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

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Date:   Fri, 19 Aug 2022 15:43:18 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nick Child <nick.child@....com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Add support for early debugging via Serial 16550
 console



On 8/19/22 15:38, Pali Rohár wrote:
> On Friday 19 August 2022 15:34:14 Randy Dunlap wrote:
>> 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?
> 
> 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...

>> 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.
-- 
~Randy