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SubjectRe: [PATCH] drivers/macintosh/smu.c: Mark expected switch fall-through
On Wed, Jul 31, 2019 at 12:28:55AM +1000, Michael Ellerman wrote:
> Stephen Rothwell <sfr@canb.auug.org.au> writes:
> > Mark switch cases where we are expecting to fall through.
> >
> > This patch fixes the following warning (Building: powerpc):
> >
> > drivers/macintosh/smu.c: In function 'smu_queue_i2c':
> > drivers/macintosh/smu.c:854:21: warning: this statement may fall through [-Wimplicit-fallthrough=]
> > cmd->info.devaddr &= 0xfe;
> > ~~~~~~~~~~~~~~~~~~^~~~~~~
> > drivers/macintosh/smu.c:855:2: note: here
> > case SMU_I2C_TRANSFER_STDSUB:
> > ^~~~
> >
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
> > ---
> > drivers/macintosh/smu.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/macintosh/smu.c b/drivers/macintosh/smu.c
> > index 276065c888bc..23f1f41c8602 100644
> > --- a/drivers/macintosh/smu.c
> > +++ b/drivers/macintosh/smu.c
> > @@ -852,6 +852,7 @@ int smu_queue_i2c(struct smu_i2c_cmd *cmd)
> > break;
> > case SMU_I2C_TRANSFER_COMBINED:
> > cmd->info.devaddr &= 0xfe;
> > + /* fall through */
> > case SMU_I2C_TRANSFER_STDSUB:
> > if (cmd->info.sublen > 3)
> > return -EINVAL;
>
> Why do we think it's an expected fall through? I can't really convince
> myself from the surrounding code that it's definitely intentional.

Yeah, good question. Just now when I went looking for who
used SMU_I2C_TRANSFER_COMBINED, I found the only caller in
arch/powerpc/platforms/powermac/low_i2c.c and it is clearly using a
fall-through for building the command for "stdsub" and "combined",
so I think that's justification enough:

switch(bus->mode) {
case pmac_i2c_mode_std:
if (subsize != 0)
return -EINVAL;
cmd->info.type = SMU_I2C_TRANSFER_SIMPLE;
break;
case pmac_i2c_mode_stdsub:
case pmac_i2c_mode_combined:
if (subsize > 3 || subsize < 1)
return -EINVAL;
cmd->info.sublen = subsize;
/* that's big-endian only but heh ! */
memcpy(&cmd->info.subaddr, ((char *)&subaddr) + (4 - subsize),
subsize);
if (bus->mode == pmac_i2c_mode_stdsub)
cmd->info.type = SMU_I2C_TRANSFER_STDSUB;
else
cmd->info.type = SMU_I2C_TRANSFER_COMBINED;


Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook

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