Re: [PATCH] powerpc: Add support for early debugging via Serial 16550 console

Le 20/08/2022 à 00:43, Randy Dunlap a écrit :

On 8/19/22 15:38, Pali Rohár wrote:

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On Friday 19 August 2022 15:34:14 Randy Dunlap wrote:

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Hi--

On 8/19/22 14:12, Pali Rohár wrote:

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Currently powerpc early debugging contains lot of platform specific
options, but does not support standard UART / serial 16550 console.

Later legacy_serial.c code supports registering UART as early debug console
from device tree but it is not early during booting, but rather later after
machine description code finishes.

So for real early debugging via UART is current code unsuitable.

Add support for new early debugging option CONFIG_PPC_EARLY_DEBUG_16550
which enable Serial 16550 console on address defined by new option
CONFIG_PPC_EARLY_DEBUG_16550_PHYSADDR and by stride by option
CONFIG_PPC_EARLY_DEBUG_16550_STRIDE.

With this change it is possible to debug powerpc machine descriptor code.
For example this early debugging code can print on serial console also
"No suitable machine description found" error which is done before
legacy_serial.c code.

Signed-off-by: Pali Rohár <pali@kernel.org>
---
Tested on P2020 board. It allowed me do debug and implement this patch series:
https://lore.kernel.org/linuxppc-dev/20220819191557.28116-1-pali@kernel.org/ (local)
---
  arch/powerpc/Kconfig.debug       | 14 ++++++++++++++
  arch/powerpc/include/asm/udbg.h  |  1 +
  arch/powerpc/kernel/udbg.c       |  2 ++
  arch/powerpc/kernel/udbg_16550.c | 33 ++++++++++++++++++++++++++++++++
  4 files changed, 50 insertions(+)

diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index 9f363c143d86..a4e7d90a45d2 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug

@@ -276,6 +276,11 @@ config PPC_EARLY_DEBUG_OPAL_HVSI
  	  Select this to enable early debugging for the PowerNV platform
  	  using an "hvsi" console
  
+config PPC_EARLY_DEBUG_16550
+	bool "Serial 16550"
+	help
+	  Select this to enable early debugging via Serial 16550 console
+
  config PPC_EARLY_DEBUG_MEMCONS
  	bool "In memory console"
  	help

@@ -355,6 +360,15 @@ config PPC_EARLY_DEBUG_CPM_ADDR
  	  platform probing is done, all platforms selected must
  	  share the same address.
  
+config PPC_EARLY_DEBUG_16550_PHYSADDR
+	hex "Early debug Serial 16550 physical address"
+	depends on PPC_EARLY_DEBUG_16550

Is there any chance that you could provide a default value here
so that 'make olddefconfig' does not end up like it does
without having a default value?

No. Because there is not any default value. Physical address of 16550 is
hardly HW dependent. Possibly also bootloader dependent (if it can remap
peripherals to different physical addresses).

User _has to_ specify correct physical address if want to use early
debug 16550 console.

OK, we'll see if it ever causes a real problem then...

It does, for instance with 83xx/mpc836x_rdk_defconfig

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CONFIG_PPC_EARLY_DEBUG_16550=y
# CONFIG_PPC_EARLY_DEBUG_MEMCONS is not set
CONFIG_PPC_EARLY_DEBUG_16550_PHYSADDR=
CONFIG_PPC_EARLY_DEBUG_16550_STRIDE=1

which then causes a kconfig prompt when starting
the build...

Cannot we set somehow that PPC_EARLY_DEBUG_16550 would be disabled by
default when upgrading defconfig?

Don't know about that.

Thanks.