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
