netdev - Re: [PATCH v7] bfin_mac: convert bfin Ethernet driver to NAPI framework

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Date:	Tue, 22 Jul 2014 16:53:30 +0800
From:	Sonic Zhang <sonic.adi@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Francois Romieu <romieu@...zoreil.com>,
	adi-buildroot-devel@...ts.sourceforge.net,
	"Zhang, Sonic" <sonic.zhang@...log.com>
Subject: Re: [PATCH v7] bfin_mac: convert bfin Ethernet driver to NAPI framework

Hi David,

On Tue, Jul 22, 2014 at 4:01 PM, David Miller <davem@...emloft.net> wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Tue, 22 Jul 2014 09:59:44 +0200
>
>> On Tue, 2014-07-22 at 15:38 +0800, Sonic Zhang wrote:
>>> From: Sonic Zhang <sonic.zhang@...log.com>
>>
>>> +    if (i < budget) {
>>> +            napi_complete(napi);
>>> +            if (lp->rx_irq_disabled--)
>>> +                    if (!lp->rx_irq_disabled)
>>> +                            enable_irq(IRQ_MAC_RX);
>>
>>
>> You wrote :
>>
>>       if (A--)
>>               if (!A)
>>                       something();
>>
>> Isnt a bit strange ???
>
> Indeed, it's very strange, how about:
>
>         if (!--lp->rx_irq_disabled)
>
> which is the canonical way to express this.

if (!--A)

doesn't equal to

if (A--)
        if (!A)

In net poll loop, because of NAPI state bit NAPI_STATE_NPSVC, the napi
poll callback may be called with A = 0 (MAC RX IRQ is enabled) if
napi_complete() other than __napi_complete() is used. See the
explanation in my last email.

In this case, the up condition decreases A to a negative number and
the interrupt handler condition "if (!A++)" can never increase A back
to 0 (disable the RX IRQ). While, the down condition only decreases A
if it is not 0.


Regards,

Sonic
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