From: Chris Packham <Chris.Packham@alliedtelesis.co.nz>
To: Wolfram Sang <wsa@kernel.org>,
"linux-i2c@vger.kernel.org" <linux-i2c@vger.kernel.org>,
"devicetree-spec@vger.kernel.org"
<devicetree-spec@vger.kernel.org>,
Andi Shyti <andi.shyti@kernel.org>,
"Rob Herring" <robh+dt@kernel.org>,
Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Subject: Re: dtschema: i2c: messy situation about timeouts
Date: Mon, 26 Feb 2024 20:20:29 +0000 [thread overview]
Message-ID: <ccb58633-5981-4b91-a6ca-a57ea1ce5e40@alliedtelesis.co.nz> (raw)
In-Reply-To: <ZdxjGwvGXlDGkYs0@shikoro>
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
>
next prev parent reply other threads:[~2024-02-26 20:20 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-26 10:08 dtschema: i2c: messy situation about timeouts Wolfram Sang
2024-02-26 14:45 ` Rob Herring
2024-02-26 21:16 ` Wolfram Sang
2024-02-26 20:20 ` Chris Packham [this message]
2024-02-26 21:24 ` Wolfram Sang
2024-02-27 0:03 ` Andi Shyti
2024-02-27 7:12 ` Wolfram Sang
2024-02-27 12:57 ` Wolfram Sang
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