Re: dtschema: i2c: messy situation about timeouts

Wolfram Sang <[email protected]> Mon, 26 Feb 2024 22:24:53 +0100

Newsgroups	org.kernel.vger.devicetree-spec,org.kernel.vger.linux-i2c
Message-ID	<Zd0Bpa2ciMHmSQrS@shikoro>

Hi Chris,

> > - "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 

That would be helpful if you could add that. I always guessed there
should be more controllers needing the flag but never encountered them
personally.

> 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).

I guess the way forward with that is we add a flag like
I2C_CLIENT_CLK_STRETCH and let the core figure out if controller and
client are compatible. Dunno what to do if not, though. Reject? Throw a
warning that there might be problems? Probably reject by default unless
overridden by something-meaning-i-know-the-risks.

> > 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.

Please wait a little with the implementation. I want to try Rob's idea
to let the core set the timeout flag initially. Dusting off the boards
for testing would be awesome, though :)

Happy hacking,

   Wolfram

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