From: Jens Rottmann <JRottmann@LiPPERTEmbedded.de>
To: Andres Salomon <dilinger@queued.net>
Cc: Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org, linux-geode@lists.infradead.org
Subject: Re: [PATCH] cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels
Date: Thu, 19 Jan 2012 13:57:31 +0100 [thread overview]
Message-ID: <4F18133B.9020801@LiPPERTEmbedded.de> (raw)
In-Reply-To: <20120111021022.16ba5c93@debxo>
Hi Andres,
and a happy new year to you. Sorry for the delay.
Andres Salomon wrote:
> Jens Rottmann <JRottmann@LiPPERTEmbedded.de> wrote:
>> cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels
>>
>> On SMP-capable kernels (e.g. generic distro kernel) the
>> cs5535-clockevt driver loads but is not actually used.
>>
>> Setting cpumask to cpu_all_mask works for UP-only kernels, but if
>> compiled for SMP - though still running on the same UP hardware -
>> kernel/time/tick-common.c:tick_check_new_device() reads this as
>> "non-cpu-local" and silently ignores the device.
>>
>> If we leave cpumask unset clockevents_register_device() will
>> initialize it and the cs5535-clockevt driver will be used no matter
>> how the kernel was compiled. Should anyone ever manage to stick a
>> CS553x in an SMP system (is this even possible?) then a warning will
>> be printed. This is fine as the cs5535-clockevt driver was never
>> written/tested for SMP.
>>
>> If bisecting led you here this patch may have exposed a pre-existing
>> MFGPT problem. Configure for UP-only and re-check.
>>
>> Signed-off-by: Jens Rottmann <JRottmann@LiPPERTEmbedded.de>
>> ---
>>
>> --- linux-3.2-rc6/drivers/clocksource/cs5535-clockevt.c
>> +++ use_mfgpt_on_smp_kernels/drivers/clocksource/cs5535-clockevt.c
>> @@ -100,7 +100,6 @@ static struct clock_event_device cs5535_
>> .set_mode = mfgpt_set_mode,
>> .set_next_event = mfgpt_next_event,
>> .rating = 250,
>> - .cpumask = cpu_all_mask,
>> .shift = 32
>> };
>>
>> _
>
> Hm, have you tried setting cpumask to cpumask_of(0), like a bunch of
> the other clock_event_device drivers do?
Yes, I've seen that, and it was my initial approach. cpumask_of(0)
isn't regarded constant, so moved it from the struct init to
cs5535_mfgpt_init() ... but then I saw
clockevents.c:clockevents_register_device():
if (!dev->cpumask) {
WARN_ON(num_possible_cpus() > 1);
dev->cpumask = cpumask_of(smp_processor_id());
}
Looks to me like meant exactly for this purpose. This will set cpumask to
cpumask_of(0), i.e. does exactly what you're suggesting.
Both do work fine, but I understand cpumask=cpumask_of(0) to mean "this timer
is hardware-tied to a specific CPU", whereas cpumask unset means more like
"wow, we never expected anybody putting this hardware in an SMP system".
And you've said it yourself:
> I've only ever tested it on UP kernels. [...] I hope all of those cs5535
> drivers that hadn't had their locking primitives tested aren't racy!
So I see the WARN_ON as a bonus.
Cheers
Jens
next prev parent reply other threads:[~2012-01-19 12:57 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-12 15:41 IRQF_TIMER | IRQF_SHARED ? Jens Rottmann
2011-12-12 20:31 ` Andres Salomon
2011-12-12 21:00 ` Martin-Éric Racine
2011-12-12 22:06 ` Andres Salomon
2011-12-12 23:38 ` Thomas Gleixner
2011-12-13 15:49 ` Jens Rottmann
2011-12-14 18:37 ` Jens Rottmann
2011-12-14 18:47 ` Andres Salomon
2011-12-21 15:42 ` [PATCH] cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels Jens Rottmann
2012-01-11 10:10 ` Andres Salomon
2012-01-19 12:57 ` Jens Rottmann [this message]
2012-01-23 10:44 ` Andres Salomon
2011-12-21 16:37 ` IRQF_TIMER | IRQF_SHARED ? Jens Rottmann
2011-12-22 16:35 ` [PATCH] cs5535-clockevt: allow the MFGPT IRQ to be shared Jens Rottmann
2012-01-11 10:15 ` Andres Salomon
2012-01-30 13:51 ` [PATCH] cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels Jens Rottmann
2012-01-30 13:59 ` [PATCH] cs5535-clockevt: allow the MFGPT IRQ to be shared Jens Rottmann
2012-02-06 8:20 ` [PATCH resend] cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels Jens Rottmann
2012-02-06 8:23 ` [PATCH resend] cs5535-clockevt: allow the MFGPT IRQ to be shared Jens Rottmann
2011-12-19 14:32 ` "clockevents: Set noop handler in clockevents_exchange_device()" causes hang with cs5535-clockevt Jens Rottmann
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