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linux-stable/include/linux/console.h
Petr Mladek a91bd6223e Revert "init/console: Use ttynull as a fallback when there is no console" 
This reverts commit 757055ae8d.

The commit caused that ttynull was used as the default console
on several systems[1][2][3]. As a result, the console was
blank even when a better alternative existed.

It happened when there was no console configured
on the command line and ttynull_init() was the first initcall
calling register_console().

Or it happened when /dev/ did not exist when console_on_rootfs()
was called. It was not able to open /dev/console even though
a console driver was registered. It tried to add ttynull console
but it obviously did not help. But ttynull became the preferred
console and was used by /dev/console when it was available later.

The commit tried to fix a historical problem that have been there
for ages. The primary motivation was the commit 3cffa06aee
("printk/console: Allow to disable console output by using console=""
 or console=null"). It provided a clean solution for a workaround
 that was widely used and worked only by chance.

This revert causes that the console="" or console=null command line
options will again work only by chance. These options will cause that
a particular console will be preferred and the default (tty) ones
will not get enabled. There will be no console registered at
all. As a result there won't be stdin, stdout, and stderr for
the init process. But it worked exactly this way even before.

The proper solution has to fulfill many conditions:

  + Register ttynull only when explicitly required or as
    the ultimate fallback.

  + ttynull should get associated with /dev/console but it must
    not become preferred console when used as a fallback.
    Especially, it must still be possible to replace it
    by a better console later.

Such a change requires clean up of the register_console() code.
Otherwise, it would be even harder to follow. Especially, the use
of has_preferred_console and CON_CONSDEV flag is tricky. The clean
up is risky. The ordering of consoles is not well defined. And
any changes tend to break existing user settings.

Do the revert at the least risky solution for now.

[1] https://lore.kernel.org/linux-kselftest/20201221144302.GR4077@smile.fi.intel.com/
[2] https://lore.kernel.org/lkml/d2a3b3c0-e548-7dd1-730f-59bc5c04e191@synopsys.com/
[3] https://patchwork.ozlabs.org/project/linux-um/patch/20210105120128.10854-1-thomas@m3y3r.de/

Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reported-by: Vineet Gupta <vgupta@synopsys.com>
Reported-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2021-01-08 11:02:18 -08:00

