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

Hi Andi,

On 7/27/2026 1:41 AM, Andi Shyti wrote:
> Hi Aniket,
> 
> On Mon, Jul 20, 2026 at 05:11:19PM +0530, Aniket Randive wrote:
>> The transfer timeout for an I2C controller should reflect the actual
>> message length and bus frequency rather than a static 1-second value.
>> A static timeout causes unnecessary delays on error paths for short
>> messages, and may be insufficient for very long transfers.
>>
>> Add i2c_update_timeout() to i2c-core which computes a transfer-specific
>> timeout and stores it directly in the standard adap->timeout field.  The
>> formula accounts for 9 bits per byte (8 data + 1 ACK) at the configured
>> bus frequency.  The caller supplies a safety multiplier and a minimum
>> floor so that each driver retains full control over its timing policy
>> without those values becoming public API.
>>
>> Storing the result in adap->timeout makes it visible to all consumers of
>> that field, including the arbitration-loss retry loop in __i2c_transfer().
> 
> this patch does not take into account a timeout set by userspace
> through the I2C_TIMEOUT ioctl. That value would be silently
> overwritten by the driver.
> 

Can we make timeout handling a kernel responsibility and avoid placing 
this burden on userspace ? Every userspace client already accesses the 
bus through the adapter/core layer, so the kernel has full visibility 
into factors such as bus frequency, transfer size, and controller 
characteristics to compute a sensible default timeout.

Also, if both a userspace provided timeout and a kernel-calculated 
timeout are specified, which one should win? Having two independent 
timeout mechanisms could easily lead to ambiguity and inconsistent behavior.

>> Signed-off-by: Aniket Randive <[email protected]>
> 
> ...
> 
>> +/**

[...]

>> +}
>> +EXPORT_SYMBOL_GPL(i2c_update_timeout);
> 
> I'm wondering whether changing the adapter timeout from the I2C
> core is the right approach.
> 
> Perhaps i2c_update_timeout() could return the calculated value
> instead, but then  do not see much value in exposing such a small
> calculation through the I2C core.
> 
> I know you were advised to move this into the core, but it does
> not seem particularly useful there and, as it stands, it looks
> somewhat controversial.
> 
> Unless there is another good reason for keeping it in the core,
> I would drop this helper and keep the calculation in the driver.
> 

There may not always be a userspace application available to configure 
the timeout.

My suggestion was that the I²C core could derive the timeout based on 
the bus frequency, transfer length, and a reasonable safety margin. This 
would provide a generic solution that works across all clients without 
requiring userspace involvement.

To account for variations in system load, we could optionally expose a 
DT property for an offset or scaling coefficient to tune the computed 
timeout. Even with such tuning, I believe this approach would be 
significantly better than the current fixed 1-second (HZ) timeout.

> Thanks,
> Andi
> 
>> +