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SubjectRE: [PATCH 05/12] mwifiex: avoid gettimeofday in ba_threshold setting
Date
Hi Arnd,

> 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 <akarwar@marvell.com>
> Cc: Nishant Sarmukadam <nishants@marvell.com>
> Cc: Kalle Valo <kvalo@codeaurora.org>
> 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;
> }
>
> /*
> --

Looks fine to me.
Acked-by: Amitkumar Karwar <akarwar@marvell.com>

Regards,
Amitkumar


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