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

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Message-ID: <20230513091358.pzlexqnff5ydhlzf@pali>
Date:   Sat, 13 May 2023 11:13:58 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Randy Dunlap <rdunlap@...radead.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 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
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.

Any other opinion?