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

Commit Message

Daniel Vetter July 6, 2017, 12:57 p.m. UTC

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 <[e-mail]>
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 <[e-mail]>
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 <[e-mail]>
Cc: Sergey Senozhatsky <[e-mail]>
Cc: Linux Fbdev development list <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Bartlomiej Zolnierkiewicz <[e-mail]>
Cc: [e-mail]
Signed-off-by: Daniel Vetter <[e-mail]>
---
v2: Switch to building fbcon code into fb.ko right away because the
cheap trick leads to a module depency loop.
---
 drivers/video/console/Kconfig                        |  2 +-
 drivers/video/console/Makefile                       |  8 --------
 drivers/video/fbdev/core/Makefile                    | 11 +++++++++++
 drivers/video/{console => fbdev/core}/bitblit.c      |  4 ----
 drivers/video/{console => fbdev/core}/fbcon.c        | 13 +++----------
 drivers/video/{console => fbdev/core}/fbcon.h        |  0
 drivers/video/{console => fbdev/core}/fbcon_ccw.c    |  4 ----
 drivers/video/{console => fbdev/core}/fbcon_cw.c     |  4 ----
 drivers/video/{console => fbdev/core}/fbcon_rotate.c |  4 ----
 drivers/video/{console => fbdev/core}/fbcon_rotate.h |  0
 drivers/video/{console => fbdev/core}/fbcon_ud.c     |  4 ----
 drivers/video/fbdev/core/fbmem.c                     |  6 ++++++
 drivers/video/{console => fbdev/core}/softcursor.c   |  4 ----
 drivers/video/{console => fbdev/core}/tileblit.c     |  5 -----
 include/linux/fbcon.h                                | 12 ++++++++++++
 15 files changed, 33 insertions(+), 48 deletions(-)
 rename drivers/video/{console => fbdev/core}/bitblit.c (98%)
 rename drivers/video/{console => fbdev/core}/fbcon.c (99%)
 rename drivers/video/{console => fbdev/core}/fbcon.h (100%)
 rename drivers/video/{console => fbdev/core}/fbcon_ccw.c (98%)
 rename drivers/video/{console => fbdev/core}/fbcon_cw.c (98%)
 rename drivers/video/{console => fbdev/core}/fbcon_rotate.c (95%)
 rename drivers/video/{console => fbdev/core}/fbcon_rotate.h (100%)
 rename drivers/video/{console => fbdev/core}/fbcon_ud.c (98%)
 rename drivers/video/{console => fbdev/core}/softcursor.c (93%)
 rename drivers/video/{console => fbdev/core}/tileblit.c (96%)
 create mode 100644 include/linux/fbcon.h

Comments

Bartlomiej Zolnierkiewicz July 12, 2017, 10:40 a.m. UTC | #1

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 <[e-mail]>
> 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 <[e-mail]>
> 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 <[e-mail]>
> Cc: Sergey Senozhatsky <[e-mail]>
> Cc: Linux Fbdev development list <[e-mail]>
> Cc: Steven Rostedt <[e-mail]>
> Cc: Bartlomiej Zolnierkiewicz <[e-mail]>
> Cc: [e-mail]
> Signed-off-by: Daniel Vetter <[e-mail]>

Acked-by: Bartlomiej Zolnierkiewicz <[e-mail]>

> ---
> v2: Switch to building fbcon code into fb.ko right away because the
> cheap trick leads to a module depency loop.
> ---

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

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Sean Paul July 13, 2017, 2:47 p.m. UTC | #2

On Thu, Jul 06, 2017 at 02:57:32PM +0200, Daniel Vetter wrote:

<snip>

> Cc: [e-mail]
> Signed-off-by: Daniel Vetter <[e-mail]>

Just 2 nits, code looks good.

