linux-kernel - [PATCH] firewire: core: suppress overflow warning when computing jiffies from isochronous cycle

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Message-ID: <20250924131140.261686-1-o-takashi@sakamocchi.jp>
Date: Wed, 24 Sep 2025 22:11:40 +0900
From: Takashi Sakamoto <o-takashi@...amocchi.jp>
To: linux1394-devel@...ts.sourceforge.net
Cc: linux-kernel@...r.kernel.org,
	kernel test robot <lkp@...el.com>
Subject: [PATCH] firewire: core: suppress overflow warning when computing jiffies from isochronous cycle

The multiplication by USEC_PER_SEC (=1000000L) may trigger an overflow
warning with 32 bit storage. In the case of the subsystem the input value
ranges between 800 and 16000, thus the result always fits within 32 bit
storage.

This commit suppresses the warning by using widening conversion to 64 bit
storage before multiplication, then using narrowing conversion to 32 bit
storage.

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509170136.b5ZHaNAV-lkp@intel.com/
Fixes: 379b870c28c6 ("firewire: core: use helper macros instead of direct access to HZ")
Signed-off-by: Takashi Sakamoto <o-takashi@...amocchi.jp>
---
 drivers/firewire/core.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firewire/core.h b/drivers/firewire/core.h
index 7f2ca93406ce..2dd715a580ac 100644
--- a/drivers/firewire/core.h
+++ b/drivers/firewire/core.h
@@ -30,7 +30,7 @@ struct fw_packet;
 // This is the arbitrary value we use to indicate a mismatched gap count.
 #define GAP_COUNT_MISMATCHED	0
 
-#define isoc_cycles_to_jiffies(cycles)	usecs_to_jiffies(cycles * USEC_PER_SEC / 8000)
+#define isoc_cycles_to_jiffies(cycles)	usecs_to_jiffies((u32)((u64)(cycles) * USEC_PER_SEC / 8000))
 
 extern __printf(2, 3)
 void fw_err(const struct fw_card *card, const char *fmt, ...);

base-commit: 19e73f65940d3d3357c637f3d7e19a59305a748f
-- 
2.48.1