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

From: Jae Hyun Yoo
Date: Thu Sep 13 2018 - 13:01:26 EST

On 9/13/2018 9:51 AM, CÃdric Le Goater wrote:
Hello !

On 09/13/2018 06:31 PM, Jae Hyun Yoo wrote:
Hi CÃdric,

On 9/12/2018 10:47 PM, CÃdric Le Goater wrote:
On 09/12/2018 06:54 PM, 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.



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)
ÂÂÂÂÂÂ spin_lock(&bus->lock);
ÂÂÂÂÂÂ irq_received = readl(bus->base + ASPEED_I2C_INTR_STS_REG);
+ÂÂÂ /* Ack all interrupt bits. */
+ÂÂÂ writel(irq_received, bus->base + ASPEED_I2C_INTR_STS_REG);
ÂÂÂÂÂÂ irq_remaining = irq_received;
@@ -584,8 +586,6 @@ static irqreturn_t aspeed_i2c_bus_irq(int irq, void *dev_id)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ "irq handled != irq. expected 0x%08x, but was 0x%08x\n",
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ irq_received, irq_handled);
-ÂÂÂ /* Ack all interrupt bits. */
-ÂÂÂ writel(irq_received, bus->base + ASPEED_I2C_INTR_STS_REG);
ÂÂÂÂÂÂ spin_unlock(&bus->lock);
ÂÂÂÂÂÂ return irq_remaining ? IRQ_NONE : IRQ_HANDLED;
ÂÂ }

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:
ÂÂ I2CD10 Interrupt Status Register
ÂÂ bit 2 Receive Done Interrupt status
ÂÂÂÂÂÂÂÂ 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.

Let me share my test result. Your code change works on 100KHz bus speed
but doesn't work well on 1MHz bus speed. Investigated that interrupt
handling is fast enough in 100KHz test but in 1MHz, most of data is
corrupted because the bit is cleared at the beginning of interrupt
handler so it allows receiving of the next data but the interrupt
handler isn't fast enough to read the data buffer on time. I checked
this problem on BMC-ME channel which ME sends lots of IPMB packets to
BMC at 1MHz speed. You could simply check the data corruption problem on
the BMC-ME channel.

My thought is, the current code is right for real Aspeed I2C hardware.
It seems that QEMU 3.0 model for witherspoon-bmc doesn't simulate the
actual Aspeed I2C hardware correctly.

That might be very well possible yes. it also misses support for the slave
mode and the DMA registers.

Yes, it would be good if qemu's Aspeed I2C model supports slave mode.
Since the current linux Aspeed I2C driver supports byte transfer mode
only, so DMA transfer mode support in qemu could be considered later.

The Aspeed SDK already does, so yes, we will need to consider it.

Yes, Aspeed SDK does but the driver in upstream doesn't at this time.
So we will need to consider it because byte mode makes too many
interrupt calls which is not good for performance.

Implementing pool mode and DMA mode for linux Aspeed I2C are in my
backlog at this moment.

Is there a QEMU model for an I2C/IPMB device ?

Not sure because I'm not familiar with QEMU. Need to check.


There is already a large IPMI framework in QEMU, that would be interesting.
to extend.