netdev - Re: macb napi strange behavior

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Date:	Wed, 17 Jun 2015 13:20:45 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Nicolae Rosia <nicolae.rosia@...il.com>
Cc:	Jaeden Amero <jaeden.amero@...com>,
	netdev <netdev@...r.kernel.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Cyrille Pitchen <cyrille.pitchen@...el.com>,
	Josh Cartwright <joshc@...com>
Subject: Re: macb napi strange behavior

2015-06-17 12:00 GMT-07:00 Nicolae Rosia <nicolae.rosia@...il.com>:
> Hi,
>
> On Wed, Jun 17, 2015 at 9:54 PM, Jaeden Amero <jaeden.amero@...com> wrote:
>> On 06/17/2015 11:09 AM, Nicolae Rosia wrote:
>> The times we've seen tons of interrupts on Ethernet with interrupts
>> routed through the PL was when the FPGA was unprogrammed (or in the
>> process of being reprogrammed), or was configured with the interrupt
>> line tied to asserted.
>>
>> In the latter case, Linux would eventually stop handling any more
>> interrupts for that port due to the interrupt storm.
>>
> This isn't the case. The FPGA is programmed, and indeed I'm using the
> second MAC routed through PL to SFP.
> The interesting thing is that I'm seeing the exact behavior on the
> other side (another Zynq7 board), with eth0 having lots of interrupts.
> It seems that the interface receiving packets doesn't have a high IRQ
> activity in contrast to the one sending packets.

Typically, NAPI is used at the receive side of the Ethernet NIC/driver
to lower the hard/soft interrupt context switch, although there is
nothing that prevent you to implement a similar scheme for the
transmit side. Usually, for transmit you will be submitting one packet
for transmission and get a completion interrupt, so without interrupt
coalescing (software or hardware) you can end-up with 1 interrupt per
packet transmitted.
-- 
Florian
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