Reviewed-by: Sean Paul <[e-mail]>

> ---
> v2: Switch to building fbcon code into fb.ko right away because the
> cheap trick leads to a module depency loop.
> ---
>  drivers/video/console/Kconfig                        |  2 +-
>  drivers/video/console/Makefile                       |  8 --------
>  drivers/video/fbdev/core/Makefile                    | 11 +++++++++++
>  drivers/video/{console => fbdev/core}/bitblit.c      |  4 ----
>  drivers/video/{console => fbdev/core}/fbcon.c        | 13 +++----------
>  drivers/video/{console => fbdev/core}/fbcon.h        |  0
>  drivers/video/{console => fbdev/core}/fbcon_ccw.c    |  4 ----
>  drivers/video/{console => fbdev/core}/fbcon_cw.c     |  4 ----
>  drivers/video/{console => fbdev/core}/fbcon_rotate.c |  4 ----
>  drivers/video/{console => fbdev/core}/fbcon_rotate.h |  0
>  drivers/video/{console => fbdev/core}/fbcon_ud.c     |  4 ----
>  drivers/video/fbdev/core/fbmem.c                     |  6 ++++++
>  drivers/video/{console => fbdev/core}/softcursor.c   |  4 ----
>  drivers/video/{console => fbdev/core}/tileblit.c     |  5 -----
>  include/linux/fbcon.h                                | 12 ++++++++++++
>  15 files changed, 33 insertions(+), 48 deletions(-)
>  rename drivers/video/{console => fbdev/core}/bitblit.c (98%)
>  rename drivers/video/{console => fbdev/core}/fbcon.c (99%)
>  rename drivers/video/{console => fbdev/core}/fbcon.h (100%)
>  rename drivers/video/{console => fbdev/core}/fbcon_ccw.c (98%)
>  rename drivers/video/{console => fbdev/core}/fbcon_cw.c (98%)
>  rename drivers/video/{console => fbdev/core}/fbcon_rotate.c (95%)
>  rename drivers/video/{console => fbdev/core}/fbcon_rotate.h (100%)
>  rename drivers/video/{console => fbdev/core}/fbcon_ud.c (98%)
>  rename drivers/video/{console => fbdev/core}/softcursor.c (93%)
>  rename drivers/video/{console => fbdev/core}/tileblit.c (96%)
>  create mode 100644 include/linux/fbcon.h
> 
> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
> index 2111d06f8c81..7f1f1fbcef9e 100644
> --- a/drivers/video/console/Kconfig
> +++ b/drivers/video/console/Kconfig
> @@ -117,7 +117,7 @@ config DUMMY_CONSOLE_ROWS
>            Select 25 if you use a 640x480 resolution by default.
>  
>  config FRAMEBUFFER_CONSOLE
> -	tristate "Framebuffer Console support"
> +	bool "Framebuffer Console support"
>  	depends on FB && !UML
>  	select VT_HW_CONSOLE_BINDING
>  	select CRC32
> diff --git a/drivers/video/console/Makefile b/drivers/video/console/Makefile
> index 43bfa485db96..eb2cbec52643 100644
> --- a/drivers/video/console/Makefile
> +++ b/drivers/video/console/Makefile
> @@ -7,13 +7,5 @@ obj-$(CONFIG_SGI_NEWPORT_CONSOLE) += newport_con.o
>  obj-$(CONFIG_STI_CONSOLE)         += sticon.o sticore.o
>  obj-$(CONFIG_VGA_CONSOLE)         += vgacon.o
>  obj-$(CONFIG_MDA_CONSOLE)         += mdacon.o
> -obj-$(CONFIG_FRAMEBUFFER_CONSOLE) += fbcon.o bitblit.o softcursor.o
> -ifeq ($(CONFIG_FB_TILEBLITTING),y)
> -obj-$(CONFIG_FRAMEBUFFER_CONSOLE)     += tileblit.o
> -endif
> -ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE_ROTATION),y)
> -obj-$(CONFIG_FRAMEBUFFER_CONSOLE)     += fbcon_rotate.o fbcon_cw.o fbcon_ud.o \
> -                                         fbcon_ccw.o
> -endif
>  
>  obj-$(CONFIG_FB_STI)              += sticore.o
> diff --git a/drivers/video/fbdev/core/Makefile b/drivers/video/fbdev/core/Makefile
> index 9e3ddf225393..0214b863ac3f 100644
> --- a/drivers/video/fbdev/core/Makefile
> +++ b/drivers/video/fbdev/core/Makefile
> @@ -4,6 +4,17 @@ obj-$(CONFIG_FB)                  += fb.o
>  fb-y                              := fbmem.o fbmon.o fbcmap.o fbsysfs.o \
>                                       modedb.o fbcvt.o
>  fb-$(CONFIG_FB_DEFERRED_IO)       += fb_defio.o
> +
> +ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE),y)
> +fb-y				  += fbcon.o bitblit.o softcursor.o

fb-$(CONFIG_FRAMEBUFFER_CONSOLE) ?

