Subject: [PATCH i2c-next v6] i2c: aspeed: Handle master/slave combined irq events properly

From: [email-masked] (Guenter Roeck)
Date: Tue, 11 Sep 2018 18:34:49 -0700

On Tue, Sep 11, 2018 at 04:58:44PM -0700, Jae Hyun Yoo wrote:
> On 9/11/2018 4:33 PM, Guenter Roeck wrote:
> >Looking into the patch, clearing the interrupt status at the end of an
> >interrupt handler is always suspicious and tends to result in race
> >conditions (because additional interrupts may have arrived while handling
> >the existing interrupts, or because interrupt handling itself may trigger
> >another interrupt). With that in mind, the following patch fixes the
> >problem for me.
> >
> >Guenter
> >
> >---
> >
> >diff --git a/drivers/i2c/busses/i2c-aspeed.c b/drivers/i2c/busses/i2c-aspeed.c
> >index c258c4d9a4c0..c488e6950b7c 100644
> >--- a/drivers/i2c/busses/i2c-aspeed.c
> >+++ b/drivers/i2c/busses/i2c-aspeed.c
> >@@ -552,6 +552,8 @@ static irqreturn_t aspeed_i2c_bus_irq(int irq, void *dev_id)
> >  	spin_lock(&bus->lock);
> >  	irq_received = readl(bus->base + ASPEED_I2C_INTR_STS_REG);
> >+	/* Ack all interrupt bits. */
> >+	writel(irq_received, bus->base + ASPEED_I2C_INTR_STS_REG);
> >  	irq_remaining = irq_received;
> >  #if IS_ENABLED(CONFIG_I2C_SLAVE)
> >@@ -584,8 +586,6 @@ static irqreturn_t aspeed_i2c_bus_irq(int irq, void *dev_id)
> >  			"irq handled != irq. expected 0x%08x, but was 0x%08x\n",
> >  			irq_received, irq_handled);
> >-	/* Ack all interrupt bits. */
> >-	writel(irq_received, bus->base + ASPEED_I2C_INTR_STS_REG);
> >  	spin_unlock(&bus->lock);
> >  	return irq_remaining ? IRQ_NONE : IRQ_HANDLED;
> >  }
> >
> 
> My intention of putting the code at the end of interrupt handler was,
> to reduce possibility of combined irq calls which is explained in this
> patch. But YES, I agree with you. It could make a potential race

Hmm, yes, but that doesn't explain why it would make sense to acknowledge
the interrupt late. The interrupt ack only means "I am going to handle these
interrupts". If additional interrupts arrive while the interrupt handler
is active, those will have to be acknowledged separately.

Sure, there is a risk that an interrupt arrives while the handler is
running, and that it is handled but not acknowledged. That can happen
with pretty much all interrupt handlers, and there are mitigations to
limit the impact (for example, read the interrupt status register in
a loop until no more interrupts are pending). But acknowledging
an interrupt that was possibly not handled is always bad idea.

Thanks,
Guenter