linux-kernel - i2c algo bit timeout question

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Message-ID: <21d7e9970904221619v7d7990bfq697cfca64a7ed6d2@mail.gmail.com>
Date:	Thu, 23 Apr 2009 09:19:21 +1000
From:	Dave Airlie <airlied@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Jean Delvare <khali@...ux-fr.org>
Subject: i2c algo bit timeout question

Hi to any i2c people,

So I've been debugging some EDID fetching failures and wanted to ask
about the use of time_after_eq in the i2c bit banging code.

EDID specification recommends 2ms timeout for the ack on the initial
read, so we set the timeout in our code to usecs_to_jiffies(2200) (10%
margin of error). On my systems this ends up as 1, and we seem to fail
to retrieve EDID one in 10-20 times. Changing the value to 2, always
gets me the EDID I want.

So looking at drivers/i2c/algos/i2c-algo-bit.c it appears it uses
time_after_eq on jiffies, start + timeout value. So if we have a 10ms
jiffie resolution and enter this at the 9ms point in the 10ms window,
we will seem to exit the loop after 1ms instead of the minimum which I
asked for which is 2.2ms. Should this code use time_after instead of
time_after_eq?

or am I missing something else.
Dave.
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