> +ifeq ($(CONFIG_FB_TILEBLITTING),y)
> +fb-y				  += tileblit.o
> +endif
> +ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE_ROTATION),y)
> +fb-y				  += fbcon_rotate.o fbcon_cw.o fbcon_ud.o \
> +				     fbcon_ccw.o
> +endif
> +endif
>  fb-objs                           := $(fb-y)
>  
>  obj-$(CONFIG_FB_CFB_FILLRECT)  += cfbfillrect.o

<snip>

> diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
> new file mode 100644
> index 000000000000..0fac6305d51c
> --- /dev/null
> +++ b/include/linux/fbcon.h

IANAL, but it seems like you're missing some GPL legalese here?

Sean

> @@ -0,0 +1,12 @@
> +#ifndef _LINUX_FBCON_H
> +#define _LINUX_FBCON_H
> +
> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE
> +void __init fb_console_init(void);
> +void __exit fb_console_exit(void);
> +#else
> +static void fb_console_init(void) {}
> +static void fb_console_exit(void) {}
> +#endif
> +
> +#endif /* _LINUX_FBCON_H */
> -- 
> 2.13.2
> 
> _______________________________________________
> dri-devel mailing list
> [e-mail]
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

Daniel Vetter July 13, 2017, 6:49 p.m. UTC | #3

On Thu, Jul 13, 2017 at 4:47 PM, Sean Paul <[e-mail]> wrote:
> On Thu, Jul 06, 2017 at 02:57:32PM +0200, Daniel Vetter wrote:
>
> <snip>
>
>> Cc: [e-mail]
>> Signed-off-by: Daniel Vetter <[e-mail]>
>
> Just 2 nits, code looks good.
>
> Reviewed-by: Sean Paul <[e-mail]>
>
>> ---
>> v2: Switch to building fbcon code into fb.ko right away because the
>> cheap trick leads to a module depency loop.
>> ---
>>  drivers/video/console/Kconfig                        |  2 +-
>>  drivers/video/console/Makefile                       |  8 --------
>>  drivers/video/fbdev/core/Makefile                    | 11 +++++++++++
>>  drivers/video/{console => fbdev/core}/bitblit.c      |  4 ----
>>  drivers/video/{console => fbdev/core}/fbcon.c        | 13 +++----------
>>  drivers/video/{console => fbdev/core}/fbcon.h        |  0
>>  drivers/video/{console => fbdev/core}/fbcon_ccw.c    |  4 ----
>>  drivers/video/{console => fbdev/core}/fbcon_cw.c     |  4 ----
>>  drivers/video/{console => fbdev/core}/fbcon_rotate.c |  4 ----
>>  drivers/video/{console => fbdev/core}/fbcon_rotate.h |  0
>>  drivers/video/{console => fbdev/core}/fbcon_ud.c     |  4 ----
>>  drivers/video/fbdev/core/fbmem.c                     |  6 ++++++
>>  drivers/video/{console => fbdev/core}/softcursor.c   |  4 ----
>>  drivers/video/{console => fbdev/core}/tileblit.c     |  5 -----
>>  include/linux/fbcon.h                                | 12 ++++++++++++
>>  15 files changed, 33 insertions(+), 48 deletions(-)
>>  rename drivers/video/{console => fbdev/core}/bitblit.c (98%)
>>  rename drivers/video/{console => fbdev/core}/fbcon.c (99%)
>>  rename drivers/video/{console => fbdev/core}/fbcon.h (100%)
>>  rename drivers/video/{console => fbdev/core}/fbcon_ccw.c (98%)
>>  rename drivers/video/{console => fbdev/core}/fbcon_cw.c (98%)
>>  rename drivers/video/{console => fbdev/core}/fbcon_rotate.c (95%)
>>  rename drivers/video/{console => fbdev/core}/fbcon_rotate.h (100%)
>>  rename drivers/video/{console => fbdev/core}/fbcon_ud.c (98%)
>>  rename drivers/video/{console => fbdev/core}/softcursor.c (93%)
>>  rename drivers/video/{console => fbdev/core}/tileblit.c (96%)
>>  create mode 100644 include/linux/fbcon.h
>>
>> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
