[PATCH 1/3] dummycon: only build module if there are users

[PATCH 1/3] dummycon: only build module if there are users

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

Dummycon is used as a fallback conswitchp for vgacon and fbcon
in the VT code, and there are no references to it if all three
are disabled, so just leave it out of the kernel image for
configurations without those.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/video/console/Kconfig | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
index bc31db6ef7d2..1c4263c164ce 100644
--- a/drivers/video/console/Kconfig
+++ b/drivers/video/console/Kconfig
@@ -47,8 +47,7 @@ config SGI_NEWPORT_CONSOLE
 	  card of your Indy.  Most people say Y here.
 
 config DUMMY_CONSOLE
-	bool
-	default y
+	def_bool VT || VGA_CONSOLE || FRAMEBUFFER_CONSOLE
 
 config DUMMY_CONSOLE_COLUMNS
 	int "Initial number of console screen columns"
-- 
2.39.5

[PATCH 2/3] dummycon: fix default rows/cols

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

dummycon fails to build on ARM/footbridge when the VGA console is
disabled, since I got the dependencies slightly wrong in a previous
patch:

drivers/video/console/dummycon.c: In function 'dummycon_init':
drivers/video/console/dummycon.c:27:25: error: 'CONFIG_DUMMY_CONSOLE_COLUMNS' undeclared (first use in this function); did you mean 'CONFIG_DUMMY_CONSOLE'?
   27 | #define DUMMY_COLUMNS   CONFIG_DUMMY_CONSOLE_COLUMNS
drivers/video/console/dummycon.c:28:25: error: 'CONFIG_DUMMY_CONSOLE_ROWS' undeclared (first use in this function); did you mean 'CONFIG_DUMMY_CONSOLE'?
   28 | #define DUMMY_ROWS      CONFIG_DUMMY_CONSOLE_ROWS

This only showed up after many thousand randconfig builds on Arm, and
doesn't matter in practice, but should still be fixed. Address it by
using the default row/columns on footbridge after all in that corner
case.

Fixes: 4293b0925149 ("dummycon: limit Arm console size hack to footbridge")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202409151512.LML1slol-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/video/console/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
index 1c4263c164ce..ea4863919eb9 100644
--- a/drivers/video/console/Kconfig
+++ b/drivers/video/console/Kconfig
@@ -51,7 +51,7 @@ config DUMMY_CONSOLE
 
 config DUMMY_CONSOLE_COLUMNS
 	int "Initial number of console screen columns"
-	depends on DUMMY_CONSOLE && !ARCH_FOOTBRIDGE
+	depends on DUMMY_CONSOLE && !(ARCH_FOOTBRIDGE && VGA_CONSOLE)
 	default 160 if PARISC
 	default 80
 	help
@@ -61,7 +61,7 @@ config DUMMY_CONSOLE_COLUMNS
 
 config DUMMY_CONSOLE_ROWS
 	int "Initial number of console screen rows"
-	depends on DUMMY_CONSOLE && !ARCH_FOOTBRIDGE
+	depends on DUMMY_CONSOLE && !(ARCH_FOOTBRIDGE && VGA_CONSOLE)
 	default 64 if PARISC
 	default 30 if ARM
 	default 25
-- 
2.39.5

[PATCH 3/3] mdacon: rework dependency list

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

mdacon has roughly the same dependencies as vgacon but expresses them
as a negative list instead of a positive list, with the only practical
difference being PowerPC/CHRP, which uses vga16fb instead of vgacon.

The CONFIG_MDA_CONSOLE description advises to only turn it on when vgacon
is also used because MDA/Hercules-only systems should be using vgacon
instead, so just change the list to enforce that directly for simplicity.

The probing was broken from 2002 to 2008, this improves on the fix
that was added then: If vgacon is a loadable module, then mdacon
cannot be built-in now, and the list of systems that support vgacon
is carried over.

