linux-kernel - Re: [RFC PATCH] powerpc: udbg: export udbg

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Message-ID: <f1e07fba-142e-2f27-6fed-c3c908deb2dd@infradead.org>
Date:   Sat, 13 May 2023 16:08:19 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        linuxppc-dev@...ts.ozlabs.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH] powerpc: udbg: export udbg_putc



On 5/13/23 16:05, Pali Rohár wrote:
> On Saturday 13 May 2023 15:53:19 Randy Dunlap wrote:
>> Hi Pali,
>>
>> On 5/13/23 02:13, Pali Rohár wrote:
>>> On Friday 12 May 2023 22:58:04 Randy Dunlap wrote:
>>>> In a randconfig with CONFIG_SERIAL_CPM=m and
>>>> CONFIG_PPC_EARLY_DEBUG_CPM=y, there is a build error:
>>>> ERROR: modpost: "udbg_putc" [drivers/tty/serial/cpm_uart/cpm_uart.ko] undefined!
>>>>
>>>> The build can be fixed by exporting "udbg_putc" in udbg.c.
>>>>
>>>> OTOH, maybe something like this is more appropriate in
>>>> arch/powerpc/Kconfig.debug?
>>>>
>>>> config PPC_EARLY_DEBUG_CPM
>>>> 	bool "Early serial debugging for Freescale CPM-based serial ports"
>>>> -	depends on SERIAL_CPM
>>>> +	depends on SERIAL_CPM=y
>>>>
>>>> Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
>>>> Cc: Michael Ellerman <mpe@...erman.id.au>
>>>> Cc: Nicholas Piggin <npiggin@...il.com>
>>>> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
>>>> Cc: linuxppc-dev@...ts.ozlabs.org
>>>> Cc: Arnd Bergmann <arnd@...db.de>
>>>> Cc: "Pali Rohár" <pali@...nel.org>
>>>> ---
>>>>  arch/powerpc/kernel/udbg.c |    1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff -- a/arch/powerpc/kernel/udbg.c b/arch/powerpc/kernel/udbg.c
>>>> --- a/arch/powerpc/kernel/udbg.c
>>>> +++ b/arch/powerpc/kernel/udbg.c
>>>> @@ -14,6 +14,7 @@
>>>>  #include <asm/udbg.h>
>>>>  
>>>>  void (*udbg_putc)(char c);
>>>> +EXPORT_SYMBOL(udbg_putc);
>>>>  void (*udbg_flush)(void);
>>>>  int (*udbg_getc)(void);
>>>>  int (*udbg_getc_poll)(void);
>>>
>>> Hello! I do not think that it is a good idea to export udbg functions
>>> for kernel modules. I have quickly looked at the cpm_uart driver file
>>
>> Sure, that was just a conversation starter.
>>
>>> drivers/tty/serial/cpm_uart/cpm_uart_core.c and it looks like that once
>>> udbg from it is registered then there is no code for unregistering it.
>>> So I have feeling that compiling cpm_uart driver as module should not be
>>> allowed when CONFIG_SERIAL_CPM_CONSOLE is enabled, and early debug
>>> should depend on CONFIG_CONSOLE_POLL || CONFIG_SERIAL_CPM_CONSOLE.
>>
>> The other change above also fixes the build error: (Option 2)
>>
>>>> config PPC_EARLY_DEBUG_CPM
>>>> 	bool "Early serial debugging for Freescale CPM-based serial ports"
>>>> -	depends on SERIAL_CPM
>>>> +	depends on SERIAL_CPM=y
>>
>>
>> Also, making SERIAL_CPM (cpm_uart driver) be restricted to not allow it
>> as a module when SERIAL_CPM_CONSOLE is enabled [how does one express that
>> in Kconfig language?] will cause a circular dependency since SERIAL_CPM_CONSOLE
>> depends on SERIAL_CPM=y. It looks like you are suggesting reversing the
>> dependencies.
> 
> I see that SERIAL_CPM_CONSOLE has already "depends on SERIAL_CPM=y".
> Am I right that this already disallow compiling cpm_uart driver as
> module when SERIAL_CPM_CONSOLE is enabled?

No, it just prevents SERIAL_CPM_CONSOLE from being set when
SERIAL_CPM=m.

>>> Any other opinion?
>>
>> Yes, please. Otherwise I prefer my option 2.
>>
>> -- 
>> ~Randy
> 
> Option 2 looks like a better solution.

Thanks.
-- 
~Randy