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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 commit50e244cc79Author: 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 commitdaee779718Author: 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>
166 lines
5.7 KiB
Plaintext
166 lines
5.7 KiB
Plaintext
#
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# Video configuration
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#
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menu "Console display driver support"
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config VGA_CONSOLE
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bool "VGA text console" if EXPERT || !X86
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depends on !4xx && !PPC_8xx && !SPARC && !M68K && !PARISC && !FRV && \
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!SUPERH && !BLACKFIN && !AVR32 && !MN10300 && !CRIS && \
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(!ARM || ARCH_FOOTBRIDGE || ARCH_INTEGRATOR || ARCH_NETWINDER) && \
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!ARM64 && !ARC && !MICROBLAZE && !OPENRISC
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default y
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help
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Saying Y here will allow you to use Linux in text mode through a
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display that complies with the generic VGA standard. Virtually
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everyone wants that.
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The program SVGATextMode can be used to utilize SVGA video cards to
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their full potential in text mode. Download it from
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<ftp://ibiblio.org/pub/Linux/utils/console/>.
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Say Y.
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config VGACON_SOFT_SCROLLBACK
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bool "Enable Scrollback Buffer in System RAM"
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depends on VGA_CONSOLE
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default n
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help
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The scrollback buffer of the standard VGA console is located in
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the VGA RAM. The size of this RAM is fixed and is quite small.
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If you require a larger scrollback buffer, this can be placed in
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System RAM which is dynamically allocated during initialization.
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Placing the scrollback buffer in System RAM will slightly slow
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down the console.
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If you want this feature, say 'Y' here and enter the amount of
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RAM to allocate for this buffer. If unsure, say 'N'.
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config VGACON_SOFT_SCROLLBACK_SIZE
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int "Scrollback Buffer Size (in KB)"
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depends on VGACON_SOFT_SCROLLBACK
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range 1 1024
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default "64"
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help
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Enter the amount of System RAM to allocate for scrollback
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buffers of VGA consoles. Each 64KB will give you approximately
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16 80x25 screenfuls of scrollback buffer.
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config VGACON_SOFT_SCROLLBACK_PERSISTENT_ENABLE_BY_DEFAULT
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bool "Persistent Scrollback History for each console by default"
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depends on VGACON_SOFT_SCROLLBACK
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default n
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help
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Say Y here if the scrollback history should persist by default when
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switching between consoles. Otherwise, the scrollback history will be
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flushed each time the console is switched. This feature can also be
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enabled using the boot command line parameter
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'vgacon.scrollback_persistent=1'.
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This feature might break your tool of choice to flush the scrollback
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buffer, e.g. clear(1) will work fine but Debian's clear_console(1)
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will be broken, which might cause security issues.
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You can use the escape sequence \e[3J instead if this feature is
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activated.
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Note that a buffer of VGACON_SOFT_SCROLLBACK_SIZE is taken for each
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created tty device.
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So if you use a RAM-constrained system, say N here.
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config MDA_CONSOLE
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depends on !M68K && !PARISC && ISA
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tristate "MDA text console (dual-headed)"
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---help---
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Say Y here if you have an old MDA or monochrome Hercules graphics
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adapter in your system acting as a second head ( = video card). You
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will then be able to use two monitors with your Linux system. Do not
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say Y here if your MDA card is the primary card in your system; the
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normal VGA driver will handle it.
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To compile this driver as a module, choose M here: the
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module will be called mdacon.
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If unsure, say N.
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config SGI_NEWPORT_CONSOLE
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tristate "SGI Newport Console support"
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depends on SGI_IP22
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select FONT_SUPPORT
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help
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Say Y here if you want the console on the Newport aka XL graphics
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card of your Indy. Most people say Y here.
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config DUMMY_CONSOLE
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bool
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depends on VGA_CONSOLE!=y || SGI_NEWPORT_CONSOLE!=y
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default y
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config DUMMY_CONSOLE_COLUMNS
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int "Initial number of console screen columns"
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depends on DUMMY_CONSOLE && !ARM
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default 160 if PARISC
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default 80
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help
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On PA-RISC, the default value is 160, which should fit a 1280x1024
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monitor.
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Select 80 if you use a 640x480 resolution by default.
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config DUMMY_CONSOLE_ROWS
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int "Initial number of console screen rows"
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depends on DUMMY_CONSOLE && !ARM
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default 64 if PARISC
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default 25
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help
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On PA-RISC, the default value is 64, which should fit a 1280x1024
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monitor.
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Select 25 if you use a 640x480 resolution by default.
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config FRAMEBUFFER_CONSOLE
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bool "Framebuffer Console support"
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depends on FB && !UML
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select VT_HW_CONSOLE_BINDING
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select CRC32
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select FONT_SUPPORT
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help
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Low-level framebuffer-based console driver.
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config FRAMEBUFFER_CONSOLE_DETECT_PRIMARY
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bool "Map the console to the primary display device"
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depends on FRAMEBUFFER_CONSOLE
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default n
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---help---
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If this option is selected, the framebuffer console will
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automatically select the primary display device (if the architecture
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supports this feature). Otherwise, the framebuffer console will
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always select the first framebuffer driver that is loaded. The latter
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is the default behavior.
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You can always override the automatic selection of the primary device
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by using the fbcon=map: boot option.
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If unsure, select n.
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config FRAMEBUFFER_CONSOLE_ROTATION
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bool "Framebuffer Console Rotation"
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depends on FRAMEBUFFER_CONSOLE
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help
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Enable display rotation for the framebuffer console. This is done
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in software and may be significantly slower than a normally oriented
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display. Note that the rotation is done at the console level only
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such that other users of the framebuffer will remain normally
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oriented.
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config STI_CONSOLE
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bool "STI text console"
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depends on PARISC
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select FONT_SUPPORT
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default y
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help
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The STI console is the builtin display/keyboard on HP-PARISC
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machines. Say Y here to build support for it into your kernel.
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The alternative is to use your primary serial port as a console.
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endmenu
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