[PATCH v2] 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.

COMPILE_TEST keeps the driver buildable on the excluded cores, where
the p14 encodings still assemble.

Fixes: 16c63f8ea49c ("drivers: char: hvc: add arm JTAG DCC console support")
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
v2: add COMPILE_TEST to keep arm build coverage, per Greg's comment

 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..51320f2f9a3f 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 || COMPILE_TEST)) || ARM64
 	select HVC_DRIVER
 	select SERIAL_CORE_CONSOLE
 	help
-- 
2.39.5 (Apple Git-154)