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SubjectRe: [RFC 1/1] drivers: i2c: omap: Add slave support
On 28 August 2016 at 07:35, Wolfram Sang <wsa@the-dreams.de> wrote:
> Well, I2C is simple, what could go wrong? :/

Actually I2C is elegant and *seems* simple, but in all its
asynchronicity there are actually a surprising number of fine details
you can trip over. Maybe that's why so many i2c controllers suck: since
i2c looks simple enough manufacturers are easily tempted to roll their
own instead of licensing a good implementation.

Having said that, most of the inconsistency and obnoxiousness of the TI
I2C controller is not even excusable by that argument. For example its
irq registers *look* like the usual set { rawstatus, status, en, dis }
that's their current standard ("Highlander") for peripherals. They do
not however *behave* like the standard set however:
1. status isn't always (rawstatus & enabled)
2. status != 0 does not always imply the irq output is asserted
3. some enable-bits also change the behaviour of rawstatus
All of these misbehaviours are unprecedented afaik.

Normally you'd also expect each irq (raw)status bit to either
a. be an event, set by hw and can be cleared by software any time, or
b. be a level status, unaffected by software attempts to set/clear.
Again the i2c controller decided this is far too little diversity.

> So, it is possible to make a proper I2C slave with OMAP, but you need
> to know those 100 gory details?

Mostly. There are some limitations such as:

* No ability to selectively ACK/NACK when addressed as slave. If you're
unable to respond for some time then you'd end up blocking the bus with
clock stretching. You could temporarily deconfigure your slave address
but the TRM states changing slave address is forbidden while bus busy.

* According to my notes it always ACKs a General Call and this cannot
even be stalled using the SBLOCK register. Since I don't care about GC
there's no more details in my notes, but if this is true then on any bus
where GC is used, irq handling will have real-time deadlines to avoid
losing track of transaction boundaries and misinterpreting data.

Finally, as my first link pointed out, various protocol errors can lock
up the peripheral's internal state machine. When operating as slave
this is basically undetectable: all registers look normal and the
bus-busy bit will continue to track start/stop, but the peripheral will
not ACK any slave address anymore until you reset it.

You could argue "well, but that requires bus protocol errors" but it is
nevertheless a direct violation of the I2C standard:

I2C-bus compatible devices must reset their bus logic on receipt
of a START or repeated START condition such that they all
anticipate the sending of a slave address, even if these START
conditions are not positioned according to the proper format.

Also, my testing showed pulsing SDA low on an idle bus sufficed to
trigger this state. It needs to pass the glitch filter of course, but
this filter is implemented by sampling the bus requiring two consecutive
samples to agree. Two small glitches with just the right timing would
therefore suffice. Rather unlikely for random noise, but having lots of
signals on your pcb that ultimately derive from the same clock source
probably makes the odds a lot more favorable.

Matthijs

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