[PATCH] tty: hvc: restrict HVC_DCC to ARMv6+ and ARM64

[PATCH] tty: hvc: restrict HVC_DCC to ARMv6+ and ARM64

[Karl Mehltretter]

hvc_dcc drives the JTAG DCC via the ARMv6/v7 CP14 debug registers
(mrc/mcr p14, 0, rX, c0, c1/c5, 0 in asm/dcc.h). That encoding is
undefined on older ARM cores, and also on ARMv7-M, but HVC_DCC only
depends on ARM, so it can be enabled on e.g. ARM926 (ARCH_MULTI_V5),
where hvc_dcc_console_init() runs __dcc_putchar() at boot and takes an
undefined-instruction trap before the console is up:

  Internal error: Oops - undefined instruction: 0 [#1] ARM
  PC is at hvc_dcc_check+0x50/0x8c
   hvc_dcc_check from hvc_dcc_console_init+0x18/0x48
   hvc_dcc_console_init from console_init+0x58/0x170
  Kernel panic - not syncing: Fatal exception

Restrict HVC_DCC to the CPUs where that encoding is valid: the
CPU_V6 || CPU_V6K || CPU_V7 set that arch/arm/include/debug/icedcc.S
guards it with, plus ARM64.

Fixes: 16c63f8ea49c ("drivers: char: hvc: add arm JTAG DCC console support")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 drivers/tty/hvc/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/hvc/Kconfig b/drivers/tty/hvc/Kconfig
index c2a4e88b328f..5866195de26a 100644
--- a/drivers/tty/hvc/Kconfig
+++ b/drivers/tty/hvc/Kconfig
@@ -79,7 +79,7 @@ config HVC_UDBG
 
 config HVC_DCC
 	bool "ARM JTAG DCC console"
-	depends on ARM || ARM64
+	depends on (ARM && (CPU_V6 || CPU_V6K || CPU_V7)) || ARM64
 	select HVC_DRIVER
 	select SERIAL_CORE_CONSOLE
 	help
-- 
2.39.5 (Apple Git-154)

Re: [PATCH] tty: hvc: restrict HVC_DCC to ARMv6+ and ARM64

[Greg Kroah-Hartman]

On Fri, Jul 17, 2026 at 09:16:16AM +0200, Karl Mehltretter wrote:
> hvc_dcc drives the JTAG DCC via the ARMv6/v7 CP14 debug registers
> (mrc/mcr p14, 0, rX, c0, c1/c5, 0 in asm/dcc.h). That encoding is
> undefined on older ARM cores, and also on ARMv7-M, but HVC_DCC only
> depends on ARM, so it can be enabled on e.g. ARM926 (ARCH_MULTI_V5),
> where hvc_dcc_console_init() runs __dcc_putchar() at boot and takes an
> undefined-instruction trap before the console is up:
> 
>   Internal error: Oops - undefined instruction: 0 [#1] ARM
>   PC is at hvc_dcc_check+0x50/0x8c
>    hvc_dcc_check from hvc_dcc_console_init+0x18/0x48
>    hvc_dcc_console_init from console_init+0x58/0x170
>   Kernel panic - not syncing: Fatal exception
> 
> Restrict HVC_DCC to the CPUs where that encoding is valid: the
> CPU_V6 || CPU_V6K || CPU_V7 set that arch/arm/include/debug/icedcc.S
> guards it with, plus ARM64.
> 
> Fixes: 16c63f8ea49c ("drivers: char: hvc: add arm JTAG DCC console support")
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
>  drivers/tty/hvc/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/hvc/Kconfig b/drivers/tty/hvc/Kconfig
> index c2a4e88b328f..5866195de26a 100644
> --- a/drivers/tty/hvc/Kconfig
> +++ b/drivers/tty/hvc/Kconfig
> @@ -79,7 +79,7 @@ config HVC_UDBG
>  
>  config HVC_DCC
>  	bool "ARM JTAG DCC console"
> -	depends on ARM || ARM64
> +	depends on (ARM && (CPU_V6 || CPU_V6K || CPU_V7)) || ARM64

What about testing builds?

thanks,

greg k-h

Re: [PATCH] tty: hvc: restrict HVC_DCC to ARMv6+ and ARM64

[Arnd Bergmann]

On Fri, Jul 17, 2026, at 09:22, Greg Kroah-Hartman wrote:
> On Fri, Jul 17, 2026 at 09:16:16AM +0200, Karl Mehltretter wrote:
>> hvc_dcc drives the JTAG DCC via the ARMv6/v7 CP14 debug registers
>> (mrc/mcr p14, 0, rX, c0, c1/c5, 0 in asm/dcc.h). That encoding is
>> undefined on older ARM cores, and also on ARMv7-M, but HVC_DCC only
>> depends on ARM, so it can be enabled on e.g. ARM926 (ARCH_MULTI_V5),
>> where hvc_dcc_console_init() runs __dcc_putchar() at boot and takes an
>> undefined-instruction trap before the console is up:
>> 
>>   Internal error: Oops - undefined instruction: 0 [#1] ARM
>>   PC is at hvc_dcc_check+0x50/0x8c
>>    hvc_dcc_check from hvc_dcc_console_init+0x18/0x48
>>    hvc_dcc_console_init from console_init+0x58/0x170
>>   Kernel panic - not syncing: Fatal exception
>> 
>> Restrict HVC_DCC to the CPUs where that encoding is valid: the
>> CPU_V6 || CPU_V6K || CPU_V7 set that arch/arm/include/debug/icedcc.S
>> guards it with, plus ARM64.
>> 
>> Fixes: 16c63f8ea49c ("drivers: char: hvc: add arm JTAG DCC console support")
>> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

I don't like having architecture specific symbols in drivers/*/Kconfig,
but this one is clearly correct, so as a quick fix this is

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

A nicer alternative would be to improve the feature detection
in the driver by probing an appropriate ID register for presence
of DCC. Unfortunately we can't use regular device probing
because the driver is a low-level debug hack that is intended
to just bypass all of that and provide a console.

Maybe checking RCP14_DBGDIDR() to see if the version is greater
or equal to '1' (arm1136) will work? If that also traps,
an __ex_table fixup could catch that to fail the driver probe.

>>  drivers/tty/hvc/Kconfig | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/tty/hvc/Kconfig b/drivers/tty/hvc/Kconfig
>> index c2a4e88b328f..5866195de26a 100644
>> --- a/drivers/tty/hvc/Kconfig
>> +++ b/drivers/tty/hvc/Kconfig
>> @@ -79,7 +79,7 @@ config HVC_UDBG
>>  
>>  config HVC_DCC
>>  	bool "ARM JTAG DCC console"
>> -	depends on ARM || ARM64
>> +	depends on (ARM && (CPU_V6 || CPU_V6K || CPU_V7)) || ARM64
>
> What about testing builds?

The driver only builds on arm32 or arm64, and there is little
value of build-testing it on armv4/v5.

      Arnd

Re: [PATCH] tty: hvc: restrict HVC_DCC to ARMv6+ and ARM64

[Karl Mehltretter]

On Fri, Jul 17, 2026 at 10:04:17AM +0100, Arnd Bergmann wrote:
> On Fri, Jul 17, 2026, at 09:22, Greg Kroah-Hartman wrote:
> >
> > What about testing builds?
> 
> The driver only builds on arm32 or arm64, and there is little
> value of build-testing it on armv4/v5.
> 
Oh I already sent a v2 with COMPILE_TEST before reading this.
I'm happy with either version.

thanks,
Karl

Re: [PATCH] tty: hvc: restrict HVC_DCC to ARMv6+ and ARM64

[Greg Kroah-Hartman]

On Fri, Jul 17, 2026 at 12:25:26PM +0200, Karl Mehltretter wrote:
> On Fri, Jul 17, 2026 at 10:04:17AM +0100, Arnd Bergmann wrote:
> > On Fri, Jul 17, 2026, at 09:22, Greg Kroah-Hartman wrote:
> > >
> > > What about testing builds?
> > 
> > The driver only builds on arm32 or arm64, and there is little
> > value of build-testing it on armv4/v5.
> > 
> Oh I already sent a v2 with COMPILE_TEST before reading this.
> I'm happy with either version.

I'll take the v1 now, thanks,

greg k-h