Date:	Mon, 17 Nov 2014 09:35:28 +0100
From:	Pali Rohár <pali.rohar@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	Steven Honeyman <stevenhoneyman@...il.com>
Subject: Re: [PATCH] i8k: Ignore temperature sensors which report invalid values

On Thursday 23 October 2014 18:45:07 Guenter Roeck wrote:
> On Thu, Oct 23, 2014 at 12:37:34PM +0200, Pali Rohár wrote:
> > On Wednesday 22 October 2014 19:10:05 Guenter Roeck wrote:
> > > On Wed, Oct 22, 2014 at 06:35:53PM +0200, Pali Rohár wrote:
> > > > On Wednesday 22 October 2014 18:19:47 Guenter Roeck 
wrote:
> > > > > On Wed, Oct 22, 2014 at 02:29:06PM +0200, Pali Rohár 
wrote:
> > > > > > On Tuesday 21 October 2014 06:27:23 Guenter Roeck 
wrote:
> > > > > > > On 10/20/2014 09:46 AM, Pali Rohár wrote:
> > > > > > > > Ok, I will describe my problem. Guenter, maybe
> > > > > > > > you can find another solution/fix for it.
> > > > > > > > 
> > > > > > > > Calling i8k_get_temp(3) on my laptop without
> > > > > > > > I8K_TEMPERATURE_BUG always returns value 193
> > > > > > > > (which is above I8K_MAX_TEMP).
> > > > > > > > 
> > > > > > > > When I8K_TEMPERATURE_BUG is enabled (by default)
> > > > > > > > then i8k_get_temp(3) returns value from prev[3]
> > > > > > > > and store new value I8K_TEMPERATURE_BUG to
> > > > > > > > prev[3]. Value in prev[3] is initialized to 0.
> > > > > > > > 
> > > > > > > > What I want to achieve is: when i8k_get_temp()
> > > > > > > > for particular sensor id always returns invalid
> > > > > > > > value (> I8K_MAX_TEMP) then we should totally
> > > > > > > > ignore sensor with that id and do not export it
> > > > > > > > via hwmon.
> > > > > > > > 
> > > > > > > > My solution is: initialize prev[id] to
> > > > > > > > I8K_MAX_TEMP, so on invalid data first call to
> > > > > > > > i8k_get_temp(id) returns I8K_MAX_TEMP. Then in
> > > > > > > > i8k_init_hwmon check if value is < I8K_MAX_TEMP
> > > > > > > > and if not ignore sensor id.
> > > > > > > > 
> > > > > > > > Guenter, it is clear now? Are you ok that we
> > > > > > > > should ignore sensor if always report value
> > > > > > > > above I8K_MAX_TEMP? If you do not like my
> > > > > > > > solution/patch for it, can you specify how
> > > > > > > > other can it be fixed?
> > > > > > > 
> > > > > > > I still don't see the point in initializing
> > > > > > > prev[].
> > > > > > 
> > > > > > Now prev[] is initialized to 0. It means that first
> > > > > > call i8k_get_temp() (with sensor id which return
> > > > > > value > I8K_MAX_TEMP) returns 0. Second and other
> > > > > > calls returns I8K_MAX_TEMP.
> > > > > > 
> > > > > > So point is to return same value for first and other
> > > > > > calls.
> > > > > 
> > > > > Yes, I realized that after I sent my previous mail.
> > > > > 
> > > > > > > Yes, I am ok with ignoring sensor values if the
> > > > > > > reported temperature is above I8K_MAX_TEMP. I am
> > > > > > > just not sure if we should check against
> > > > > > > I8K_MAX_TEMP or against, say, 192. Reason is that
> > > > > > > we do know that the sensor can erroneously return
> > > > > > > 0x99 on some systems once in a while. We would
> > > > > > > not want to ignore those sensors just because
> > > > > > > they happen to report 0x99 during initialization.
> > > > > > > 
> > > > > > > So maybe make it
> > > > > > > 
> > > > > > > 	if (err >= 0 && err < 192)
> > > > > > > 
> > > > > > > and add a note before the first if(), explaining
> > > > > > > that higher values suggest that there is no
> > > > > > > sensor attached.
