poll broken (for can) (was: Re: Multiple programs trying to access the socket)
From: Marc Kleine-Budde <hidden>
Date: 2011-03-28 16:13:36
On 03/28/2011 05:55 PM, Wolfgang Grandegger wrote:
quoted
BTW: I figured out why poll() wakes you up but the next write will fail with -ENOBUFS again.Ah, I'm curious? I also did realize that poll does burn CPU cycles (instead of waiting).
The poll callback checks if the used memory is less than the half of per socket snd buffer (IIRC ~60K). See: datagram_poll (http://lxr.linux.no/linux+v2.6.38/net/core/datagram.c#L737) sock_writeable (http://lxr.linux.no/linux+v2.6.38/include/net/sock.h#L1618) Because the size of a can frame (+the skb overhead) is much less then the ethernet frame (+overhead) the default value for the snd buffer is too big for can. We get the -ENOBUF from write() if the tx_queue_len (default 10) is exceeded. http://lxr.linux.no/linux+v2.6.38/drivers/net/can/dev.c#L435 http://lxr.linux.no/linux+v2.6.38/net/can/af_can.c#L268 cheers, Marc -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |