linux-kernel - Re: [PATCH 2/2] i2c: designware: slave should do WRITE_REQUESTED before WRITE

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Date:   Thu, 5 Nov 2020 11:13:14 +0200
From:   Jarkko Nikula <jarkko.nikula@...ux.intel.com>
To:     Michael.Wu@...ics.com, wsa@...nel.org
Cc:     andriy.shevchenko@...ux.intel.com, mika.westerberg@...ux.intel.com,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        morgan.chang@...ics.com
Subject: Re: [PATCH 2/2] i2c: designware: slave should do WRITE_REQUESTED
 before WRITE_RECEIVED

On 11/4/20 12:17 PM, Michael.Wu@...ics.com wrote:
> Hi Wolfram,
> 
>>> dev->status can be used to record the current state, especially Designware
>>> I2C controller has no interrupts to identify a write-request. This patch
>>
>> Just double-checking: the designware HW does not raise an interrupt when
>> its own address + RW bit has been received?
> 
> Not exactly. There're an interrupt state name "RD_REQ" but no one named
> like "WR_REQ".
> 
> For read-request, the slave will get a RD_REQ interrupt.
> For write-request, the slave won't be interrupted until data arrived to
> trigger interrupt "RX_FULL".
> 
> I tried to use GPIO to simulate an I2C master. I only sent its own
> address + W bit without any data and then I got only a STOP_DET interrupt.
> If I sent its own address + W bit + one byte data and then I got one
> RX_FULL and a STOP_DET.
> 
> It seems the controller doesn't interrupt when RW bit is W, but R does.
> What do you think, Jarkko?
> 
Yes, the datasheet has a flowchart for slave mode and it shows for a 
write only RX_FULL interrupt followed by read from IC_DATA_CMD to 
retrieve received byte. Which I believe won't occur if there is no 
incoming data byte and only STOP_DET happens as you have observed. The 
flowchart however doesn't include the STOP_DET flow.

Jarkko