>> index 2111d06f8c81..7f1f1fbcef9e 100644
>> --- a/drivers/video/console/Kconfig
>> +++ b/drivers/video/console/Kconfig
>> @@ -117,7 +117,7 @@ config DUMMY_CONSOLE_ROWS
>>            Select 25 if you use a 640x480 resolution by default.
>>
>>  config FRAMEBUFFER_CONSOLE
>> -     tristate "Framebuffer Console support"
>> +     bool "Framebuffer Console support"
>>       depends on FB && !UML
>>       select VT_HW_CONSOLE_BINDING
>>       select CRC32
>> diff --git a/drivers/video/console/Makefile b/drivers/video/console/Makefile
>> index 43bfa485db96..eb2cbec52643 100644
>> --- a/drivers/video/console/Makefile
>> +++ b/drivers/video/console/Makefile
>> @@ -7,13 +7,5 @@ obj-$(CONFIG_SGI_NEWPORT_CONSOLE) += newport_con.o
>>  obj-$(CONFIG_STI_CONSOLE)         += sticon.o sticore.o
>>  obj-$(CONFIG_VGA_CONSOLE)         += vgacon.o
>>  obj-$(CONFIG_MDA_CONSOLE)         += mdacon.o
>> -obj-$(CONFIG_FRAMEBUFFER_CONSOLE) += fbcon.o bitblit.o softcursor.o
>> -ifeq ($(CONFIG_FB_TILEBLITTING),y)
>> -obj-$(CONFIG_FRAMEBUFFER_CONSOLE)     += tileblit.o
>> -endif
>> -ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE_ROTATION),y)
>> -obj-$(CONFIG_FRAMEBUFFER_CONSOLE)     += fbcon_rotate.o fbcon_cw.o fbcon_ud.o \
>> -                                         fbcon_ccw.o
>> -endif
>>
>>  obj-$(CONFIG_FB_STI)              += sticore.o
>> diff --git a/drivers/video/fbdev/core/Makefile b/drivers/video/fbdev/core/Makefile
>> index 9e3ddf225393..0214b863ac3f 100644
>> --- a/drivers/video/fbdev/core/Makefile
>> +++ b/drivers/video/fbdev/core/Makefile
>> @@ -4,6 +4,17 @@ obj-$(CONFIG_FB)                  += fb.o
>>  fb-y                              := fbmem.o fbmon.o fbcmap.o fbsysfs.o \
>>                                       modedb.o fbcvt.o
>>  fb-$(CONFIG_FB_DEFERRED_IO)       += fb_defio.o
>> +
>> +ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE),y)
>> +fb-y                           += fbcon.o bitblit.o softcursor.o
>
> fb-$(CONFIG_FRAMEBUFFER_CONSOLE) ?

The ifeq is longer than just this line ...

>> +ifeq ($(CONFIG_FB_TILEBLITTING),y)
>> +fb-y                           += tileblit.o
>> +endif
>> +ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE_ROTATION),y)
>> +fb-y                           += fbcon_rotate.o fbcon_cw.o fbcon_ud.o \
>> +                                  fbcon_ccw.oli
>> +endif
>> +endif
>>  fb-objs                           := $(fb-y)
>>
>>  obj-$(CONFIG_FB_CFB_FILLRECT)  += cfbfillrect.o
>
> <snip>
>
>> diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
>> new file mode 100644
>> index 000000000000..0fac6305d51c
>> --- /dev/null
>> +++ b/include/linux/fbcon.h
>
> IANAL, but it seems like you're missing some GPL legalese here?

There's a pile of headers which don't have the GPL header (like
linux/fb.h), I went with the local style ... not strictly necessary I
think.
-Daniel

>
> Sean
>
>> @@ -0,0 +1,12 @@
>> +#ifndef _LINUX_FBCON_H
>> +#define _LINUX_FBCON_H
>> +
>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE
>> +void __init fb_console_init(void);
>> +void __exit fb_console_exit(void);
>> +#else
>> +static void fb_console_init(void) {}
>> +static void fb_console_exit(void) {}
>> +#endif
>> +
>> +#endif /* _LINUX_FBCON_H */
>> --
>> 2.13.2
>>
>> _______________________________________________
>> dri-devel mailing list
>> [e-mail]
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Sean Paul, Software Engineer, Google / Chromium OS