Fixes: 0b9cf3aa6b1e ("mdacon messing up default vc's - set default to vc13-16 again")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
I have no idea when the last time was that someone actually tried using
dualhead vgacon/mdacon with two ISA cards, or if it still works. We may
be better off removing the driver altogether, but I don't see anything
immediately wrong it with it.
---
 drivers/video/console/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
index ea4863919eb9..12f54480f57f 100644
--- a/drivers/video/console/Kconfig
+++ b/drivers/video/console/Kconfig
@@ -24,7 +24,7 @@ config VGA_CONSOLE
 	  Say Y.
 
 config MDA_CONSOLE
-	depends on !M68K && !PARISC && ISA
+	depends on VGA_CONSOLE && ISA
 	tristate "MDA text console (dual-headed)"
 	help
 	  Say Y here if you have an old MDA or monochrome Hercules graphics
-- 
2.39.5

Re: [PATCH 1/3] dummycon: only build module if there are users

[Thomas Zimmermann]

Hi Arnd

Am 25.02.25 um 17:44 schrieb Arnd Bergmann:
> From: Arnd Bergmann <arnd@arndb.de>
>
> Dummycon is used as a fallback conswitchp for vgacon and fbcon
> in the VT code, and there are no references to it if all three
> are disabled, so just leave it out of the kernel image for
> configurations without those.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   drivers/video/console/Kconfig | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
> index bc31db6ef7d2..1c4263c164ce 100644
> --- a/drivers/video/console/Kconfig
> +++ b/drivers/video/console/Kconfig
> @@ -47,8 +47,7 @@ config SGI_NEWPORT_CONSOLE
>   	  card of your Indy.  Most people say Y here.
>   
>   config DUMMY_CONSOLE
> -	bool
> -	default y
> +	def_bool VT || VGA_CONSOLE || FRAMEBUFFER_CONSOLE

What about MDA_CONSOLE and STI_CONSOLE. Don't they require this as fallback?

Best regards
Thomas

>   
>   config DUMMY_CONSOLE_COLUMNS
>   	int "Initial number of console screen columns"

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH 1/3] dummycon: only build module if there are users

[Thomas Zimmermann]

Am 26.02.25 um 08:48 schrieb Thomas Zimmermann:
> Hi Arnd
>
> Am 25.02.25 um 17:44 schrieb Arnd Bergmann:
>> From: Arnd Bergmann <arnd@arndb.de>
>>
>> Dummycon is used as a fallback conswitchp for vgacon and fbcon
>> in the VT code, and there are no references to it if all three
>> are disabled, so just leave it out of the kernel image for
>> configurations without those.
>>
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

>> ---
>>   drivers/video/console/Kconfig | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/video/console/Kconfig 
>> b/drivers/video/console/Kconfig
>> index bc31db6ef7d2..1c4263c164ce 100644
>> --- a/drivers/video/console/Kconfig
>> +++ b/drivers/video/console/Kconfig
>> @@ -47,8 +47,7 @@ config SGI_NEWPORT_CONSOLE
>>         card of your Indy.  Most people say Y here.
>>     config DUMMY_CONSOLE
>> -    bool
>> -    default y
>> +    def_bool VT || VGA_CONSOLE || FRAMEBUFFER_CONSOLE
>
> What about MDA_CONSOLE and STI_CONSOLE. Don't they require this as 
> fallback?

I did some grepping in the kernel sources and found the answer to this 
question. It's still surprising that the other consoles don't use 
dummycon. They are never unloaded, it seems.

Best regards
Thomas

>
> Best regards
> Thomas
>
>>     config DUMMY_CONSOLE_COLUMNS
>>       int "Initial number of console screen columns"
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH 2/3] dummycon: fix default rows/cols

[Thomas Zimmermann]

