I'm looking to enable -Wmissing-variable-declarations behind W=1. 0day
bot spotted the following instance:

  drivers/i2c/busses/i2c-amd756.c:286:20: warning: no previous extern
  declaration for non-static variable 'amd756_smbus'
  [-Wmissing-variable-declarations]
  286 | struct i2c_adapter amd756_smbus = {
      |                    ^
  drivers/i2c/busses/i2c-amd756.c:286:1: note: declare 'static' if the
  variable is not intended to be used outside of this translation unit
  286 | struct i2c_adapter amd756_smbus = {
      | ^

This symbol is referenced by more than one translation unit, so create
then include the correct header for their declarations.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/llvm/202308081000.tTL1ElTr-lkp@intel.com/
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 drivers/i2c/busses/i2c-amd756-s4882.c | 3 +--
 drivers/i2c/busses/i2c-amd756.c       | 1 +
 drivers/i2c/busses/i2c-amd756.h       | 3 +++
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-amd756-s4882.c b/drivers/i2c/busses/i2c-amd756-s4882.c
index 063274388a75..8156cfc43df3 100644
--- a/drivers/i2c/busses/i2c-amd756-s4882.c
+++ b/drivers/i2c/busses/i2c-amd756-s4882.c
@@ -26,8 +26,7 @@
 #include <linux/init.h>
 #include <linux/i2c.h>
 #include <linux/mutex.h>
-
-extern struct i2c_adapter amd756_smbus;
+#include "i2c-amd756.h"
 
 static struct i2c_adapter *s4882_adapter;
 static struct i2c_algorithm *s4882_algo;
diff --git a/drivers/i2c/busses/i2c-amd756.c b/drivers/i2c/busses/i2c-amd756.c
index ef1307a258e9..af77374d2ab3 100644
--- a/drivers/i2c/busses/i2c-amd756.c
+++ b/drivers/i2c/busses/i2c-amd756.c
@@ -31,6 +31,7 @@
 #include <linux/i2c.h>
 #include <linux/acpi.h>
 #include <linux/io.h>
+#include "i2c-amd756.h"
 
 /* AMD756 SMBus address offsets */
 #define SMB_ADDR_OFFSET		0xE0
diff --git a/drivers/i2c/busses/i2c-amd756.h b/drivers/i2c/busses/i2c-amd756.h
new file mode 100644
index 000000000000..88698266d6d8
--- /dev/null
+++ b/drivers/i2c/busses/i2c-amd756.h
@@ -0,0 +1,3 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#include <linux/i2c.h>
+extern struct i2c_adapter amd756_smbus;

---
base-commit: 14f9643dc90adea074a0ffb7a17d337eafc6a5cc
change-id: 20230808-i2c-amd_static-81e5c27a84ce

Best regards,
-- 
Nick Desaulniers <ndesaulniers@google.com>

Hi Nick,

On Tue, Aug 08, 2023 at 09:56:16AM -0700, Nick Desaulniers wrote:
> I'm looking to enable -Wmissing-variable-declarations behind W=1. 0day
> bot spotted the following instance:
> 
>   drivers/i2c/busses/i2c-amd756.c:286:20: warning: no previous extern
>   declaration for non-static variable 'amd756_smbus'
>   [-Wmissing-variable-declarations]
>   286 | struct i2c_adapter amd756_smbus = {
>       |                    ^
>   drivers/i2c/busses/i2c-amd756.c:286:1: note: declare 'static' if the
>   variable is not intended to be used outside of this translation unit
>   286 | struct i2c_adapter amd756_smbus = {
>       | ^
> 
> This symbol is referenced by more than one translation unit, so create
> then include the correct header for their declarations.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/llvm/202308081000.tTL1ElTr-lkp@intel.com/
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

You might want to have a Fixes tag here and

Cc: Jean Delvare <jdelvare@suse.com>

[...]

> --- /dev/null
> +++ b/drivers/i2c/busses/i2c-amd756.h
> @@ -0,0 +1,3 @@
> +/* SPDX-License-Identifier: GPL-2.0 */

Please, leave a space here.

And you might also want to have something like:

#ifndef __I2C_AMD_756_H__
#define __I2C_AMD_756_H__

> +#include <linux/i2c.h>

space here.

> +extern struct i2c_adapter amd756_smbus;

#endif /* __I2C_AMD_756_H__ */

Jean, any opinion on this patch, I don't really know this driver,
but is there a way to avoid this extern declaration.

Thanks,
Andi

Hi Andi, Nick,

On Wed, 09 Aug 2023 21:13:10 +0200, Andi Shyti wrote:
> On Tue, Aug 08, 2023 at 09:56:16AM -0700, Nick Desaulniers wrote:
> > I'm looking to enable -Wmissing-variable-declarations behind W=1. 0day
> > bot spotted the following instance:
> > 
> >   drivers/i2c/busses/i2c-amd756.c:286:20: warning: no previous extern
> >   declaration for non-static variable 'amd756_smbus'
> >   [-Wmissing-variable-declarations]
> >   286 | struct i2c_adapter amd756_smbus = {
> >       |                    ^
> >   drivers/i2c/busses/i2c-amd756.c:286:1: note: declare 'static' if the
> >   variable is not intended to be used outside of this translation unit
> >   286 | struct i2c_adapter amd756_smbus = {
> >       | ^
> > 
> > This symbol is referenced by more than one translation unit, so create
> > then include the correct header for their declarations.
> > 
> > Reported-by: kernel test robot <lkp@intel.com>
> > Closes: https://lore.kernel.org/llvm/202308081000.tTL1ElTr-lkp@intel.com/
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>  
> 
> You might want to have a Fixes tag here and
> 
> Cc: Jean Delvare <jdelvare@suse.com>

Fixes tag would cause unnecessary worry, with people backporting the
patch while it doesn't actually fix anything. No need for that.

> 
> [...]
> 
> > --- /dev/null
> > +++ b/drivers/i2c/busses/i2c-amd756.h
> > @@ -0,0 +1,3 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */  
> 
> Please, leave a space here.
> 
> And you might also want to have something like:
> 
> #ifndef __I2C_AMD_756_H__
> #define __I2C_AMD_756_H__
> 
> > +#include <linux/i2c.h>  
> 
> space here.
> 
> > +extern struct i2c_adapter amd756_smbus;  
> 
> #endif /* __I2C_AMD_756_H__ */
> 
> Jean, any opinion on this patch, I don't really know this driver,
> but is there a way to avoid this extern declaration.

Thanks for your review. I would personally not bother with a header
file, this is unnecessary burden. Just add the extern declaration to
i2c-amd756.c as was already done in i2c-amd756-s4882.c. The whole thing
is a horrible hack anyway and there won't be more occurrences of it, so
the more simple it stays, the better.

Some history: I wrote the pseudo-driver i2c-amd756-s4882 when the
i2c-core did not yet support multiplexing. Nowadays, proper support
would be implemented using i2c-mux-* and i2c-amd756-s4882.c would go
away entirely. Actually, I very much doubt any of these 2007 Tyan
server boards is still in activity today, so maybe we should just
delete the driver.

Note that i2c-nforce2.c will need the same fix as struct i2c_adapter
nforce2_smbus is extern as well.

-- 
Jean Delvare
SUSE L3 Support