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

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Date:   Fri, 19 Aug 2022 15:34:14 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Pali Rohár <pali@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nick Child <nick.child@....com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Add support for early debugging via Serial 16550
 console

Hi--

On 8/19/22 14:12, Pali Rohár wrote:
> 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@...nel.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/
> ---
>  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?

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...

> +
> +config PPC_EARLY_DEBUG_16550_STRIDE
> +	int "Early debug Serial 16550 stride"
> +	depends on PPC_EARLY_DEBUG_16550
> +	default 1
> +
>  config FAIL_IOMMU
>  	bool "Fault-injection capability for IOMMU"
>  	depends on FAULT_INJECTION

Thanks.
-- 
~Randy