Re: [PATCH] dell-smm-hwmon: Cache fan_type() calls and use fan_status() for fan detection

["Pali Rohár" <pali.rohar@gmail.com>]

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On Sunday 22 May 2016 02:28:00 Pali Rohár wrote:
> On Sunday 22 May 2016 02:19:48 Guenter Roeck wrote:
> > On 05/21/2016 07:46 AM, Pali Rohár wrote:
> > > On more Dell machines (e.g. Dell Precision M3800)
> > > I8K_SMM_GET_FAN_TYPE call is too expensive (CPU is too long in
> > > SMM mode) and cause kernel to hang. This patch cache type for
> > > each fan (as it should not change) and change the way how fan
> > > presense is detected. It revert and use function fan_status() as
> > > was before commit f989e55452c7 ("i8k: Add support for fan
> > > labels").
> > > 
> > > Moreover, kernel hangs for 2 - 3 seconds only sometimes and only
> > > on some Dell machines. When kernel hangs fan speed is at max. So
> > > it was hard to debug and bisect where is root of this problem.
> > > It looks like this is bug in Dell BIOS which implement fan type
> > > SMM code... and there is no way how to fix it in kernel.
> > > 
> > > Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
> > > Reviewed-by: Jean Delvare <jdelvare@suse.de>
> > > Reported-and-tested-by: Tolga Cakir <cevelnet@gmail.com>
> > > Fixes: f989e55452c7 ("i8k: Add support for fan labels")
> > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=112021
> > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=100121
> > > Cc: stable@vger.kernel.org # v4.0+, will need backport
> > 
> > Should this patch be applied, or do you wait for more testing ?
> 
> I would like to hear some confirmation from people with affected
> machine.
> 

Ok, now after testing we know that kernel should prevent calling 
I8K_SMM_GET_FAN_TYPE on affected buggy Dell machines.

Looks like there are two different bugs in Dell SMM with 
I8K_SMM_GET_FAN_TYPE call.

First bug cause that kernel freeze for 2 - 3 seconds when 
I8K_SMM_GET_FAN_TYPE is issued.

Second bug cause that fan goes randomly up and down (that is controlled 
by Dell SMM) when I8K_SMM_GET_FAN_TYPE is issued. Normal behaviour is 
returned after machine reboots.

Some Dell machines are affected by first bug, some by second bug. And 
there are Dell machines without both bugs.

This my patch just partially fix first bug and prevent calling that call 
at boot time. But can be issued by sysfs (+value is cached, so it is 
called only once).

So question is: is my patch enough for fixing first bug?

And second question: how to fix second bug? I see only one option: 
Create machine blacklist with broken Dell SMM firmware and disallow 
calling I8K_SMM_GET_FAN_TYPE for them.

-- 
Pali Rohár
pali.rohar@gmail.com

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