From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Mon, 14 May 2007 20:36:35 +0200
From: Jan Kiszka <jan.kiszka@domain.hid>
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Subject: Re: [Xenomai-core] [Adeos-main] [BUG] pit clocksource broken under
 Xenomai
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Jan Kiszka wrote:
> Jan Kiszka wrote:
>> Hi,
>>
>> just checked on real hardware (found in qemu originally):
>>
>> When Linux uses the pit clocksource AND Xenomai is attached, the syste=
m
>> time no longer makes progress. Observed under 2.6.20 with 1.7-03 and
>> newer patches, Xenomai trunk.
>>
>> I guess not many people will be affected by this and I'm lacking time =
to
>> debug, so I'm just dumping the report and running away...
>=20
> Somehow I hate open issues...
>=20
> I recalled some private thread with you, Philippe, about this particula=
r
> line in pid_read(). A look at it again revealed the reason for this bug=
=2E
>=20
> Jan
>=20
> PS: pending_patches++ ;)
>=20
>=20
> ---
>  arch/i386/kernel/i8253.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>=20
> Index: linux-2.6.20/arch/i386/kernel/i8253.c
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> --- linux-2.6.20.orig/arch/i386/kernel/i8253.c
> +++ linux-2.6.20/arch/i386/kernel/i8253.c
> @@ -47,7 +47,8 @@ static cycle_t pit_read(void)
> =20
>  #ifdef CONFIG_IPIPE
>  	if (!__ipipe_pipeline_head_p(ipipe_root_domain))
> -		return 0;	/* We don't really own the PIT. */
> +		/* We don't own the PIT, fall back to jiffies. */
> +		return (cycle_t)(jiffies * LATCH);

I think even more correct would be

	return (cycle_t)(jiffies * LATCH) + (LATCH - 1) - old_count;

Should avoid the theoretical chance of a clock backward jump when
Xenomai attaches. I leave it up to the maintainer to decide. :)

Jan


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