Regression: at24 eeprom writing

Peter Rosin peda at
Tue Oct 13 03:38:39 PDT 2015

On 2015-10-12 18:13, Cyrille Pitchen wrote:
> Le 12/10/2015 17:13, Peter Rosin a écrit :
>> On 2015-10-05 17:09, Peter Rosin wrote:
>>> But what trouble does the i2c bus driver see? Admittedly I only
>>> have a simple logic level bus viewer, and not a full-blown
>>> oscilloscope, so there might be something analogue going on?
>>> I don't think so though, those signals looked fine last time we
>>> looked (but we obviously didn't have these issues then, and
>>> didn't really look that closely). I'll see if I can recheck
>>> with a real scope too.
>> We redid the tests with a real scope, and the signal looks nice
>> and square, so it is not that.
>> Speculating further on the cause of the long ACKs, I think that
>> the i2c driver gets confused by an interrupt that marks the
>> transfer complete, and thinks it's a NACK interrupt instead. Is that
>> plausible?
>> If the peripheral unit is such that it generates a stop automatically
>> on NACKs, then this makes perfect sense. I.e. the TWI sees that the
>> transfer is complete, generates an interrupt, and waits for further
>> data or a stop command. Meanwhile the driver thinks it's a NACK and
>> that a stop condition has already been sent to the bus, and just
>> notifies the i2c consumer (the eeprom driver in this case) of the
>> failure and frees up the bus for any future user.
>> This also matches what I see when I turn on some more traffic on the
>> bus, that is interleaved with the eeprom traffic. AFAICT, it can be
>> any command that gets chewed up by the eeprom if it is sent to the
>> i2c driver during the long ACK.
>> Are you Atmel people making any progress on this data corrupting
>> regression? Is there anything else I can do to help?
>> Cheers,
>> Peter
> Hi Peter,
> I have sent a patch to Ludovic for a first internal review before publishing to
> mainline. The patch should fix your issue since it fixes it on my sama5d36ek
> board with an at24 eeprom.
> More details on the reason of this bug would be provided in both the commit
> message and comments in the code provided by the reviewed patch but I you want
> an early fix just read the Status Register (AT91_TWI_SR) at the beginning of
> at91_do_twi_transfer(). This read clears the NACK bit in the Status Register.
> Then the following source code can safely enable the NACK interrupt, otherwise
> in some cases a pending NACK interrupt would rise immediately after the line:
> at91_twi_write(dev, AT91_TWI_IER, AT91_TWI_NACK);
> hence breaking the sequence of operations to be done because the interrupt
> handler would call complete() too early so wait_for_completion_timeout()
> also exits too early.
> So reading the Status Register at the beginning of at91_do_twi_transfer()
> should be enough to fix the issue.

Yes, I see no more long ACKs after that reading the Status Register there.

> Another mistake is in the interrupt handler itself, ie atmel_twi_interrupt():
> we should check the TWI_TXRDY status bit before calling
> at91_twi_write_next_byte() only if both the TWI_TXCOMP and TWI_NACK status bits
> are clear. Otherwise, writing a new byte into the THR tells the I2C controller
> to start a new transfer. Then the I2C slave, the at24 eeprom, is likely to
> also reply by a second NACK. Hence the NACK bit is already set into the Status
> Register on the next call of at91_do_twi_transfer().
> This is what I saw on my scope for PIO transfers.

I interpret this as a proposed solution for the strange double NACKs?

Anyway, I find it unnecessarily hard to grasp exactly what you mean
(wasteful policy you are apparently suffering from where it is OK to
publish a patch written in English, but apparently a big no-no to
send a diff until it passes some internal review???). I interpreted
your "patch" in English as:

-	else if (irqstatus & AT_TWI_TXRDY)
+	else if ((irqstatus & (AT91_TWI_TXCOMP | AT91_TWI_TXRDY | AT91_TWI_NACK)) == AT91_TWI_TXRDY)

But still see some double NACKs. Not always though, and it doesn't wreak
any havoc. But it still looks strange and I can't explain them when looking
at what the eeprom driver requests. Does this mean that there are more
races present?

Or, did I just parse your English "patch" badly?

> By the way, in my case, the first NACK occurs because the at24 driver tries to
> perform a second write transfer too quickly and the eeprom is not ready yet,
> then replies with a NACK.

Yes, I believe this is by design. Noone wants to encode the exact delays
needed since they are hard to predict. So, the eeprom driver polls.

> However while debugging, placing debug traces in the I2C controller driver
> changed the timing: this small delay was enough to make the first NACK
> disappear because the eeprom was now ready to process the second write
> operation.

Yes, w/o NACKs everything is going smooth regardless of kernel version.

> For DMA transfer, you have to know that when NACK error is detected, the I2C
> controller sets the TWI_TXCOMP, TWI_NACK and TWI_TXRDY bits all together in
> the Status Register:
> - The TWI_NACK is cleared by reading the Status Register.
> - The TWI_TXRDY triggers the DMA controller to write the next byte in the THR,
>   which should also tell the I2C controller to start a new transfer.
> - The TWI_TXCOMP bit is cleared by writing into the THR: this is why the
>   interrupt handler may fail to detect NACK if it relies on TWI_TXCOMP.
> Best Regards,
> Cyrille

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