linux-kernel - Re: [PATCH] staging: rtl8723bs: Mark ACPI table declaration as used

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Date:   Thu, 27 Sep 2018 13:08:33 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        devel@...verdev.osuosl.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: rtl8723bs: Mark ACPI table declaration as used

On Thu, Sep 27, 2018 at 1:05 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Wed, Sep 26, 2018 at 10:16 PM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > On Wed, Sep 26, 2018 at 04:20:55PM -0700, Nathan Chancellor wrote:
> > > Clang emits the following warning:
> > >
> > > drivers/staging/rtl8723bs/os_dep/sdio_intf.c:25:36: warning: variable
> > > 'acpi_ids' is not needed and will not be emitted
> > > [-Wunneeded-internal-declaration]
> > > static const struct acpi_device_id acpi_ids[] = {
> > >                                    ^
> > > 1 warning generated.
> > >
> > > Mark acpi_ids with the attribute __used, which makes it clear to Clang
> > > that we don't want this warning while not inhibiting Clang's dead code
> > > elimination from removing the unreferenced internal symbol when moving
> > > the data to the globally available symbol with MODULE_DEVICE_TABLE.
> > >
> > > $ nm -S drivers/staging/rtl8723bs/os_dep/sdio_intf.o | grep acpi
> > > 0000000000000000 0000000000000040 R __mod_acpi__acpi_ids_device_table
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/169
> > > Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > > ---
> > >  drivers/staging/rtl8723bs/os_dep/sdio_intf.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/staging/rtl8723bs/os_dep/sdio_intf.c b/drivers/staging/rtl8723bs/os_dep/sdio_intf.c
> > > index 6d02904de63f..7c03b69b8ed3 100644
> > > --- a/drivers/staging/rtl8723bs/os_dep/sdio_intf.c
> > > +++ b/drivers/staging/rtl8723bs/os_dep/sdio_intf.c
> > > @@ -22,7 +22,7 @@ static const struct sdio_device_id sdio_ids[] =
> > >       { SDIO_DEVICE(0x024c, 0xb723), },
> > >       { /* end: all zeroes */                         },
> > >  };
> > > -static const struct acpi_device_id acpi_ids[] = {
> > > +static const struct acpi_device_id acpi_ids[] __used = {
> >
> > Can't we put __used somewhere in the MODULE_DEVICE_TABLE() macro so we
> > don't have to go adding it to all individual entries?
>
> Maybe; I'm still trying to think of what we could do in that macro to
> make its arg appear to be used while still being no functional change.

Sorry, I meant to say:
No, the __used needs to go on the declaration of the struct array, not
the alias being created within MODULE_DEVICE_TABLE().  I'm still
trying to think of what we can do *in* MODULE_DEVICE_TABLE() to
prevent this problem tree-wide, without having to modify any call
sites.

>
> This is not a million-little-commits problem; more like 2 drivers in
> the whole tree have this problem.
>
> Note that this change that Nathan has DOES NOT need to applied to
> every driver that uses MODULE_DEVICE_TABLE.  Only when the struct has
> no other references other than JUST being passed to
> MODULE_DEVICE_TABLE() is this a problem.  For an allyesconfig, I only
> see 2 instances of this case in the whole tree:
>
> https://github.com/ClangBuiltLinux/linux/issues/169 <- this patch is addressing
> https://github.com/ClangBuiltLinux/linux/issues/178 <- waiting on the
> results of this patch
>
> If you're ok with this change to two drivers, then:
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers