Re: dtschema: i2c: messy situation about timeouts

On 26/02/24 23:08, Wolfram Sang wrote:
> 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'.

Hmm I know of a few adapters that should probably set 
I2C_AQ_NO_CLK_STRETCH based on some Errata. Probably just a 
documentation exercise. It would be nice to reject clients that need to 
do clock stretching but often it happens as a side effect rather than 
being intentional (I've seen this with i2c clients implemented in 
microcontrollers).

>   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.

Sounds like a good idea. We'd obviously need to keep support for the 
existing property but it wouldn't be hard to add 
"i2c-transfer-timeout-us". I'll try to whip up a patch for that sometime 
this week, just need to dust off my Freescale boards.

> 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
>