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/*
* linux/include/linux/console.h
*
* Copyright (C) 1993 Hamish Macdonald
*
* This file is subject to the terms and conditions of the GNU General Public
* License. See the file COPYING in the main directory of this archive
* for more details.
*
* Changed:
* 10-Mar-94: Arno Griffioen: Conversion for vt100 emulator port from PC LINUX
*/
#ifndef _LINUX_CONSOLE_H_
#define _LINUX_CONSOLE_H_ 1
#include <linux/atomic.h>
#include <linux/types.h>
struct vc_data;
struct console_font_op;
struct console_font;
struct module;
struct tty_struct;
struct notifier_block;
enum con_scroll {
SM_UP,
SM_DOWN,
};
enum vc_intensity;
/**
* struct consw - callbacks for consoles
*
* @con_scroll: move lines from @top to @bottom in direction @dir by @lines.
* Return true if no generic handling should be done.
* Invoked by csi_M and printing to the console.
* @con_set_palette: sets the palette of the console to @table (optional)
* @con_scrolldelta: the contents of the console should be scrolled by @lines.
* Invoked by user. (optional)
*/
struct consw {
struct module *owner;
const char *(*con_startup)(void);
void (*con_init)(struct vc_data *vc, int init);
void (*con_deinit)(struct vc_data *vc);
void (*con_clear)(struct vc_data *vc, int sy, int sx, int height,
int width);
void (*con_putc)(struct vc_data *vc, int c, int ypos, int xpos);
void (*con_putcs)(struct vc_data *vc, const unsigned short *s,
int count, int ypos, int xpos);
void (*con_cursor)(struct vc_data *vc, int mode);
bool (*con_scroll)(struct vc_data *vc, unsigned int top,
unsigned int bottom, enum con_scroll dir,
unsigned int lines);
int (*con_switch)(struct vc_data *vc);
int (*con_blank)(struct vc_data *vc, int blank, int mode_switch);
int (*con_font_set)(struct vc_data *vc, struct console_font *font,
unsigned int flags);
int (*con_font_get)(struct vc_data *vc, struct console_font *font);
int (*con_font_default)(struct vc_data *vc,
struct console_font *font, char *name);
int (*con_resize)(struct vc_data *vc, unsigned int width,
unsigned int height, unsigned int user);
void (*con_set_palette)(struct vc_data *vc,
const unsigned char *table);
void (*con_scrolldelta)(struct vc_data *vc, int lines);
int (*con_set_origin)(struct vc_data *vc);
void (*con_save_screen)(struct vc_data *vc);
u8 (*con_build_attr)(struct vc_data *vc, u8 color,
enum vc_intensity intensity,
bool blink, bool underline, bool reverse, bool italic);
void (*con_invert_region)(struct vc_data *vc, u16 *p, int count);
u16 *(*con_screen_pos)(const struct vc_data *vc, int offset);
unsigned long (*con_getxy)(struct vc_data *vc, unsigned long position,
int *px, int *py);
/*
* Flush the video console driver's scrollback buffer
*/
void (*con_flush_scrollback)(struct vc_data *vc);
/*
* Prepare the console for the debugger. This includes, but is not
* limited to, unblanking the console, loading an appropriate
* palette, and allowing debugger generated output.
*/
int (*con_debug_enter)(struct vc_data *vc);
/*
* Restore the console to its pre-debug state as closely as possible.
*/
int (*con_debug_leave)(struct vc_data *vc);
};
extern const struct consw *conswitchp;
extern const struct consw dummy_con; /* dummy console buffer */
extern const struct consw vga_con; /* VGA text console */
extern const struct consw newport_con; /* SGI Newport console */
int con_is_bound(const struct consw *csw);
int do_unregister_con_driver(const struct consw *csw);
int do_take_over_console(const struct consw *sw, int first, int last, int deflt);
void give_up_console(const struct consw *sw);
#ifdef CONFIG_HW_CONSOLE
int con_debug_enter(struct vc_data *vc);
int con_debug_leave(void);
#else
static inline int con_debug_enter(struct vc_data *vc)
{
return 0;
}
static inline int con_debug_leave(void)
{
return 0;
}
#endif
/* cursor */
#define CM_DRAW (1)
#define CM_ERASE (2)
#define CM_MOVE (3)
/*
* The interface for a console, or any other device that wants to capture
* console messages (printer driver?)
*
* If a console driver is marked CON_BOOT then it will be auto-unregistered
* when the first real console is registered. This is for early-printk drivers.
*/
#define CON_PRINTBUFFER (1)
#define CON_CONSDEV (2) /* Preferred console, /dev/console */
#define CON_ENABLED (4)
#define CON_BOOT (8)
#define CON_ANYTIME (16) /* Safe to call when cpu is offline */
#define CON_BRL (32) /* Used for a braille device */
#define CON_EXTENDED (64) /* Use the extended output format a la /dev/kmsg */
struct console {
char name[16];
void (*write)(struct console *, const char *, unsigned);
int (*read)(struct console *, char *, unsigned);
struct tty_driver *(*device)(struct console *, int *);
void (*unblank)(void);
int (*setup)(struct console *, char *);
int (*exit)(struct console *);
int (*match)(struct console *, char *name, int idx, char *options);
short flags;
short index;
int cflag;
void *data;
struct console *next;
};
/*
* for_each_console() allows you to iterate on each console
*/
#define for_each_console(con) \
for (con = console_drivers; con != NULL; con = con->next)
extern int console_set_on_cmdline;
extern struct console *early_console;
enum con_flush_mode {
CONSOLE_FLUSH_PENDING,
CONSOLE_REPLAY_ALL,
};
extern int add_preferred_console(char *name, int idx, char *options);
extern void register_console(struct console *);
extern int unregister_console(struct console *);
extern struct console *console_drivers;
extern void console_lock(void);
extern int console_trylock(void);
extern void console_unlock(void);
extern void console_conditional_schedule(void);
extern void console_unblank(void);
extern void console_flush_on_panic(enum con_flush_mode mode);
extern struct tty_driver *console_device(int *);
extern void console_stop(struct console *);
extern void console_start(struct console *);
extern int is_console_locked(void);
extern int braille_register_console(struct console *, int index,
char *console_options, char *braille_options);
extern int braille_unregister_console(struct console *);
#ifdef CONFIG_TTY
extern void console_sysfs_notify(void);
#else
static inline void console_sysfs_notify(void)
{ }
#endif
extern bool console_suspend_enabled;
/* Suspend and resume console messages over PM events */
extern void suspend_console(void);
extern void resume_console(void);
int mda_console_init(void);
void vcs_make_sysfs(int index);
void vcs_remove_sysfs(int index);
/* Some debug stub to catch some of the obvious races in the VT code */
#define WARN_CONSOLE_UNLOCKED() \
WARN_ON(!atomic_read(&ignore_console_lock_warning) && \
!is_console_locked() && !oops_in_progress)
/*
* Increment ignore_console_lock_warning if you need to quiet
* WARN_CONSOLE_UNLOCKED() for debugging purposes.
*/
extern atomic_t ignore_console_lock_warning;
/* VESA Blanking Levels */
#define VESA_NO_BLANKING 0
#define VESA_VSYNC_SUSPEND 1
#define VESA_HSYNC_SUSPEND 2
#define VESA_POWERDOWN 3
#ifdef CONFIG_VGA_CONSOLE
extern bool vgacon_text_force(void);
#else
static inline bool vgacon_text_force(void) { return false; }
#endif
extern void console_init(void);
/* For deferred console takeover */
void dummycon_register_output_notifier(struct notifier_block *nb);
void dummycon_unregister_output_notifier(struct notifier_block *nb);
#endif /* _LINUX_CONSOLE_H */
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