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

[Guenter Roeck <linux@roeck-us.net>]

On Wed, Sep 12, 2018 at 09:54:51AM -0700, Jae Hyun Yoo wrote:
> On 9/11/2018 6:34 PM, Guenter Roeck wrote:
> >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.
> 
> Well, that's generally right but not always. Sometimes that depends on
> hardware and Aspeed I2C is the case.
> 
> This is a description from Aspeed AST2500 datasheet:
>   I2CD10 Interrupt Status Register
>   bit 2 Receive Done Interrupt status
>         S/W needs to clear this status bit to allow next data receiving.
> 
> It means, driver should hold this bit to prevent transition of hardware
> state machine until the driver handles received data, so the bit should
> be cleared at the end of interrupt handler.
> 
That makes sense. Does that apply to the other status bits as well ?
Reason for asking is that the current code actually gets stuck
in transmit, not receive.

Thanks,
Guenter