Bartlomiej Zolnierkiewicz Aug. 1, 2017, 3:43 p.m. UTC | #4

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 <[e-mail]>
> > 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 <[e-mail]>
> > 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 <[e-mail]>
> > Cc: Sergey Senozhatsky <[e-mail]>
> > Cc: Linux Fbdev development list <[e-mail]>
> > Cc: Steven Rostedt <[e-mail]>
> > Cc: Bartlomiej Zolnierkiewicz <[e-mail]>
> > Cc: [e-mail]
> > Signed-off-by: Daniel Vetter <[e-mail]>

Patch queued for 4.14, thanks.

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

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Patch

diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
index 2111d06f8c81..7f1f1fbcef9e 100644
--- a/drivers/video/console/Kconfig
+++ b/drivers/video/console/Kconfig
@@ -117,7 +117,7 @@  config DUMMY_CONSOLE_ROWS
           Select 25 if you use a 640x480 resolution by default.
 
 config FRAMEBUFFER_CONSOLE
-	tristate "Framebuffer Console support"
+	bool "Framebuffer Console support"
 	depends on FB && !UML
 	select VT_HW_CONSOLE_BINDING
 	select CRC32
diff --git a/drivers/video/console/Makefile b/drivers/video/console/Makefile
index 43bfa485db96..eb2cbec52643 100644
--- a/drivers/video/console/Makefile
+++ b/drivers/video/console/Makefile
@@ -7,13 +7,5 @@  obj-$(CONFIG_SGI_NEWPORT_CONSOLE) += newport_con.o
 obj-$(CONFIG_STI_CONSOLE)         += sticon.o sticore.o
 obj-$(CONFIG_VGA_CONSOLE)         += vgacon.o
 obj-$(CONFIG_MDA_CONSOLE)         += mdacon.o
-obj-$(CONFIG_FRAMEBUFFER_CONSOLE) += fbcon.o bitblit.o softcursor.o
-ifeq ($(CONFIG_FB_TILEBLITTING),y)
-obj-$(CONFIG_FRAMEBUFFER_CONSOLE)     += tileblit.o
-endif
-ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE_ROTATION),y)
-obj-$(CONFIG_FRAMEBUFFER_CONSOLE)     += fbcon_rotate.o fbcon_cw.o fbcon_ud.o \
-                                         fbcon_ccw.o
-endif
 
 obj-$(CONFIG_FB_STI)              += sticore.o
diff --git a/drivers/video/fbdev/core/Makefile b/drivers/video/fbdev/core/Makefile
index 9e3ddf225393..0214b863ac3f 100644
--- a/drivers/video/fbdev/core/Makefile
+++ b/drivers/video/fbdev/core/Makefile
@@ -4,6 +4,17 @@  obj-$(CONFIG_FB)                  += fb.o
 fb-y                              := fbmem.o fbmon.o fbcmap.o fbsysfs.o \
                                      modedb.o fbcvt.o
 fb-$(CONFIG_FB_DEFERRED_IO)       += fb_defio.o
+
+ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE),y)
+fb-y				  += fbcon.o bitblit.o softcursor.o
+ifeq ($(CONFIG_FB_TILEBLITTING),y)
+fb-y				  += tileblit.o
+endif
+ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE_ROTATION),y)
+fb-y				  += fbcon_rotate.o fbcon_cw.o fbcon_ud.o \
+				     fbcon_ccw.o
+endif
+endif
 fb-objs                           := $(fb-y)
 
 obj-$(CONFIG_FB_CFB_FILLRECT)  += cfbfillrect.o
diff --git a/drivers/video/console/bitblit.c b/drivers/video/fbdev/core/bitblit.c
similarity index 98%
rename from drivers/video/console/bitblit.c
rename to drivers/video/fbdev/core/bitblit.c
index dbfe4eecf12e..99f3a1c3d093 100644
--- a/drivers/video/console/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -416,7 +416,3 @@  void fbcon_set_bitops(struct fbcon_ops *ops)
 
 EXPORT_SYMBOL(fbcon_set_bitops);
 
-MODULE_AUTHOR("Antonino Daplas <[e-mail]>");
-MODULE_DESCRIPTION("Bit Blitting Operation");
-MODULE_LICENSE("GPL");
-
diff --git a/drivers/video/console/fbcon.c b/drivers/video/fbdev/core/fbcon.c
similarity index 99%
rename from drivers/video/console/fbcon.c
rename to drivers/video/fbdev/core/fbcon.c
index 12ded23f1aaf..86b3bcbd01a8 100644
--- a/drivers/video/console/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -68,6 +68,7 @@ 
 #include <linux/kd.h>
 #include <linux/slab.h>
 #include <linux/fb.h>
+#include <linux/fbcon.h>
 #include <linux/vt_kern.h>
 #include <linux/selection.h>
 #include <linux/font.h>
@@ -3606,7 +3607,7 @@  static void fbcon_exit(void)
 	fbcon_has_exited = 1;
 }
 
-static int __init fb_console_init(void)
+void __init fb_console_init(void)
 {
 	int i;
 
@@ -3628,11 +3629,8 @@  static int __init fb_console_init(void)
 
 	console_unlock();
 	fbcon_start();
-	return 0;
 }
 
-fs_initcall(fb_console_init);
-
 #ifdef MODULE
 
 static void __exit fbcon_deinit_device(void)
@@ -3647,7 +3645,7 @@  static void __exit fbcon_deinit_device(void)
 	}
 }
 
-static void __exit fb_console_exit(void)
+void __exit fb_console_exit(void)
 {
 	console_lock();
 	fb_unregister_client(&fbcon_event_notifier);
@@ -3657,9 +3655,4 @@  static void __exit fb_console_exit(void)
 	do_unregister_con_driver(&fb_con);
 	console_unlock();
 }	
-
-module_exit(fb_console_exit);
-
 #endif
-
-MODULE_LICENSE("GPL");
diff --git a/drivers/video/console/fbcon.h b/drivers/video/fbdev/core/fbcon.h
similarity index 100%
rename from drivers/video/console/fbcon.h
rename to drivers/video/fbdev/core/fbcon.h
diff --git a/drivers/video/console/fbcon_ccw.c b/drivers/video/fbdev/core/fbcon_ccw.c
similarity index 98%
rename from drivers/video/console/fbcon_ccw.c
rename to drivers/video/fbdev/core/fbcon_ccw.c
index 5a3cbf6dff4d..2eeefa97bd4a 100644
--- a/drivers/video/console/fbcon_ccw.c
+++ b/drivers/video/fbdev/core/fbcon_ccw.c
@@ -418,7 +418,3 @@  void fbcon_rotate_ccw(struct fbcon_ops *ops)
 	ops->update_start = ccw_update_start;
 }
 EXPORT_SYMBOL(fbcon_rotate_ccw);
-
-MODULE_AUTHOR("Antonino Daplas <[e-mail]>");
-MODULE_DESCRIPTION("Console Rotation (270 degrees) Support");
-MODULE_LICENSE("GPL");
diff --git a/drivers/video/console/fbcon_cw.c b/drivers/video/fbdev/core/fbcon_cw.c
similarity index 98%
rename from drivers/video/console/fbcon_cw.c
rename to drivers/video/fbdev/core/fbcon_cw.c
index e7ee44db4e98..c321f7c59e5c 100644
--- a/drivers/video/console/fbcon_cw.c
+++ b/drivers/video/fbdev/core/fbcon_cw.c
@@ -401,7 +401,3 @@  void fbcon_rotate_cw(struct fbcon_ops *ops)
 	ops->update_start = cw_update_start;
 }
 EXPORT_SYMBOL(fbcon_rotate_cw);
-
-MODULE_AUTHOR("Antonino Daplas <[e-mail]>");
-MODULE_DESCRIPTION("Console Rotation (90 degrees) Support");
-MODULE_LICENSE("GPL");
diff --git a/drivers/video/console/fbcon_rotate.c b/drivers/video/fbdev/core/fbcon_rotate.c
similarity index 95%
rename from drivers/video/console/fbcon_rotate.c
rename to drivers/video/fbdev/core/fbcon_rotate.c
index db6528f2d3f2..8a51e4d95cc5 100644
--- a/drivers/video/console/fbcon_rotate.c
+++ b/drivers/video/fbdev/core/fbcon_rotate.c
@@ -110,7 +110,3 @@  void fbcon_set_rotate(struct fbcon_ops *ops)
 	}
 }
 EXPORT_SYMBOL(fbcon_set_rotate);
-
-MODULE_AUTHOR("Antonino Daplas <[e-mail]>");
-MODULE_DESCRIPTION("Console Rotation Support");
-MODULE_LICENSE("GPL");
diff --git a/drivers/video/console/fbcon_rotate.h b/drivers/video/fbdev/core/fbcon_rotate.h
similarity index 100%
rename from drivers/video/console/fbcon_rotate.h
rename to drivers/video/fbdev/core/fbcon_rotate.h
diff --git a/drivers/video/console/fbcon_ud.c b/drivers/video/fbdev/core/fbcon_ud.c
similarity index 98%
rename from drivers/video/console/fbcon_ud.c
rename to drivers/video/fbdev/core/fbcon_ud.c
index 19e3714abfe8..c0b605d49cb3 100644
--- a/drivers/video/console/fbcon_ud.c
+++ b/drivers/video/fbdev/core/fbcon_ud.c
@@ -446,7 +446,3 @@  void fbcon_rotate_ud(struct fbcon_ops *ops)
 	ops->update_start = ud_update_start;
 }
 EXPORT_SYMBOL(fbcon_rotate_ud);
-
-MODULE_AUTHOR("Antonino Daplas <[e-mail]>");
-MODULE_DESCRIPTION("Console Rotation (180 degrees) Support");
-MODULE_LICENSE("GPL");
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 069fe7960df1..283d57cf8526 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -32,6 +32,7 @@ 
 #include <linux/device.h>
 #include <linux/efi.h>
 #include <linux/fb.h>
+#include <linux/fbcon.h>
 
