linux-kernel - Re: [PATCH v5 3/6] i2c: npcm: Modify timeout evaluation mechanism

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Message-ID: <Zvv1m3RT916dyYRC@smile.fi.intel.com>
Date: Tue, 1 Oct 2024 16:14:03 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Tyrone Ting <warp5tw@...il.com>
Cc: avifishman70@...il.com, tmaimon77@...il.com, tali.perry1@...il.com,
	venture@...gle.com, yuenn@...gle.com, benjaminfair@...gle.com,
	andi.shyti@...nel.org, wsa@...nel.org, rand.sec96@...il.com,
	wsa+renesas@...g-engineering.com, tali.perry@...oton.com,
	Avi.Fishman@...oton.com, tomer.maimon@...oton.com,
	KWLIU@...oton.com, JJLIU0@...oton.com, kfting@...oton.com,
	openbmc@...ts.ozlabs.org, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/6] i2c: npcm: Modify timeout evaluation mechanism

On Tue, Oct 01, 2024 at 02:28:52PM +0800, Tyrone Ting wrote:
> From: Tyrone Ting <kfting@...oton.com>
> 
> The users want to connect a lot of masters on the same bus.
> This timeout is used to determine the time it takes to take bus ownership.
> The transactions are very long, so waiting 35ms is not enough.
> 
> Increase the timeout and treat it as the total timeout, including retries.
> The total timeout is 2 seconds now.
> 
> The i2c core layer will have chances to retry to call the i2c driver
> transfer function if the i2c driver reports that the bus is busy and
> returns EAGAIN.

-EAGAIN

...

> +		/*
> +		 * Adaptive TimeOut: estimated time in usec + 100% margin:
> +		 * 2: double the timeout for clock stretching case
> +		 * 9: bits per transaction (including the ack/nack)
> +		 */
> +		timeout_usec = (2 * 9 * USEC_PER_SEC / bus->bus_freq) * (2 + nread + nwrite);

Side note (as I see it was in the original code), from physics
point of view the USEC_PER_SEC here should be simply MICRO
(as 1/Hz == s, and here it will be read as s^2 in the result),
but if one finds the current more understandable, okay then.

-- 
With Best Regards,
Andy Shevchenko