Re: Accesing vt(4) framebuffer

On Wed Sep 3, 2025 at 6:23 PM +0300, Adrian Chadd wrote:
> hi!
>
> We put code review requests in the tool at https://reviews.freebsd.org/ .
> If you've not heard back about this
> then please feel free to poke me directly.
>
> Having direct fb access from userland would be great. I thought we already
> did?!

We can access it through mmap (see vd_fb_mmap). This patch just seems to
expose some framebuffer attributes. Although I think (?) that this info
is available through FBIOGTYPE.

>
>
>
> -adrian
>
> On Tue, 26 Aug 2025 at 07:10, Eduardo Morrás <emorras@emorras.eu> wrote:
>
>>
>> Hello, I'm developing a gui/widget over vt(4) framebuffer, no X11 nor
>> Wayland.
>>
>> For this I have created a new ioctl that allows access to framebuffer
>> information and change it's content. For this I modified
>> /usr/src/sys/dev/vt/hw/fb/vt_fb.c and .h, patches at end of this mail.
>>
>> Is this a correct way? Should I do it different? Any security problems?
>>
>>
>> Thanks, Good Day.
>>
>>
>> --- vt_fb.h_ORIG        2025-08-21 19:01:01.192050000 +0200
>> +++ vt_fb.h     2025-08-21 19:04:58.447415000 +0200
>> @@ -49,4 +49,20 @@
>>  vd_fb_ioctl_t          vt_fb_ioctl;
>>  vd_fb_mmap_t           vt_fb_mmap;
>>
>> +#ifdef _KERNEL
>> +#include <sys/ioccom.h>
>> +#endif
>> +
>> +/* Struct to expose framebuffer to userland*/
>> +struct vt_fb_userinfo {
>> +    uint32_t width;
>> +    uint32_t height;
>> +    uint32_t depth;
>> +    uint32_t stride;
>> +    uint32_t size;
>> +};
>> +
>> +/* IOCTL code to work with vt_fb_userinfo */
>> +#define VT_FB_GETINFO _IOR('v', 1, struct vt_fb_userinfo)
>> +
>>  #endif /* _DEV_VT_HW_FB_VT_FB_H_ */
>>
>>
>> --- vt_fb.c_ORIG        2025-08-21 19:01:07.096190000 +0200
>> +++ vt_fb.c     2025-08-21 19:10:22.228787000 +0200
>> @@ -42,6 +42,8 @@
>>  #include <vm/vm.h>
>>  #include <vm/pmap.h>
>>
>> +#include <sys/ioccom.h> /* Userland access framebuffer */
>> +
>>  static struct vt_driver vt_fb_driver = {
>>         .vd_name = "fb",
>>         .vd_init = vt_fb_init,
>> @@ -87,6 +89,24 @@
>>         *(uint32_t *)(sc->fb_vbase + o) = v;
>>  }
>>
>> +static int
>> +vt_fb_userioctl(struct vt_device *vd, u_long cmd, caddr_t data)
>> +{
>> +       struct vt_fb_userinfo *ui = (struct vt_fb_userinfo *)data;
>> +       struct fb_info *info = vd->vd_softc;
>> +
>> +       if (cmd != VT_FB_GETINFO)
>> +               return (ENOIOCTL);
>> +
>> +       ui->width  = info->fb_width;
>> +       ui->height = info->fb_height;
>> +       ui->depth  = info->fb_bpp * 8;
>> +       ui->stride = info->fb_stride;
>> +       ui->size   = info->fb_size;
>> +
>> +       return (0);
>> +}
>> +
>>  int
>>  vt_fb_ioctl(struct vt_device *vd, u_long cmd, caddr_t data, struct thread
>> *td)
>>  {
>> @@ -125,6 +145,10 @@
>>                         return (ENOTTY);
>>                 memcpy((struct fb_rgboffs *)data, &info->fb_rgboffs,
>>                     sizeof(struct fb_rgboffs));
>> +               break;
>> +
>> +       case  VT_FB_GETINFO:   /* userland framebuffer */
>> +               return vt_fb_userioctl(vd, cmd, data);
>>                 break;
>>
>>         default:
>>
>>
>> ---   ---
>> Eduardo Morrás <emorras@emorras.eu>
>>
>>