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linux-next/drivers/video/console/Kconfig
Daniel Vetter 6104c37094 fbcon: Make fbcon a built-time depency for fbdev 
There's a bunch of folks who're trying to make printk less
contended and faster, but there's a problem: printk uses the
console_lock, and the console lock has become the BKL for all things
fbdev/fbcon, which in turn pulled in half the drm subsystem under that
lock. That's awkward.

There reasons for that is probably just a historical accident:

- fbcon is a runtime option of fbdev, i.e. at runtime you can pick
  whether your fbdev driver instances are used as kernel consoles.
  Unfortunately this wasn't implemented with some module option, but
  through some module loading magic: As long as you don't load
  fbcon.ko, there's no fbdev console support, but loading it (in any
  order wrt fbdev drivers) will create console instances for all fbdev
  drivers.

- This was implemented through a notifier chain. fbcon.ko enumerates
  all fbdev instances at load time and also registers itself as
  listener in the fbdev notifier. The fbdev core tries to register new
  fbdev instances with fbcon using the notifier.

- On top of that the modifier chain is also used at runtime by the
  fbdev subsystem to e.g. control backlights for panels.

- The problem is that the notifier puts a mutex locking context
  between fbdev and fbcon, which mixes up the locking contexts for
  both the runtime usage and the register time usage to notify fbcon.
  And at runtime fbcon (through the fbdev core) might call into the
  notifier from a printk critical section while console_lock is held.

- This means console_lock must be an outer lock for the entire fbdev
  subsystem, which also means it must be acquired when registering a
  new framebuffer driver as the outermost lock since we might call
  into fbcon (through the notifier) which would result in a locking
  inversion if fbcon would acquire the console_lock from its notifier
  callback (which it needs to register the console).

- console_lock can be held anywhere, since printk can be called
  anywhere, and through the above story, plus drm/kms being an fbdev
  driver, we pull in a shocking amount of locking hiercharchy
  underneath the console_lock. Which makes cleaning up printk really
  hard (not even splitting console_lock into an rwsem is all that
  useful due to this).

There's various ways to address this, but the cleanest would be to
make fbcon a compile-time option, where fbdev directly calls the fbcon
register functions from register_framebuffer, or dummy static inline
versions if fbcon is disabled. Maybe augmented with a runtime knob to
disable fbcon, if that's needed (for debugging perhaps).

But this could break some users who rely on the magic "loading
fbcon.ko enables/disables fbdev framebuffers at runtime" thing, even
if that's unlikely. Hence we must be careful:

1. Create a compile-time dependency between fbcon and fbdev in the
least minimal way. This is what this patch does.

2. Wait at least 1 year to give possible users time to scream about
how we broke their setup. Unlikely, since all distros make fbcon
compile-in, and embedded platforms only compile stuff they know they
need anyway. But still.

3. Convert the notifier to direct functions calls, with dummy static
inlines if fbcon is disabled. We'll still need the fb notifier for the
other uses (like backlights), but we can probably move it into the fb
core (atm it must be built-into vmlinux).

4. Push console_lock down the call-chain, until it is down in
console_register again.

5. Finally start to clean up and rework the printk/console locking.

For context of this saga see

commit 50e244cc79
Author: Alan Cox <alan@linux.intel.com>
Date:   Fri Jan 25 10:28:15 2013 +1000

    fb: rework locking to fix lock ordering on takeover

plus the pile of commits on top that tried to make this all work
without terminally upsetting lockdep. We've uncovered all this when
console_lock lockdep annotations where added in

commit daee779718
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Sep 22 19:52:11 2012 +0200

    console: implement lockdep support for console_lock

On the patch itself:
- Switch CONFIG_FRAMEBUFFER_CONSOLE to be a boolean, using the overall
  CONFIG_FB tristate to decided whether it should be a module or
  built-in.

- At first I thought I could force the build depency with just a dummy
  symbol that fbcon.ko exports and fb.ko uses. But that leads to a
  module depency cycle (it works fine when built-in).

  Since this tight binding is the entire goal the simplest solution is
  to move all the fbcon modules (and there's a bunch of optinal
  source-files which are each modules of their own, for no good
  reason) into the overall fb.ko core module. That's a bit more than
  what I would have liked to do in this patch, but oh well.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
2017-08-01 17:32:07 +02:00

