Re: poll broken (for can)

On 28.03.2011 18:13, Marc Kleine-Budde wrote:

On 03/28/2011 05:55 PM, Wolfgang Grandegger wrote:

quoted

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

What would be your suggestion? Decreasing the socket send buffer for CAN by
default?

Regards,
Oliver