netdev - poll broken (for can) (was: Re: [Socketcan-users] Multiple programs trying to access the socket)

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Message-ID: <4D90B3B0.2010401@pengutronix.de>
Date:	Mon, 28 Mar 2011 18:13:36 +0200
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Wolfgang Grandegger <wg@...ndegger.com>
CC:	socketcan-users@...ts.berlios.de, Netdev@...r.kernel.org
Subject: poll broken (for can)  (was: Re: [Socketcan-users] Multiple programs
 trying to access the socket)

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