 #include <asm/fb.h>
 
@@ -1888,6 +1889,9 @@  fbmem_init(void)
 		fb_class = NULL;
 		goto err_class;
 	}
+
+	fb_console_init();
+
 	return 0;
 
 err_class:
@@ -1902,6 +1906,8 @@  module_init(fbmem_init);
 static void __exit
 fbmem_exit(void)
 {
+	fb_console_exit();
+
 	remove_proc_entry("fb", NULL);
 	class_destroy(fb_class);
 	unregister_chrdev(FB_MAJOR, "fb");
diff --git a/drivers/video/console/softcursor.c b/drivers/video/fbdev/core/softcursor.c
similarity index 93%
rename from drivers/video/console/softcursor.c
rename to drivers/video/fbdev/core/softcursor.c
index 46dd8f5d2e9e..fc93f254498e 100644
--- a/drivers/video/console/softcursor.c
+++ b/drivers/video/fbdev/core/softcursor.c
@@ -76,7 +76,3 @@  int soft_cursor(struct fb_info *info, struct fb_cursor *cursor)
 }
 
 EXPORT_SYMBOL(soft_cursor);
-
-MODULE_AUTHOR("James Simmons <[e-mail]>");
-MODULE_DESCRIPTION("Generic software cursor");
-MODULE_LICENSE("GPL");
diff --git a/drivers/video/console/tileblit.c b/drivers/video/fbdev/core/tileblit.c
similarity index 96%
rename from drivers/video/console/tileblit.c
rename to drivers/video/fbdev/core/tileblit.c
index 15e8e1a89c45..4636a6110c27 100644
--- a/drivers/video/console/tileblit.c
+++ b/drivers/video/fbdev/core/tileblit.c
@@ -152,8 +152,3 @@  void fbcon_set_tileops(struct vc_data *vc, struct fb_info *info)
 }
 
 EXPORT_SYMBOL(fbcon_set_tileops);
-
-MODULE_AUTHOR("Antonino Daplas <[e-mail]>");
-MODULE_DESCRIPTION("Tile Blitting Operation");
-MODULE_LICENSE("GPL");
-
diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
new file mode 100644
index 000000000000..0fac6305d51c
--- /dev/null
+++ b/include/linux/fbcon.h
@@ -0,0 +1,12 @@ 
+#ifndef _LINUX_FBCON_H
+#define _LINUX_FBCON_H
+
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE
+void __init fb_console_init(void);
+void __exit fb_console_exit(void);
+#else
+static void fb_console_init(void) {}
+static void fb_console_exit(void) {}
+#endif
+
+#endif /* _LINUX_FBCON_H */