Am 25.02.25 um 17:44 schrieb Arnd Bergmann:
> From: Arnd Bergmann <arnd@arndb.de>
>
> dummycon fails to build on ARM/footbridge when the VGA console is
> disabled, since I got the dependencies slightly wrong in a previous
> patch:
>
> drivers/video/console/dummycon.c: In function 'dummycon_init':
> drivers/video/console/dummycon.c:27:25: error: 'CONFIG_DUMMY_CONSOLE_COLUMNS' undeclared (first use in this function); did you mean 'CONFIG_DUMMY_CONSOLE'?
>     27 | #define DUMMY_COLUMNS   CONFIG_DUMMY_CONSOLE_COLUMNS
> drivers/video/console/dummycon.c:28:25: error: 'CONFIG_DUMMY_CONSOLE_ROWS' undeclared (first use in this function); did you mean 'CONFIG_DUMMY_CONSOLE'?
>     28 | #define DUMMY_ROWS      CONFIG_DUMMY_CONSOLE_ROWS
>
> This only showed up after many thousand randconfig builds on Arm, and
> doesn't matter in practice, but should still be fixed. Address it by
> using the default row/columns on footbridge after all in that corner
> case.
>
> Fixes: 4293b0925149 ("dummycon: limit Arm console size hack to footbridge")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202409151512.LML1slol-lkp@intel.com/
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

> ---
>   drivers/video/console/Kconfig | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
> index 1c4263c164ce..ea4863919eb9 100644
> --- a/drivers/video/console/Kconfig
> +++ b/drivers/video/console/Kconfig
> @@ -51,7 +51,7 @@ config DUMMY_CONSOLE
>   
>   config DUMMY_CONSOLE_COLUMNS
>   	int "Initial number of console screen columns"
> -	depends on DUMMY_CONSOLE && !ARCH_FOOTBRIDGE
> +	depends on DUMMY_CONSOLE && !(ARCH_FOOTBRIDGE && VGA_CONSOLE)
>   	default 160 if PARISC
>   	default 80
>   	help
> @@ -61,7 +61,7 @@ config DUMMY_CONSOLE_COLUMNS
>   
>   config DUMMY_CONSOLE_ROWS
>   	int "Initial number of console screen rows"
> -	depends on DUMMY_CONSOLE && !ARCH_FOOTBRIDGE
> +	depends on DUMMY_CONSOLE && !(ARCH_FOOTBRIDGE && VGA_CONSOLE)
>   	default 64 if PARISC
>   	default 30 if ARM
>   	default 25

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH 1/3] dummycon: only build module if there are users

[Arnd Bergmann]

On Wed, Feb 26, 2025, at 08:48, Thomas Zimmermann wrote:
> Am 25.02.25 um 17:44 schrieb Arnd Bergmann:
>> From: Arnd Bergmann <arnd@arndb.de>
>>
>> Dummycon is used as a fallback conswitchp for vgacon and fbcon
>> in the VT code, and there are no references to it if all three
>> are disabled, so just leave it out of the kernel image for
>> configurations without those.
>>
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>> ---
>>   drivers/video/console/Kconfig | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
>> index bc31db6ef7d2..1c4263c164ce 100644
>> --- a/drivers/video/console/Kconfig
>> +++ b/drivers/video/console/Kconfig
>> @@ -47,8 +47,7 @@ config SGI_NEWPORT_CONSOLE
>>   	  card of your Indy.  Most people say Y here.
>>   
>>   config DUMMY_CONSOLE
>> -	bool
>> -	default y
>> +	def_bool VT || VGA_CONSOLE || FRAMEBUFFER_CONSOLE
>
> What about MDA_CONSOLE and STI_CONSOLE. Don't they require this as fallback?
>

MDA_CONSOLE clearly does not, because that is only the second
console when VGA_CONSOLE is the main one.

For sti_console, I don't see how it would do use it: when CONFIG_VT
is enabled, the line above turns on DUMMY_CONSOLE, but without
CONFIG_VT there seems to be no reference to it after
58a5c67aadde ("parisc/sticon: Always register sticon console
driver"). I also see that CONFIG_STI_CONSOLE is a 'bool' symbol,
so there is no dynamic loading/unloading of the driver.

