioctl.c | 13 +++++++++++++
> >  include/linux/console.h   |  3 +++
> >  include/linux/vt_kern.h   |  1 +
> >  include/uapi/linux/kd.h   | 14 ++++++++++++++
> >  5 files changed, 55 insertions(+)
> > 
> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > index 9b5b98dfc8b4..e8db0e9ea674 100644
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -4851,6 +4851,30 @@ int con_font_op(struct vc_data *vc, struct console_font_op *op)
> >  	return -ENOSYS;
> >  }
> >  
> > +int con_font_info(struct vc_data *vc, struct console_font_info *info)
> > +{
> > +	int rc;
> > +
> > +	info->min_height = 0;
> > +	info->max_height = max_font_height;
> > +
> > +	info->min_width = 0;
> > +	info->max_width = max_font_width;
> > +
> > +	info->flags = KD_FONT_INFO_FLAG_LOW_SIZE | KD_FONT_INFO_FLAG_HIGH_SIZE;
> > +
> > +	console_lock();
> > +	if (vc->vc_mode != KD_TEXT)
> > +		rc = -EINVAL;
> > +	else if (vc->vc_sw->con_font_info)
> > +		rc = vc->vc_sw->con_font_info(vc, info);
> > +	else
> > +		rc = -ENOSYS;
> > +	console_unlock();
> > +
> > +	return rc;
> > +}
> > +
> >  /*
> >   *	Interface exported to selection and vcs.
> >   */
> > diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
> > index 4b91072f3a4e..9a2f8081f650 100644
> > --- a/drivers/tty/vt/vt_ioctl.c
> > +++ b/drivers/tty/vt/vt_ioctl.c
> > @@ -479,6 +479,19 @@ static int vt_k_ioctl(struct tty_struct *tty, unsigned int cmd,
> >  		break;
> >  	}
> >  
> > +	case KDFONTINFO: {
> > +		struct console_font_info fnt_info;
> > +
> > +		memset(&fnt_info, 0, sizeof(fnt_info));
> > +
> > +		ret = con_font_info(vc, &fnt_info);
> 
> Shouldn't con_font_info() memset it first?  No need to do it in the
> caller.
> 
> > +		if (ret)
> > +			return ret;
> > +		if (copy_to_user(up, &fnt_info, sizeof(fnt_info)))
> > +			return -EFAULT;
> > +		break;
> > +	}
> > +
> >  	default:
> >  		return -ENOIOCTLCMD;
> >  	}
> > diff --git a/include/linux/console.h b/include/linux/console.h
> > index 31a8f5b85f5d..4b798322aa01 100644
> > --- a/include/linux/console.h
> > +++ b/include/linux/console.h
> > @@ -21,6 +21,7 @@
> >  #include <linux/vesa.h>
> >  
> >  struct vc_data;
> > +struct console_font_info;
> >  struct console_font_op;
> >  struct console_font;
> >  struct module;
> > @@ -102,6 +103,8 @@ struct consw {
> >  	bool	(*con_switch)(struct vc_data *vc);
> >  	bool	(*con_blank)(struct vc_data *vc, enum vesa_blank_mode blank,
> >  			     bool mode_switch);
> > +	int	(*con_font_info)(struct vc_data *vc,
> > +				 struct console_font_info *info);
> 
> To make the names more obvious, how about:
> 	con_font_info_get()?
> 
> >  	int	(*con_font_set)(struct vc_data *vc,
> >  				const struct console_font *font,
> >  				unsigned int vpitch, unsigned int flags);
> > diff --git a/include/linux/vt_kern.h b/include/linux/vt_kern.h
> > index d008c3d0a9bb..383b3a4f6113 100644
> > --- a/include/linux/vt_kern.h
> > +++ b/include/linux/vt_kern.h
> > @@ -33,6 +33,7 @@ void do_blank_screen(int entering_gfx);
> >  void do_unblank_screen(int leaving_gfx);
> >  void poke_blanked_console(void);
> >  int con_font_op(struct vc_data *vc, struct console_font_op *op);
> > +int con_font_info(struct vc_data *vc, struct console_font_info *info);
> >  int con_set_cmap(unsigned char __user *cmap);
> >  int con_get_cmap(unsigned char __user *cmap);
> >  void scrollback(struct vc_data *vc);
> > diff --git a/include/uapi/linux/kd.h b/include/uapi/linux/kd.h
> > index 8ddb2219a84b..68b715ad4d5c 100644
> > --- a/include/uapi/linux/kd.h
> > +++ b/include/uapi/linux/kd.h
> > @@ -185,6 +185,20 @@ struct console_font {
> >  
> >  #define KD_FONT_FLAG_DONT_RECALC 	1	/* Don't recalculate hw charcell size [compat] */
> >  
> > +/* font information */
> > +
> > +#define KD_FONT_INFO_FLAG_LOW_SIZE	_BITUL(0) /* 256 */
> > +#define KD_FONT_INFO_FLAG_HIGH_SIZE	_BITUL(1) /* 512 */
> 
> I don't understand why bit 0 and bit 1 have those comments after them.
> That's confusing (i.e. bit 0 is NOT 256...)
> 
> > +
> > +struct console_font_info {
> > +	__u32  flags;			/* KD_FONT_INFO_FLAG_* */
> 
> Why are there flags if you are only setting these 2 values?  What are
> the flags for?
> 
> If this is going to be a "multiplexed" type of structure, then make it a
> union?  Or maybe we are totally over thinking this whole thing.
> 
> All you want is the min/max font size of the console, right?  So perhaps
> the whole structure is just:
> 
> > +	__u32 min_width, min_height;	/* minimal font size */
> > +	__u32 max_width, max_height;	/* maximum font size */
> 
> Those 4 variables?  Why have anything else here at all?  For any new
> thing you wish to discover, have it be a new ioctl?
> 
> > +	__u32 reserved[5];		/* This field is reserved for future use. Must be 0. */
> 
> I understand the "must be 0" but this is a read-only structure, so
> saying "it will be set to 0" might be better?"  Or something like that?
> 
> > +};
> > +
> > +#define KDFONTINFO	_IOR(KD_IOCTL_BASE, 0x73, struct console_font_info)
> 
> As mentioned above how about KDFONTINFOGET?
> 
> thanks,
> 
> greg k-h
> 

-- 
Rgrds, legion