netdev - Re: [PATCH 1/2] net: fec: don't ack masked interrupt events

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Date:   Wed, 06 Dec 2017 15:47:37 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     l.stach@...gutronix.de
Cc:     fugang.duan@....com, netdev@...r.kernel.org,
        patchwork-lst@...gutronix.de, kernel@...gutronix.de
Subject: Re: [PATCH 1/2] net: fec: don't ack masked interrupt events

From: Lucas Stach <l.stach@...gutronix.de>
Date: Wed,  6 Dec 2017 18:24:58 +0100

> The FEC doesn't have a real interrupt status register, that takes
> into account the mask status of the IRQ. The driver reads the raw
> interrupt event register, which also reports events for masked
> IRQs.
> 
> The driver needs to apply the current mask itself, to avoid acking
> IRQs that are currently masked, as NAPI relies on the masking to
> hide the IRQs. The current behavior of just acking all interrupts
> regardless of their mask status opens the driver up the "rotting
> packet" race-window, as described in the original NAPI-HOWTO, which
> has been observed in the wild.
> 
> Signed-off-by: Lucas Stach <l.stach@...gutronix.de>
> ---
>  drivers/net/ethernet/freescale/fec_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> index 610573855213..0b70c07eb703 100644
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c
> @@ -1584,7 +1584,8 @@ fec_enet_interrupt(int irq, void *dev_id)
>  	uint int_events;
>  	irqreturn_t ret = IRQ_NONE;
>  
> -	int_events = readl(fep->hwp + FEC_IEVENT);
> +	int_events = readl_relaxed(fep->hwp + FEC_IEVENT) &
> +	             readl_relaxed(fep->hwp + FEC_IMASK);

Adding a full new MMIO register read to every interrupt is going to make
interrupts significantly more expensive.

You should cache this mask in software in order to avoid the expensive
MMIO read.

Thanks.