    Arnd

Re: [PATCH 3/3] mdacon: rework dependency list

[Thomas Zimmermann]

Am 25.02.25 um 17:44 schrieb Arnd Bergmann:
> From: Arnd Bergmann <arnd@arndb.de>
>
> mdacon has roughly the same dependencies as vgacon but expresses them
> as a negative list instead of a positive list, with the only practical
> difference being PowerPC/CHRP, which uses vga16fb instead of vgacon.
>
> The CONFIG_MDA_CONSOLE description advises to only turn it on when vgacon
> is also used because MDA/Hercules-only systems should be using vgacon
> instead, so just change the list to enforce that directly for simplicity.
>
> The probing was broken from 2002 to 2008, this improves on the fix
> that was added then: If vgacon is a loadable module, then mdacon
> cannot be built-in now, and the list of systems that support vgacon
> is carried over.
>
> Fixes: 0b9cf3aa6b1e ("mdacon messing up default vc's - set default to vc13-16 again")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

> ---
> I have no idea when the last time was that someone actually tried using
> dualhead vgacon/mdacon with two ISA cards, or if it still works. We may
> be better off removing the driver altogether, but I don't see anything
> immediately wrong it with it.
> ---
>   drivers/video/console/Kconfig | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
> index ea4863919eb9..12f54480f57f 100644
> --- a/drivers/video/console/Kconfig
> +++ b/drivers/video/console/Kconfig
> @@ -24,7 +24,7 @@ config VGA_CONSOLE
>   	  Say Y.
>   
>   config MDA_CONSOLE
> -	depends on !M68K && !PARISC && ISA
> +	depends on VGA_CONSOLE && ISA
>   	tristate "MDA text console (dual-headed)"
>   	help
>   	  Say Y here if you have an old MDA or monochrome Hercules graphics

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH 1/3] dummycon: only build module if there are users

[Thomas Zimmermann]

Hi

Am 26.02.25 um 08:55 schrieb Arnd Bergmann:
> On Wed, Feb 26, 2025, at 08:48, Thomas Zimmermann wrote:
>> Am 25.02.25 um 17:44 schrieb Arnd Bergmann:
>>> From: Arnd Bergmann <arnd@arndb.de>
>>>
>>> Dummycon is used as a fallback conswitchp for vgacon and fbcon
>>> in the VT code, and there are no references to it if all three
>>> are disabled, so just leave it out of the kernel image for
>>> configurations without those.
>>>
>>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>>> ---
>>>    drivers/video/console/Kconfig | 3 +--
>>>    1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
>>> index bc31db6ef7d2..1c4263c164ce 100644
>>> --- a/drivers/video/console/Kconfig
>>> +++ b/drivers/video/console/Kconfig
>>> @@ -47,8 +47,7 @@ config SGI_NEWPORT_CONSOLE
>>>    	  card of your Indy.  Most people say Y here.
>>>    
>>>    config DUMMY_CONSOLE
>>> -	bool
>>> -	default y
>>> +	def_bool VT || VGA_CONSOLE || FRAMEBUFFER_CONSOLE
>> What about MDA_CONSOLE and STI_CONSOLE. Don't they require this as fallback?
>>
> MDA_CONSOLE clearly does not, because that is only the second
> console when VGA_CONSOLE is the main one.
>
> For sti_console, I don't see how it would do use it: when CONFIG_VT
> is enabled, the line above turns on DUMMY_CONSOLE, but without
> CONFIG_VT there seems to be no reference to it after
> 58a5c67aadde ("parisc/sticon: Always register sticon console
> driver"). I also see that CONFIG_STI_CONSOLE is a 'bool' symbol,
> so there is no dynamic loading/unloading of the driver.

Thanks.

Here's another general question. vgacon and fbcon only seem usable with 
CONFIG_VT=y. Wouldn't it make sense to have them depend on CONFIG_VT=y? 
dummycon could then be implemented as part of the vt code, maybe even 
become a vt-internal thing. The console code is complex, so I'm probably 
missing something here?

Best regards
Thomas

>
>      Arnd

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH 1/3] dummycon: only build module if there are users

[Arnd Bergmann]

On Wed, Feb 26, 2025, at 09:16, Thomas Zimmermann wrote:
> Am 26.02.25 um 08:55 schrieb Arnd Bergmann:
> Here's another general question. vgacon and fbcon only seem usable with 
> CONFIG_VT=y. Wouldn't it make sense to have them depend on CONFIG_VT=y? 
> dummycon could then be implemented as part of the vt code, maybe even 
> become a vt-internal thing. The console code is complex, so I'm probably 
> missing something here?

