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

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Date:   Mon, 11 Jul 2022 14:29:32 -0700
From:   Justin Stitt <justinstitt@...gle.com>
To:     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>
Cc:     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, Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] mediatek: mt7601u: fix clang -Wformat warning

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`.

example:
```
uint8_t a = 4, b = 7;
int size = sizeof(a + b - 1);
printf("%d\n", size);
// output: 4
```

See more:
(https://wiki.sei.cmu.edu/confluence/display/c/INT02-C.+Understand+integer+conversion+rules)
"Integer types smaller than int are promoted when an operation is
performed on them. If all values of the original type can be represented
as an int, the value of the smaller type is converted to an int;
otherwise, it is converted to an unsigned int."

Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
Note: This patch silences the -Wformat warning for this file (which is
the goal) but in reality all instances of `%hh[dux]` should be converted
to `%[dux]` for this file and probably every file. That's a bit larger
scope than the goal of enabling -Wformat for Clang builds, though.

 drivers/net/wireless/mediatek/mt7601u/debugfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wireless/mediatek/mt7601u/debugfs.c b/drivers/net/wireless/mediatek/mt7601u/debugfs.c
index 20669eacb66e..230b0e1061a7 100644
--- 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);
 
 	seq_puts(file, "Per rate power:\n");
-- 
2.37.0.144.g8ac04bfd2-goog