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#
# Video configuration
#
menu "Console display driver support"
config VGA_CONSOLE
bool "VGA text console" if EXPERT || !X86
depends on !4xx && !PPC_8xx && !SPARC && !M68K && !PARISC && !FRV && \
!SUPERH && !BLACKFIN && !AVR32 && !MN10300 && !CRIS && \
(!ARM || ARCH_FOOTBRIDGE || ARCH_INTEGRATOR || ARCH_NETWINDER) && \
!ARM64 && !ARC && !MICROBLAZE && !OPENRISC
default y
help
Saying Y here will allow you to use Linux in text mode through a
display that complies with the generic VGA standard. Virtually
everyone wants that.
The program SVGATextMode can be used to utilize SVGA video cards to
their full potential in text mode. Download it from
<ftp://ibiblio.org/pub/Linux/utils/console/>.
Say Y.
config VGACON_SOFT_SCROLLBACK
bool "Enable Scrollback Buffer in System RAM"
depends on VGA_CONSOLE
default n
help
The scrollback buffer of the standard VGA console is located in
the VGA RAM. The size of this RAM is fixed and is quite small.
If you require a larger scrollback buffer, this can be placed in
System RAM which is dynamically allocated during initialization.
Placing the scrollback buffer in System RAM will slightly slow
down the console.
If you want this feature, say 'Y' here and enter the amount of
RAM to allocate for this buffer. If unsure, say 'N'.
config VGACON_SOFT_SCROLLBACK_SIZE
int "Scrollback Buffer Size (in KB)"
depends on VGACON_SOFT_SCROLLBACK
range 1 1024
default "64"
help
Enter the amount of System RAM to allocate for scrollback
buffers of VGA consoles. Each 64KB will give you approximately
16 80x25 screenfuls of scrollback buffer.
config VGACON_SOFT_SCROLLBACK_PERSISTENT_ENABLE_BY_DEFAULT
bool "Persistent Scrollback History for each console by default"
depends on VGACON_SOFT_SCROLLBACK
default n
help
Say Y here if the scrollback history should persist by default when
switching between consoles. Otherwise, the scrollback history will be
flushed each time the console is switched. This feature can also be
enabled using the boot command line parameter
'vgacon.scrollback_persistent=1'.
This feature might break your tool of choice to flush the scrollback
buffer, e.g. clear(1) will work fine but Debian's clear_console(1)
will be broken, which might cause security issues.
You can use the escape sequence \e[3J instead if this feature is
activated.
Note that a buffer of VGACON_SOFT_SCROLLBACK_SIZE is taken for each
created tty device.
So if you use a RAM-constrained system, say N here.
config MDA_CONSOLE
depends on !M68K && !PARISC && ISA
tristate "MDA text console (dual-headed)"
---help---
Say Y here if you have an old MDA or monochrome Hercules graphics
adapter in your system acting as a second head ( = video card). You
will then be able to use two monitors with your Linux system. Do not
say Y here if your MDA card is the primary card in your system; the
normal VGA driver will handle it.
To compile this driver as a module, choose M here: the
module will be called mdacon.
If unsure, say N.
config SGI_NEWPORT_CONSOLE
tristate "SGI Newport Console support"
depends on SGI_IP22
select FONT_SUPPORT
help
Say Y here if you want the console on the Newport aka XL graphics
card of your Indy. Most people say Y here.
config DUMMY_CONSOLE
bool
depends on VGA_CONSOLE!=y || SGI_NEWPORT_CONSOLE!=y
default y
config DUMMY_CONSOLE_COLUMNS
int "Initial number of console screen columns"
depends on DUMMY_CONSOLE && !ARM
default 160 if PARISC
default 80
help
On PA-RISC, the default value is 160, which should fit a 1280x1024
monitor.
Select 80 if you use a 640x480 resolution by default.
config DUMMY_CONSOLE_ROWS
int "Initial number of console screen rows"
depends on DUMMY_CONSOLE && !ARM
default 64 if PARISC
default 25
help
On PA-RISC, the default value is 64, which should fit a 1280x1024
monitor.
Select 25 if you use a 640x480 resolution by default.
config FRAMEBUFFER_CONSOLE
bool "Framebuffer Console support"
depends on FB && !UML
select VT_HW_CONSOLE_BINDING
select CRC32
select FONT_SUPPORT
help
Low-level framebuffer-based console driver.
config FRAMEBUFFER_CONSOLE_DETECT_PRIMARY
bool "Map the console to the primary display device"
depends on FRAMEBUFFER_CONSOLE
default n
---help---
If this option is selected, the framebuffer console will
automatically select the primary display device (if the architecture
supports this feature). Otherwise, the framebuffer console will
always select the first framebuffer driver that is loaded. The latter
is the default behavior.
You can always override the automatic selection of the primary device
by using the fbcon=map: boot option.
If unsure, select n.
config FRAMEBUFFER_CONSOLE_ROTATION
bool "Framebuffer Console Rotation"
depends on FRAMEBUFFER_CONSOLE
help
Enable display rotation for the framebuffer console. This is done
in software and may be significantly slower than a normally oriented
display. Note that the rotation is done at the console level only
such that other users of the framebuffer will remain normally
oriented.
config STI_CONSOLE
bool "STI text console"
depends on PARISC
select FONT_SUPPORT
default y
help
The STI console is the builtin display/keyboard on HP-PARISC
machines. Say Y here to build support for it into your kernel.
The alternative is to use your primary serial port as a console.
endmenu
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