I think in theory one may have a system use fbcon purely to get the
boot logo, but not actually support VT.  I had also assumed there might
be a way to use fbcon as the console (i.e. printk) but not register
the tty, but it looks like the console code still requires vt.

After I looked at the vt and conswitchp code some more, I wonder
if we could go the other way and instead of integrating it more
make the conswitchp logic optional: most of the complexity here
deals with switching between text console and fbcon dynamically,
but having any text console support is getting very rare (vga
on alpha/mips/x86-32, newport on mips-ip22, sti on parisc).

If we do this, the conswitchp code could be merged with dummycon
in drivers/video/console, with the simpler alternative just
calling into fbcon functions. I'm not sure if we can already drop
vgacon from normal x86-64 distro configs, i.e. if there are cases
that are not already covered by any of efi-earlycon, efifb,
vga16fb, vesafb/uvesafb or a PCI DRM driver.

    Arnd

Re: [PATCH 1/3] dummycon: only build module if there are users

[Javier Martinez Canillas]

"Arnd Bergmann" <arnd@arndb.de> writes:

Hello Arnd,

> On Wed, Feb 26, 2025, at 09:16, Thomas Zimmermann wrote:
>> Am 26.02.25 um 08:55 schrieb Arnd Bergmann:
>> Here's another general question. vgacon and fbcon only seem usable with 
>> CONFIG_VT=y. Wouldn't it make sense to have them depend on CONFIG_VT=y? 
>> dummycon could then be implemented as part of the vt code, maybe even 
>> become a vt-internal thing. The console code is complex, so I'm probably 
>> missing something here?
>
> I think in theory one may have a system use fbcon purely to get the
> boot logo, but not actually support VT.  I had also assumed there might
> be a way to use fbcon as the console (i.e. printk) but not register
> the tty, but it looks like the console code still requires vt.
>
> After I looked at the vt and conswitchp code some more, I wonder
> if we could go the other way and instead of integrating it more
> make the conswitchp logic optional: most of the complexity here
> deals with switching between text console and fbcon dynamically,
> but having any text console support is getting very rare (vga
> on alpha/mips/x86-32, newport on mips-ip22, sti on parisc).
>
> If we do this, the conswitchp code could be merged with dummycon

This sounds like a much better approach indeed.

> in drivers/video/console, with the simpler alternative just
> calling into fbcon functions. I'm not sure if we can already drop
> vgacon from normal x86-64 distro configs, i.e. if there are cases
> that are not already covered by any of efi-earlycon, efifb,
> vga16fb, vesafb/uvesafb or a PCI DRM driver.
>

I believe vgacon is still useful for x86 with legacy BIOS, because
vesafb (and simpledrm) only works if the user defines a mode using
the vga= kernel cmdline parameter.

>     Arnd
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat

Re: [PATCH 1/3] dummycon: only build module if there are users

[Arnd Bergmann]

On Wed, Feb 26, 2025, at 13:05, Javier Martinez Canillas wrote:
> "Arnd Bergmann" <arnd@arndb.de> writes:
>> in drivers/video/console, with the simpler alternative just
>> calling into fbcon functions. I'm not sure if we can already drop
>> vgacon from normal x86-64 distro configs, i.e. if there are cases
>> that are not already covered by any of efi-earlycon, efifb,
>> vga16fb, vesafb/uvesafb or a PCI DRM driver.
>>
>
> I believe vgacon is still useful for x86 with legacy BIOS, because
> vesafb (and simpledrm) only works if the user defines a mode using
> the vga= kernel cmdline parameter.

Right, at the minimum, a configuration without vgacon would
have to pick a graphical mode in arch/x86/boot/video*.c if one
wasn't already set by grub.

Looking through the git history of that code, it seems that there
are lots of corner cases with weird 32-bit hardware.
Anything from the past 20 years is probably reasonably safe,
but there still needs to be a way to configure vgacon back in
to be safe.

