Re: [PATCH v6 1/2] i2c: core: Add i2c_update_timeout() helper for dynamic transfer timeouts

On 7/29/2026 8:49 PM, Wolfram Sang wrote:
> Hi,
> 
>> I agree that the precise timeout is platform dependent and cannot be derived
>> exactly from the transfer parameters alone. My intention is not to determine
>> the perfect value, but rather to provide a reasonable kernel-side default
>> for cases where no timeout has been configured explicitly.
> 
> We have that already. From the I2C core:
> 
> 1572         /* Set default timeout to 1 second if not already set */
> 1573         if (adap->timeout == 0)
> 1574                 adap->timeout = HZ;
> 
> This may not meet your definition of 'reasonable', though, I understand
> that. But you need to be aware that you immediately enter
> regression-area if you change this behaviour.
> 
Yes, 1HZ it's not reasonable for smaller transfers. Agree too that 
changing this may cause regression to few others.

>> Since kernel-space clients have no generic mechanism to tune adapter
>> timeouts on a per-system basis, deriving a baseline from the transfer length
> 
> This would be easy to add. We could introduce
> i2c_client_request_timeout_margin(client, desired_timeout) or something alike
> with basically doing:
> 
>    client->adapter->timeout = max(client->adapter->timeout, desired_timeout);
> 
> Or? Then we would get the theoretical value of a client. Which is maybe
> exceeded by the board specific timeout set by the board designer. It
> gets tricky, though, with userspace. Who has precedence then?
> 

Thinking to give precedence to user space here in such case. if no 
userspace setting timeout, then default will continue with core set timeout.

>> I am also suggesting let userspace add something on top of this if the core
>> derived final timeout is not sufficient.
> 
> Why can't userspace set an absolute value like now?
> 

So, does it mean user space can override kernel/core calculated timeout 
? if yes, i agree to this idea.

>>
>> This is an option for userspace. Should we expose device attributes for
>> kernel space ?
> 
> See above. adap->timeout is easily accessible.
> 
>> Yes, and I fully support keeping I2C_TIMEOUT as the mechanism for userspace
>> adjustment. What I am proposing is complementary rather than a replacement.
>> The core could calculate a baseline timeout from the transfer
>> characteristics and apply a conservative margin, while I2C_TIMEOUT would
>> remain available for systems that require additional headroom beyond the
>> default calculation.
> 
> If you have two ways of setting a timeout, people might get confused.
> 

In that case, let's decide if userspace configured timeout wins. If not 
set by user, then set calculated timeout by core layer.

I was thinking, user space may not always set the timeout but core layer 
will always need some timeout value based on formulae aniket has kept.

>>>> Do you see cases where a transfer-time-based timeout with a generous
>>>> system-latency margin would still be insufficient?
>>>
>>> Regressions. You could time out too early on boards which worked before.
>>
>> That is a valid concern. My assumption is that any calculated timeout would
>> include a sufficiently conservative margin, based on measurements across a
>> range of systems, so that existing working platforms would not regress.
> 
> You simply cannot guarantee this.
> 

Understood now, it may cause regression.

>> platform still requires significantly larger values due to exceptional
>> latency characteristics, I would expect that requirement to be addressed
>> through the existing timeout override mechanism rather than by forcing every
>> client to use a large fixed timeout.
> 
> The only way to deal with this is 'opt_in', not 'opt_out'. If you want
> to provide different defaults than the existing ones, I think you should
> make this available via a kernel config option, so somebody has to make
> an active decision "I want that and I know it can regress".
> 
> I am still not convinced this is all worth the hazzle, but let's keep
> discussing...
> 

This looks like a reasonable compromise to discuss and converge on. A 
polished version:

1. Userspace-configured timeout takes precedence over any timeout 
configured by the kernel.
2. When userspace does not configure a timeout, the kernel-computed 
timeout is used.
3. To avoid regressions, introduce a Kconfig option for formula-based 
timeout calculation:
	A. If the Kconfig option is enabled, derive the timeout using
            the proposed formula-based approach.
	B. If the Kconfig option is disabled, retain the existing
            default timeout behavior (currently 1 second) to preserve
            backward compatibility.


> Happy hacking,
> 
>     Wolfram
>