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

[Guenter Roeck]

On 06/13/2016 11:52 AM, Pali RohÃr wrote:
> On Sunday 22 May 2016 02:28:00 Pali RohÃr wrote:
> > On Sunday 22 May 2016 02:19:48 Guenter Roeck wrote:
> > > On 05/21/2016 07:46 AM, Pali RohÃr 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 RohÃr <pali.rohar@xxxxxxxxx>
> > > > Reviewed-by: Jean Delvare <jdelvare@xxxxxxx>
> > > > Reported-and-tested-by: Tolga Cakir <cevelnet@xxxxxxxxx>
> > > > 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@xxxxxxxxxxxxxxx # 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.
Or maybe a whitelist with known working systems ?

Guenter