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input, fb, vt, sysfs: the kernel calls Xorg makes

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Xorg on a framebuffer -- the fbdev video driver, the evdev input driver,
libpciaccess -- asks the kernel a fixed set of questions before it draws
anything, and gives up on the first wrong answer. These are those
answers.

evdev: EVIOCGVERSION, GID, GNAME, GPHYS, GUNIQ, GPROP, GBIT, GKEY, GABS,
GREP and the accepted setters. Xorg and libinput decide what a device
*is* from the capability bitmaps; /dev/input/eventN answered only
EVIOCGRAB, which reads as no device at all. Each device's bitmaps are
its defaults plus whatever it has reported, so a USB tablet makes the
pointer absolute.

fbdev: FBIOPUT_VSCREENINFO succeeds for the mode already set, as on
Linux -- the fbdev driver sets it every time it starts -- and still
refuses any other. FBIOPAN_DISPLAY to the origin; FBIOBLANK.

The console: KDSETMODE(KD_GRAPHICS) stops it drawing on a terminal,
KDSKBMODE(K_OFF) stops keys reaching its tty, and VT_PROCESS now does
the switch handshake: leaving the terminal sends relsig and waits for
VT_RELDISP, arriving sends acqsig. A terminal whose owner has died goes
back to the kernel whole, so a crashed X cannot leave the console dead.

/sys: boot_vga, a writable enable, and graphics/fb0 under the display
device, which is how the fbdev driver pairs a card with /dev/fb0.

ports/linux-headers: <linux/*.h> into musl's prefix, from sabotage-
linux's kernel-headers 4.19.88-2. Its checksum is recorded from the
first download; upstream publishes none.

cmd/xready makes each call and checks the answer, including the switch
handshake on tty2 and an owner that dies holding it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
This commit is contained in:
Nexoniarz 2026-10-24 12:06:04 +02:00
commit b1323e193a
14 changed files with 860 additions and 32 deletions
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10
include/dev/vt.h
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@ -134,6 +134,12 @@ struct vt {
// VT_AUTO or VT_PROCESS. In VT_PROCESS a program has claimed switching and
// the kernel stops acting on the chord -- see vt_kernel_switches().
int mode;
int owner; // pid that set VT_PROCESS
int relsig, acqsig; // what to send it; see vt_switch()
int leaving_to; // a switch waiting on VT_RELDISP, or -1
int kdmode; // KD_TEXT, or KD_GRAPHICS: not drawn
int kbmode; // K_UNICODE..., or K_OFF: no keys
};
// The active terminal, and the one the framebuffer is showing.
@ -160,5 +166,9 @@ void vt_repaint(struct vt *v); // grid -> framebuffer, if active
uint32 vt_rgb(uint8 idx); // palette index -> RGB
int vt_index(struct vt *v); // 0-based number of a terminal
int vt_kernel_switches(void); // 0 if a program owns switching
int vt_reldisp(struct vt *v, int arg); // VT_RELDISP
void vt_set_process(struct vt *v, int mode, int relsig, int acqsig);
void vt_set_kdmode(struct vt *v, int mode);
int vt_takes_keys(struct vt *v); // 0 in K_OFF or a raw mode
#endif // VT_H