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From: Pavel Machek on 6 Sep 2009 02:00
On Sat 2009-09-05 20:19:46, Marcin Slusarz wrote:
> Norbert van Bolhuis wrote:
> >
> > The problem occurs when e.g. drivers use time_after(jiffes, timeout).
> >
> > CONFIG_NO_HZ could make jiffies advance by more than 1.
> > This is done by:
> > tick_nohz_update_jiffies->tick_do_update_jiffies64->do_timer
> >
> > If drivers use a timeout value of jiffies+1,
> > "time_after(jiffies, timeout)" will be true after 1 interrupt
> > (given that it advances jiffies by at least 2).
> >
> > This is exactly what happens in cfi_cmdset_0002.c:do_write_buffer
> > for our case (Powerpc MPC8313, linux-2.6.28, CONFIG_HZ=250,
> > CONFIG_NO_HZ=y).
> >
> > do_write_buffer does the following:
> > unsigned long uWriteTimeout = ( HZ / 1000 ) + 1;
> > ...
> > timeo = jiffies + uWriteTimeout;
> > ...
> > for (;;) {
> > ...
> > if (time_after(jiffies, timeo) && !chip_ready(map, adr))
> > break;
> > if (chip_ready(map, adr)) {
> > xip_enable(map, chip, adr);
> > goto op_done;
> > }
> > UDELAY(map, chip, adr, 1);
> > }
> > /* software timeout */
> > ret = -EIO;
> > opdone:
> > ...
> >
> > I've seen a few software timeouts after the for-loop
> > looped only 13 times (= 13 us delay, i.s.o. the expected 1 ms). Typically
>
> Are you sure? UDELAY may call schedule(), which can return to this thread
> after much longer time than 13us...

Too long wait is expected, but AFAICS he's complaining about too short
delay and that's a hard bug.

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From: Marcin Slusarz on 6 Sep 2009 06:20
Pavel Machek wrote:
> On Sat 2009-09-05 20:19:46, Marcin Slusarz wrote:
>> Norbert van Bolhuis wrote:
>>> The problem occurs when e.g. drivers use time_after(jiffes, timeout).
>>>
>>> CONFIG_NO_HZ could make jiffies advance by more than 1.
>>> This is done by:
>>> tick_nohz_update_jiffies->tick_do_update_jiffies64->do_timer
>>>
>>> If drivers use a timeout value of jiffies+1,
>>> "time_after(jiffies, timeout)" will be true after 1 interrupt
>>> (given that it advances jiffies by at least 2).
>>>
>>> This is exactly what happens in cfi_cmdset_0002.c:do_write_buffer
>>> for our case (Powerpc MPC8313, linux-2.6.28, CONFIG_HZ=250,
>>> CONFIG_NO_HZ=y).
>>>
>>> do_write_buffer does the following:
>>> unsigned long uWriteTimeout = ( HZ / 1000 ) + 1;
>>> ...
>>> timeo = jiffies + uWriteTimeout;
>>> ...
>>> for (;;) {
>>> ...
>>> if (time_after(jiffies, timeo) && !chip_ready(map, adr))
>>> break;
>>> if (chip_ready(map, adr)) {
>>> xip_enable(map, chip, adr);
>>> goto op_done;
>>> }
>>> UDELAY(map, chip, adr, 1);
>>> }
>>> /* software timeout */
>>> ret = -EIO;
>>> opdone:
>>> ...
>>>
>>> I've seen a few software timeouts after the for-loop
>>> looped only 13 times (= 13 us delay, i.s.o. the expected 1 ms). Typically
>> Are you sure? UDELAY may call schedule(), which can return to this thread
>> after much longer time than 13us...
>
> Too long wait is expected, but AFAICS he's complaining about too short
> delay and that's a hard bug.

Yeah, I know. But conclusion is a bit fishy - 13 iterations don't necessarily mean 13us.
Bug might be elsewhere.

Marcin

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