Re: [PATCH 1/4] fbcon: Make fbcon a built-time depency for fbdev

Bartlomiej Zolnierkiewicz
2017-08-01T15:43:15Z

On Wednesday, July 12, 2017 12:40:45 PM Bartlomiej Zolnierkiewicz wrote:
> On Thursday, July 06, 2017 02:57:32 PM Daniel Vetter wrote:
> > 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 50e244cc793d511b86adea24972f3a7264cae114
> > 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 daee779718a319ff9f83e1ba3339334ac650bb22
> > 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.
> >
> > Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
> > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
> > Cc: Linux Fbdev development list <linux-fbdev@vger.kernel.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> > Cc: dri-devel@lists.freedesktop.org
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Patch queued for 4.14, thanks.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics