Subject: Re: [PATCH] i2c: aspeed: fix master pending state handling

From: Jae Hyun Yoo <[email-masked]>
Date: Thu, 10 Oct 2019 16:16:17 -0700

On 10/10/2019 4:11 PM, Tao Ren wrote:
> On 10/10/19 3:04 PM, Jae Hyun Yoo wrote:
>> On 10/10/2019 2:20 PM, Tao Ren wrote:
>>> On 10/9/19 2:20 PM, Jae Hyun Yoo wrote:
>> [...]
>>>>            /*
>>>>             * If a peer master starts a xfer immediately after it queues a
>>>> -         * master command, change its state to 'pending' then H/W will
>>>> -         * continue the queued master xfer just after completing the
>>>> -         * slave mode session.
>>>> +         * master command, clear the queued master command and change
>>>> +         * its state to 'pending'. To simplify handling of pending
>>>> +         * cases, it uses S/W solution instead of H/W command queue
>>>> +         * handling.
>>>>             */
>>>>            if (unlikely(irq_status & ASPEED_I2CD_INTR_SLAVE_MATCH)) {
>>>> +            writel(readl(bus->base + ASPEED_I2C_CMD_REG) &
>>>> +                ~ASPEED_I2CD_MASTER_CMDS_MASK,
>>>> +                   bus->base + ASPEED_I2C_CMD_REG);
>>>
>>> Sorry for the late comments (just noticed this line while testing the patch):
>>>
>>> I assume this line is aimed at stopping the running master commands, but as per
>>> AST2500 datasheet, it's NOP to write 0 to MASTER_STOP/MASTER_RX/MASTER_TX bits.
>>> Maybe all we need is writing 1 to MASTER_STOP field?
>>
>> There could be two pending cases:
>> 1. Master goes to pending before it triggers a command if a slave
>>     operation is already initiated.
>> 2. Master goes to pending after it triggered a command if a peer
>>     master immediately sends something just after the master command
>>     triggering.
>>
>> Above code is for the latter case. H/W handles the case priority based
>> so the slave event will be handled first, and then the master command
>> will be handled when the slave operation is completed. Problem is,
>> this H/W shares the same buffer for master and slave operations so
>> it's unreliable. Above code just removes the master command from the
>> command register to prevent this H/W command handling of pending events.
>> Instead, it restarts the master command using a call of aspeed_i2c_do_start when the slave operation is completed.
> 
> Thanks for the clarify, Jae. I mean clearing these bits has no effect to
> hardware according to aspeed datasheet; in other word, master command cannot
> be removed from command register by this statement.
> 
> For example, below is the description for MASTER_STOP_CMD(I2CD14, bit 5):
> 
>    0: NOP
>    1: Issue Master Stop Command
>    This register will be automatically cleared by H/W when Stop Command has
>    been issues.

It's removing before H/W fetches the the command so the pending command
isn't cleared by H/W at the timing. If we send a stop command at here, 
the bus will be messed up.

Thanks,

Jae

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