On 5/13/19 2:44 PM, Oleksij Rempel wrote:
> If retry timeout is equal for all systems and/or sockets, it will
> collide at some point. In this case, one sender will always have
> a chance to send some thing and make the CAN queue full, the other
> one will always get -ENOBUS response.
> Even if we would implement a scheduler for local system, we can't solve
> same problem for multiple identical systems on same bus.
>
> Since the protocol has no other way to avoid this kind of collisions,
> we can use random number for retry timer and reduce probability of
> colliding retries.
>
> Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
> ---
> net/can/j1939/transport.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
> index f469d9e131c7..978f2e194f32 100644
> --- a/net/can/j1939/transport.c
> +++ b/net/can/j1939/transport.c
> @@ -116,7 +116,6 @@ enum j1939_xtp_abort {
> };
>
> static unsigned int j1939_tp_block = 255;
> -static unsigned int j1939_tp_retry_ms = 20;
> static unsigned int j1939_tp_packet_delay;
> static unsigned int j1939_tp_padding = 1;
>
> @@ -873,8 +872,12 @@ static enum hrtimer_restart j1939_tp_txtimer(struct hrtimer *hrtimer)
> int ret;
>
> ret = j1939_tp_txnext(session);
> - if (ret < 0)
> - j1939_tp_schedule_txtimer(session, j1939_tp_retry_ms ?: 20);
> + if (ret < 0) {
> + u32 random = get_random_u32();
It's probably save to use prandom_u32_max():
https://elixir.bootlin.com/linux/v5.1/source/include/linux/random.h#L137
> +
> + j1939_tp_schedule_txtimer(session, 10 + (random & 0xf));
> +
> + }
> j1939_session_put(session);
>
> return HRTIMER_NORESTART;
>
Marc
--
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