netdev - Re: r8169: improve spurious interrupt detection

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Date:   Sat, 15 Dec 2018 19:44:35 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     nic_swsd@...ltek.com, netdev@...r.kernel.org
Subject: Re: r8169: improve spurious interrupt detection

On 15.12.2018 19:35, David Miller wrote:
> From: Heiner Kallweit <hkallweit1@...il.com>
> Date: Sat, 15 Dec 2018 16:24:02 +0100
> 
>> @@ -6405,8 +6405,9 @@ static irqreturn_t rtl8169_interrupt(int irq, void *dev_instance)
>>  {
>>  	struct rtl8169_private *tp = dev_instance;
>>  	u16 status = rtl_get_events(tp);
>> +	u16 irq_mask = RTL_R16(tp, IntrMask);
>>  
>> -	if (status == 0xffff || !(status & tp->irq_mask))
>> +	if (status == 0xffff || !(status & irq_mask))
>>  		return IRQ_NONE;
> 
> Heiner, this is really expensive.  Why is the tp->irq_mask value out of
> sync with the value programmed into IntrMask?  Is there some way we can
> fix that instead?
> 
tp->irq_mask holds the chip-specific interrupt mask. It doesn't say
whether interrupts are enabled or not. rtl_get_events() reads via
PCI(e) anyway, so I was under the impression that one more PCI(e) read
doesn't really matter.
We could introduce a flag shadowing the "interrupts are enabled" state
and use it here. But I'm not sure whether it's worth it.
Alternatively we could also go with the readw_relaxed() version to
get the values, this would eliminate the memory barrier at least.