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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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17
Makefile
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@ -128,7 +128,7 @@ KERN_NOENTRY = $(filter-out $(BUILD)/kernel/arch/x86_64/entry.o, $(KERNOBJS))
# Third-party software does NOT go here — see ports/.
# -----------------------------------------------------------------------
ULIB = $(addprefix $(BUILD)/lib/, start.o glue.o usys.o)
CMDS = schinit schinitd fbinfo shmtest socktest inputtest nettest reflink schattr ping chvt loadkeys sigtest memfdtest pcitest vgpud schdbg schdbgd resolvconf httpd schmode netecho ldlsd tone hwreport dynhello evtest
CMDS = schinit schinitd fbinfo shmtest socktest inputtest nettest reflink schattr ping chvt loadkeys sigtest memfdtest pcitest vgpud schdbg schdbgd resolvconf httpd schmode netecho ldlsd tone hwreport dynhello evtest xready
CMDBINS = $(addprefix $(BUILD)/cmd/, $(CMDS))
# -----------------------------------------------------------------------
@ -1058,6 +1058,21 @@ DYNLD = -Wl,-dynamic-linker,/lib/ld-musl-x86_64.so.1 -Wl,-rpath,/lib
$(MUSLPREFIX)/lib/libc.so:
$(MAKE) -C ports musl CC=gcc
# <linux/*.h>, for programs that spell ioctls the way Linux does. See
# ports/linux-headers/Makefile.
LINUXHDR = $(MUSLPREFIX)/include/linux/input.h
$(LINUXHDR): $(MUSLPREFIX)/lib/libc.so
$(MAKE) -C ports linux-headers
# xready: the kernel calls Xorg makes before drawing, checked. Musl and
# dynamic, like evtest.
$(BUILD)/cmd/xready: cmd/xready/main.c $(MUSLPREFIX)/lib/libc.so $(LINUXHDR)
@mkdir -p $(BUILD)/cmd
$(DYNCC) -fno-pic -c $< -o $(BUILD)/cmd/xready.o
$(DYNCC) -no-pie -nostdlib $(DYNLD) -o $@ \
$(MUSLPREFIX)/lib/crt1.o $(MUSLPREFIX)/lib/crti.o $(BUILD)/cmd/xready.o \
-L$(MUSLPREFIX)/lib -lc $(PORTEND)
$(BUILD)/cmd/libdynhello.so: cmd/dynhello/lib.c $(MUSLPREFIX)/lib/libc.so
@mkdir -p $(BUILD)/cmd
$(DYNCC) -fPIC -c $< -o $(BUILD)/cmd/libdynhello.o