From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH v2] sh_eth: use RNC mode for R8A7790/R87791
Date: Mon, 02 Jun 2014 11:53:03 -0700 (PDT)
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From: Ben Dooks <ben.dooks@codethink.co.uk>
Date: Mon,  2 Jun 2014 18:17:36 +0100

> The current behaviour of the sh_eth driver is not to use the RNC bit
> for the receive ring. This means that every packet recieved is not only
> generating an IRQ but it also stops the receive ring DMA as well until
> the driver re-enables it after unloading the packet.
> 
> This means that a number of the following errors are generated due to
> the receive packet FIFO overflowing due to nowhere to put packets:
> 
> 	net eth0: Receive FIFO Overflow
> 
> I have tested the RMCR_RNC configuration with NFS root filesystem and
> the driver has not failed yet.  There are further test reports from
> Sergei Shtylov and others for both the R8A7790 and R8A7791.
> 
> There is also feedback fron Cao Minh Hiep[1] which reports the
> same issue in (http://comments.gmane.org/gmane.linux.network/316285)
> showing this fixes issues with losing UDP datagrams under iperf.
> 
> Tested-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
> Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>

Given the description, I can't fathom a reason why this wouldn't be
set always, for every chip.

Do some chips not implement this bit at all?