Re: [PATCH i2c-next v6] i2c: aspeed: Handle master/slave combined irq events properly
From: Jae Hyun Yoo
Date: Wed Sep 12 2018 - 16:11:11 EST
On 9/12/2018 12:58 PM, Guenter Roeck wrote:
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.
Only bit 2 has that description in datasheet. Is slave config enabled
for QEMU build? Does that get stuck in master sending or slave
receiving?
Thanks,
Jae