linux-kernel - Re: [PATCH] mediatek: mt7601u: fix clang -Wformat warning

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Date:   Fri, 15 Jul 2022 17:08:34 -0700
From:   Justin Stitt <justinstitt@...gle.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Jakub Kicinski <kubakici@...pl>, Kalle Valo <kvalo@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] mediatek: mt7601u: fix clang -Wformat warning

On Thu, Jul 14, 2022 at 11:33 PM Joe Perches <joe@...ches.com> wrote:
>
> On Mon, 2022-07-11 at 14:29 -0700, Justin Stitt wrote:
> > When building with Clang we encounter this warning:
> > > drivers/net/wireless/mediatek/mt7601u/debugfs.c:92:6: error: format
> > > specifies type 'unsigned char' but the argument has type 'int'
> > > [-Werror,-Wformat] dev->ee->reg.start + dev->ee->reg.num - 1);
> >
> > The format specifier used is `%hhu` which describes a u8. Both
> > `dev->ee->reg.start` and `.num` are u8 as well. However, the expression
> > as a whole is promoted to an int as you cannot get smaller-than-int from
> > addition. Therefore, to fix the warning, use the promoted-to-type's
> > format specifier -- in this case `%d`.
>
> I think whenever a sizeof(unsigned type) that is less than sizeof(int) is
> emitted with vsprintf, the preferred format specifier should be %u not %d.

I believe the standard recommends using the promoted-to-type's format
specifier, in this case integer. I agree, though, that it is quite
bizarre to represent unsigned types with signed types. I'd be
interested in hearing more opinions on the matter.

> > diff --git a/drivers/net/wireless/mediatek/mt7601u/debugfs.c b/drivers/net/wireless/mediatek/mt7601u/debugfs.c
> []
> > @@ -88,7 +88,7 @@ mt7601u_eeprom_param_show(struct seq_file *file, void *data)
> >                  dev->ee->rssi_offset[0], dev->ee->rssi_offset[1]);
> >       seq_printf(file, "Reference temp: %hhx\n", dev->ee->ref_temp);
> >       seq_printf(file, "LNA gain: %hhx\n", dev->ee->lna_gain);
> > -     seq_printf(file, "Reg channels: %hhu-%hhu\n", dev->ee->reg.start,
> > +     seq_printf(file, "Reg channels: %hhu-%d\n", dev->ee->reg.start,
> >                  dev->ee->reg.start + dev->ee->reg.num - 1);
>
> And this is not a promotion of an argument to int via varargs.
> The arithmetic did the promotion.

Right, I noted in my patch message that the type promotion was due to addition.

>
> I suggest s/%hh/%/ for all the uses here, not just this one.

I also contemplated this change but I think it might be a bit out of
scope for https://github.com/ClangBuiltLinux/linux/issues/378  -- What
do you think?
It could be argued that every single instance of %hh[dux] should be
replaced with %[dux].

> checkpatch could do this somewhat automatically.
> Of course any changes it suggests need human review.
>
> $ ./scripts/checkpatch.pl -f drivers/net/wireless/mediatek/mt7601u/debugfs.c --show-types --types=unnecessary_modifier --fix-inplace
> $ git diff --stat -p drivers/net/wireless/mediatek/mt7601u/debugfs.c
> ---
>  drivers/net/wireless/mediatek/mt7601u/debugfs.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/wireless/mediatek/mt7601u/debugfs.c b/drivers/net/wireless/mediatek/mt7601u/debugfs.c
> index 20669eacb66ea..b7a6376e3352e 100644
> --- a/drivers/net/wireless/mediatek/mt7601u/debugfs.c
> +++ b/drivers/net/wireless/mediatek/mt7601u/debugfs.c
> @@ -83,28 +83,28 @@ mt7601u_eeprom_param_show(struct seq_file *file, void *data)
>         struct tssi_data *td = &dev->ee->tssi_data;
>         int i;
>
> -       seq_printf(file, "RF freq offset: %hhx\n", dev->ee->rf_freq_off);
> -       seq_printf(file, "RSSI offset: %hhx %hhx\n",
> +       seq_printf(file, "RF freq offset: %x\n", dev->ee->rf_freq_off);
> +       seq_printf(file, "RSSI offset: %x %x\n",
>                    dev->ee->rssi_offset[0], dev->ee->rssi_offset[1]);
> -       seq_printf(file, "Reference temp: %hhx\n", dev->ee->ref_temp);
> -       seq_printf(file, "LNA gain: %hhx\n", dev->ee->lna_gain);
> -       seq_printf(file, "Reg channels: %hhu-%hhu\n", dev->ee->reg.start,
> +       seq_printf(file, "Reference temp: %x\n", dev->ee->ref_temp);
> +       seq_printf(file, "LNA gain: %x\n", dev->ee->lna_gain);
> +       seq_printf(file, "Reg channels: %u-%u\n", dev->ee->reg.start,
>                    dev->ee->reg.start + dev->ee->reg.num - 1);
>
>         seq_puts(file, "Per rate power:\n");
>         for (i = 0; i < 2; i++)
> -               seq_printf(file, "\t raw:%02hhx bw20:%02hhx bw40:%02hhx\n",
> +               seq_printf(file, "\t raw:%02x bw20:%02x bw40:%02x\n",
>                            rp->cck[i].raw, rp->cck[i].bw20, rp->cck[i].bw40);
>         for (i = 0; i < 4; i++)
> -               seq_printf(file, "\t raw:%02hhx bw20:%02hhx bw40:%02hhx\n",
> +               seq_printf(file, "\t raw:%02x bw20:%02x bw40:%02x\n",
>                            rp->ofdm[i].raw, rp->ofdm[i].bw20, rp->ofdm[i].bw40);
>         for (i = 0; i < 4; i++)
> -               seq_printf(file, "\t raw:%02hhx bw20:%02hhx bw40:%02hhx\n",
> +               seq_printf(file, "\t raw:%02x bw20:%02x bw40:%02x\n",
>                            rp->ht[i].raw, rp->ht[i].bw20, rp->ht[i].bw40);
>
>         seq_puts(file, "Per channel power:\n");
>         for (i = 0; i < 7; i++)
> -               seq_printf(file, "\t tx_power  ch%u:%02hhx ch%u:%02hhx\n",
> +               seq_printf(file, "\t tx_power  ch%u:%02x ch%u:%02x\n",
>                            i * 2 + 1, dev->ee->chan_pwr[i * 2],
>                            i * 2 + 2, dev->ee->chan_pwr[i * 2 + 1]);
>
> @@ -112,8 +112,8 @@ mt7601u_eeprom_param_show(struct seq_file *file, void *data)
>                 return 0;
>
>         seq_puts(file, "TSSI:\n");
> -       seq_printf(file, "\t slope:%02hhx\n", td->slope);
> -       seq_printf(file, "\t offset=%02hhx %02hhx %02hhx\n",
> +       seq_printf(file, "\t slope:%02x\n", td->slope);
> +       seq_printf(file, "\t offset=%02x %02x %02x\n",
>                    td->offset[0], td->offset[1], td->offset[2]);
>         seq_printf(file, "\t delta_off:%08x\n", td->tx0_delta_offset);
>
>