dtschema: i2c: messy situation about timeouts

Wolfram Sang <[email protected]> Mon, 26 Feb 2024 11:08:27 +0100

Newsgroups	org.kernel.vger.devicetree-spec,org.kernel.vger.linux-i2c
Message-ID	<ZdxjGwvGXlDGkYs0@shikoro>

Hey guys,

we have quite a messy situation regarding I2C timeouts in the dtschema.
Partly because I was too busy to pay detailed attention, partly because
reviewing dtschema changes happen on Github which I totally missed. No
complaining, though, here are my observations and suggestions to get it
straight. Comments are more than welcome.

- "i2c-transfer-timeout-us"

Description says "Number of microseconds to wait before considering an
I2C transfer has failed."

To me, this binding is very descriptive and makes sense. We should keep
it. Sadly, it is the newest one and we already have others.


- "i2c-scl-has-clk-low-timeout"

AFAIU this binding tells that the controller can do clock stretching.
But what for? I don't see why this is important for clients. If
anything, then it would be interesting if the *client* can do clock
stretching and if the controller can actually handle that. But no need
to describe it in DT, we have this as an adapter quirk already
'I2C_AQ_NO_CLK_STRETCH'. Two controllers use it, but no client checks
for it so far. Coming back to this binding, it is also unused in the
kernel.

Suggestion: let's remove it


- "i2c-scl-clk-low-timeout-us"

The description says "Number of microseconds the clock line needs to be
pulled down in order to force a waiting state." What does "forcing a
waiting state" mean here? I don't understand this description.

It is used in the i2c-mpc driver. The use case is simply to put it into
the 'struct i2c_adapter.timeout' member. That timeout is used to
determine if a transfer failed. So, to me, "i2c-transfer-timeout-us"
makes a lot more sense to use here.

Suggestion: let's remove this binding and conver i2c-mpc to
"i2c-transfer-timeout-us". Yes, not nice to have two deprecated
bindings, but things happened.


So, these are my thoughts about the current situation. I might have
missed something, so if you have anything to add, I am all ears.
Comments really welcome!

Happy hacking,

   Wolfram

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