Re: [PATCH 05/12] mwifiex: avoid gettimeofday in ba_threshold setting

From: Arnd Bergmann [mailto:arnd@arndb.de]
Sent: Wednesday, September 30, 2015 4:57 PM
To: netdev@vger.kernel.org
Cc: y2038@lists.linaro.org; linux-kernel@vger.kernel.org; David S.
Miller; Arnd Bergmann; Amitkumar Karwar; Nishant Sarmukadam; Kalle Valo;
linux-wireless@vger.kernel.org
Subject: [PATCH 05/12] mwifiex: avoid gettimeofday in ba_threshold
setting

mwifiex_get_random_ba_threshold() uses a complex homegrown
implementation to generate a pseudo-random number from the current time
as returned from do_gettimeofday().

This currently requires two 32-bit divisions plus a couple of other
computations that are eventually discarded as only eight bits of the
microsecond portion are used at all.

We could replace this with a call to get_random_bytes(), but that might
drain the entropy pool too fast if this is called for each packet.

Instead, this patch converts it to use ktime_get_ns(), which is a bit
faster than do_gettimeofday(), and then uses a similar algorithm as
before, but in a way that takes both the nanosecond and second portion
into account for slightly-more-but-still-not-very-random
pseudorandom number.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Amitkumar Karwar <redacted>
Cc: Nishant Sarmukadam <redacted>
Cc: Kalle Valo <redacted>
Cc: linux-wireless@vger.kernel.org
---
 drivers/net/wireless/mwifiex/wmm.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/drivers/net/wireless/mwifiex/wmm.c

b/drivers/net/wireless/mwifiex/wmm.c
index 173d3663c2e0..878d358063dc 100644

--- a/drivers/net/wireless/mwifiex/wmm.c
+++ b/drivers/net/wireless/mwifiex/wmm.c

@@ -117,22 +117,15 @@ mwifiex_wmm_allocate_ralist_node(struct

mwifiex_adapter *adapter, const u8 *ra)
  */
 static u8 mwifiex_get_random_ba_threshold(void)
 {
-	u32 sec, usec;
-	struct timeval ba_tstamp;
-	u8 ba_threshold;
-
+	u64 ns;
 	/* setup ba_packet_threshold here random number between
 	 * [BA_SETUP_PACKET_OFFSET,
 	 * BA_SETUP_PACKET_OFFSET+BA_SETUP_MAX_PACKET_THRESHOLD-1]
 	 */
+	ns = ktime_get_ns();
+	ns += (ns >> 32) + (ns >> 16);

-	do_gettimeofday(&ba_tstamp);
-	sec = (ba_tstamp.tv_sec & 0xFFFF) + (ba_tstamp.tv_sec >> 16);
-	usec = (ba_tstamp.tv_usec & 0xFFFF) + (ba_tstamp.tv_usec >> 16);
-	ba_threshold = (((sec << 16) + usec) %
BA_SETUP_MAX_PACKET_THRESHOLD)
-						      + BA_SETUP_PACKET_OFFSET;
-
-	return ba_threshold;
+	return ((u8)ns % BA_SETUP_MAX_PACKET_THRESHOLD) +
+BA_SETUP_PACKET_OFFSET;
 }

 /*
--