[PATCH/RFC 1/2] ALSA: fireface: Fix integer overflow in transmit_midi_msg()

[Geert Uytterhoeven]

As snd_ff.rx_bytes[] is unsigned int, and NSEC_PER_SEC is 1000000000L,
the second multiplication in

    ff->rx_bytes[port] * 8 * NSEC_PER_SEC / 31250

always overflows on 32-bit platforms, truncating the result.  Fix this
by precalculating "NSEC_PER_SEC / 31250", which is an integer constant.

Note that this assumes ff->rx_bytes[port] <= 16777.

Fixes: 19174295788de77d ("ALSA: fireface: add transaction support")
Signed-off-by: Geert Uytterhoeven <geert+renesas at glider.be>
---
Compile-tested only.

I don't know the maximum transfer length of MIDI, but given it's an old
standard, I guess it's rather small.  If it is larger than 16777, the
constant "8" should be replaced by "8ULL", to force 64-bit arithmetic.
---
 sound/firewire/fireface/ff-transaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/firewire/fireface/ff-transaction.c b/sound/firewire/fireface/ff-transaction.c
index 7f82762ccc8c80ba..ee7122c461d46f44 100644
--- a/sound/firewire/fireface/ff-transaction.c
+++ b/sound/firewire/fireface/ff-transaction.c
@@ -88,7 +88,7 @@ static void transmit_midi_msg(struct snd_ff *ff, unsigned int port)
 
 	/* Set interval to next transaction. */
 	ff->next_ktime[port] = ktime_add_ns(ktime_get(),
-				ff->rx_bytes[port] * 8 * NSEC_PER_SEC / 31250);
+			ff->rx_bytes[port] * 8 * (NSEC_PER_SEC / 31250));
 
 	if (quad_count == 1)
 		tcode = TCODE_WRITE_QUADLET_REQUEST;
-- 
2.25.1