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Subject: Re: [PATCH i2c-next v6] i2c: aspeed: Handle master/slave combined irq
 events properly
To:     Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>,
        Guenter Roeck <linux@roeck-us.net>
Cc:     Joel Stanley <joel@jms.id.au>, linux-aspeed@lists.ozlabs.org,
        Vernon Mauery <vernon.mauery@linux.intel.com>,
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        Brendan Higgins <brendanhiggins@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-i2c@vger.kernel.org, jarkko.nikula@linux.intel.com,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        James Feist <james.feist@linux.intel.com>
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From:   =?UTF-8?Q?C=c3=a9dric_Le_Goater?= <clg@kaod.org>
Message-ID: <798fd5fe-475f-f633-c830-62a3e4f1692d@kaod.org>
Date:   Thu, 13 Sep 2018 18:51:04 +0200
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Hello ! 

On 09/13/2018 06:31 PM, Jae Hyun Yoo wrote:
> Hi Cédric,
> 
> On 9/12/2018 10:47 PM, Cédric 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.
>>>>>>
>>>>>> 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.
>>>
>>> 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.
>>
>> OK.
>>  
>>> 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.

> 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 ? 

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

Thanks,

C.