From: Jean Delvare <khali@linux-fr.org>
To: Dave Airlie <airlied@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Linux I2C <linux-i2c@vger.kernel.org>
Subject: Re: i2c algo bit timeout question
Date: Thu, 23 Apr 2009 08:57:28 +0200 [thread overview]
Message-ID: <20090423085728.18d0c9cf@hyperion.delvare> (raw)
In-Reply-To: <21d7e9970904221619v7d7990bfq697cfca64a7ed6d2@mail.gmail.com>
Hi Dave,
On Thu, 23 Apr 2009 09:19:21 +1000, Dave Airlie wrote:
> Hi to any i2c people,
i2c people tend to live on the linux-i2c list, Cc'd.
> 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?
Yes, I think it should. This bug has been there pretty much since
forever. I suppose people didn't notice because they usually use a
large timeout value.
Please send a patch fixing this and I'll apply it.
--
Jean Delvare
WARNING: multiple messages have this Message-ID (diff)
From: Jean Delvare <khali-PUYAD+kWke1g9hUCZPvPmw@public.gmane.org>
To: Dave Airlie <airlied-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
Linux I2C <linux-i2c-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Subject: Re: i2c algo bit timeout question
Date: Thu, 23 Apr 2009 08:57:28 +0200 [thread overview]
Message-ID: <20090423085728.18d0c9cf@hyperion.delvare> (raw)
In-Reply-To: <21d7e9970904221619v7d7990bfq697cfca64a7ed6d2-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
Hi Dave,
On Thu, 23 Apr 2009 09:19:21 +1000, Dave Airlie wrote:
> Hi to any i2c people,
i2c people tend to live on the linux-i2c list, Cc'd.
> 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?
Yes, I think it should. This bug has been there pretty much since
forever. I suppose people didn't notice because they usually use a
large timeout value.
Please send a patch fixing this and I'll apply it.
--
Jean Delvare
next prev parent reply other threads:[~2009-04-23 6:57 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-04-22 23:19 i2c algo bit timeout question Dave Airlie
2009-04-23 6:57 ` Jean Delvare [this message]
2009-04-23 6:57 ` Jean Delvare
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