    Arnd

Re: [PATCH 1/3] dummycon: only build module if there are users

[Helge Deller]

On 2/26/25 12:46, Arnd Bergmann wrote:
> On Wed, Feb 26, 2025, at 09:16, Thomas Zimmermann wrote:
>> Am 26.02.25 um 08:55 schrieb Arnd Bergmann:
>> Here's another general question. vgacon and fbcon only seem usable with
>> CONFIG_VT=y. Wouldn't it make sense to have them depend on CONFIG_VT=y?
>> dummycon could then be implemented as part of the vt code, maybe even
>> become a vt-internal thing. The console code is complex, so I'm probably
>> missing something here?
>
> I think in theory one may have a system use fbcon purely to get the
> boot logo, but not actually support VT.  I had also assumed there might
> be a way to use fbcon as the console (i.e. printk) but not register
> the tty, but it looks like the console code still requires vt.
>
> After I looked at the vt and conswitchp code some more, I wonder
> if we could go the other way and instead of integrating it more
> make the conswitchp logic optional: most of the complexity here
> deals with switching between text console and fbcon dynamically,
> but having any text console support is getting very rare (vga
> on alpha/mips/x86-32, newport on mips-ip22, sti on parisc).

Yes, it's rare. But on parisc, if no supported fbdev or drm
graphic card is found, it needs to stays on sticon (which always works).
Otherwise - if a card was found - the kernel switches dynamically to fbcon.

> If we do this, the conswitchp code could be merged with dummycon
> in drivers/video/console, with the simpler alternative just
> calling into fbcon functions.

As mentioned above, that should be optional then.

> I'm not sure if we can already drop
> vgacon from normal x86-64 distro configs, i.e. if there are cases
> that are not already covered by any of efi-earlycon, efifb,
> vga16fb, vesafb/uvesafb or a PCI DRM driver.

Helge

Re: [PATCH 1/3] dummycon: only build module if there are users

[Thomas Zimmermann]

Hi

Am 26.02.25 um 12:46 schrieb Arnd Bergmann:
> On Wed, Feb 26, 2025, at 09:16, Thomas Zimmermann wrote:
>> Am 26.02.25 um 08:55 schrieb Arnd Bergmann:
>> Here's another general question. vgacon and fbcon only seem usable with
>> CONFIG_VT=y. Wouldn't it make sense to have them depend on CONFIG_VT=y?
>> dummycon could then be implemented as part of the vt code, maybe even
>> become a vt-internal thing. The console code is complex, so I'm probably
>> missing something here?
> I think in theory one may have a system use fbcon purely to get the
> boot logo, but not actually support VT.  I had also assumed there might
> be a way to use fbcon as the console (i.e. printk) but not register
> the tty, but it looks like the console code still requires vt.
>
> After I looked at the vt and conswitchp code some more, I wonder
> if we could go the other way and instead of integrating it more
> make the conswitchp logic optional: most of the complexity here
> deals with switching between text console and fbcon dynamically,
> but having any text console support is getting very rare (vga
> on alpha/mips/x86-32, newport on mips-ip22, sti on parisc).
>
> If we do this, the conswitchp code could be merged with dummycon
> in drivers/video/console, with the simpler alternative just
> calling into fbcon functions. I'm not sure if we can already drop
> vgacon from normal x86-64 distro configs, i.e. if there are cases
> that are not already covered by any of efi-earlycon, efifb,
> vga16fb, vesafb/uvesafb or a PCI DRM driver.

Thanks for the lengthy answer. I don't want to add work to your todo 
list. For vgacon, I wouldn't remove it yet, as there still exisits 
x86_64 systems with plain VGA support.

Best regards
Thomas


>
>      Arnd
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH 1/3] dummycon: only build module if there are users

[Helge Deller]

On 2/25/25 17:44, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
>
> Dummycon is used as a fallback conswitchp for vgacon and fbcon
> in the VT code, and there are no references to it if all three
> are disabled, so just leave it out of the kernel image for
> configurations without those.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   drivers/video/console/Kconfig | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)


whole series applied to fbdev git tree.

Thanks!
Helge