netdev - Re: poll broken (for can)

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Date:	Mon, 28 Mar 2011 19:53:27 +0200
From:	Oliver Hartkopp <socketcan@...tkopp.net>
To:	Marc Kleine-Budde <mkl@...gutronix.de>
CC:	Wolfgang Grandegger <wg@...ndegger.com>,
	socketcan-users@...ts.berlios.de, Netdev@...r.kernel.org
Subject: 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:
>>> 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

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