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

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Message-ID: <20230513230532.ojjf2xkmkx4v4zdo@pali>
Date:   Sun, 14 May 2023 01:05:32 +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 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?

> > Any other opinion?
> 
> Yes, please. Otherwise I prefer my option 2.
> 
> -- 
> ~Randy

Option 2 looks like a better solution.