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

[Russell King - ARM Linux]

On Fri, Aug 26, 2016 at 05:53:38PM +0200, Lucas Stach wrote:
> Sorry, please ignore the FEC patches. Those are test patches still
> residing in my to-send folder. Sorry for the noise.

This patch actually looks correct: you are indeed correct that the
driver can end up with a packet sitting waiting to be received.
All it will take is a correctly timed MII interrupt event with a
NAPI just coming to the end of its processing.

So, I'd encourage this to be applied - but obviously when sent to
netdev rather than the DRI mailing list!

> 
> Am Freitag, den 26.08.2016, 17:49 +0200 schrieb Lucas Stach:
> > 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 at pengutronix.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 01f7e811739b..498264969e89 100644
> > --- a/drivers/net/ethernet/freescale/fec_main.c
> > +++ b/drivers/net/ethernet/freescale/fec_main.c
> > @@ -1572,7 +1572,8 @@ 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);
> >  	writel(int_events, fep->hwp + FEC_IEVENT);
> >  	fec_enet_collect_events(fep, int_events);
> >  
> 
> 

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