> > > > > > > 
> > > > > > > Thanks,
> > > > > > > Guenter
> > > > > > 
> > > > > > Right, now we need to decide which magic constant to
> > > > > > use...
> > > > > > 
> > > > > > And now I found another problem :-)
> > > > > > 
> > > > > > On my laptop i8k_get_temp(3) not always return value
> > > > > > 193. It is only when AMD graphics card is turned
> > > > > > off. When card is on i8k_get_temp(3) returns same
> > > > > > value as temperature hwmon part from radeon DRM
> > > > > > driver.
> > > > > 
> > > > > Can you turn the GPU on or off during runtime ?
> > > > > That would make it really tricky to handle the
> > > > > situation.
> > > > 
> > > > Yes. New laptops with Nvidia Optimus or AMD PowerXpress
> > > > or Enduro technology are designed to automatically turn
> > > > off secondary GPU when is not in use. And
> > > > nouveau/radeon drivers together with vga_switcheroo
> > > > implements this dynamic power on/off.
> > > > 
> > > > > > So it looks like that on my laptop i8k sensor with
> > > > > > id 3 reports GPU temperature.
> > > > > > 
> > > > > > When card is turned off radeon driver reports
> > > > > > -EINVAL for temperature hwmon sysnode.
> > > > > > 
> > > > > > So now I think i8k could not ignore sensor totally
> > > > > > as it can be mapped to some HW which can be
> > > > > > dynamically turned on/off (like my graphics card).
> > > > > > 
> > > > > > So what do you think about reporting -EINVAL instead
> > > > > > I8K_MAX_TEMP when dell SMM returns value above
> > > > > > I8K_MAX_TEMP?
> > > > > 
> > > > > -EINVAL is supposed to mean "Invalid Argument", so it
> > > > > really has ia different semantics. We could use
> > > > > -ENXIO, "No such device or address", which seems more
> > > > > appropriate.
> > > > 
> > > > I prefer to use -EINVAL because other driver (radeon) is
> > > > using it and userspace "sensors" programs handle EINVAL
> > > > and show "N/A" in output instead reporting some error
> > > > about reading value. If nothing else consistency (with
> > > > other drivers) is my argument.
> > > 
> > > Ok, if sensors implements it that way then let's do it.
> > > 
> > > > > Overall, I think the entire error handling is broken
> > > > > and should be replaced. One option would be to
> > > > > explicitly check for 0x99 and, if detected, go to
> > > > > sleep for, say, 100ms and try again. If it still
> > > > > fails, and for all other bad values, return -ENXIO.
> > > > > Then the calling code can either return the error to
> > > > > user space in the show function, or not install the
> > > > > sensor in the probe function.
> > > > > 
> > > > > Does that make sense ?
> > > > 
> > > > Yes, replacing error handling with retry call (after
> > > > some sleep) is better then current code (which returns
> > > > previous value or returns I8K_MAX_TEMP).
> > > > 
> > > > But your solution not install the sensor if probe fails
> > > > on bad value does not solve problem that i8k.ko is
> > > > loading at time when GPU card is turned off.
> > > 
> > > Yes, the dynamics in that situation makes it a bit
> > > difficult to handle the situation.
> > > 
> > > > I think current check for installing sensor (err < 0) is
> > > > enough and when invalid value (> I8K_MAX_TEMP) is
> > > > returned just do not show this value for userspace in
> > > > hwmon sysnode.
> > > 
> > > Ok with me, and agreed.
> > > 
> > > Thanks,
> > > Guenter
> > 
> > Ok, are you going to fix i8k_get_temp() function (with
> > sleeping)?
> 
> I had hoped you would find the time to do it ;-).
> 
> Sure, I can do it, but I am kind of busy right now, so it will
> take a while.
> 
> Thanks,
> Guenter

Ok. Will you do that for 3.19 kernel? Meanwhile I can prepare 
patch for temperature labels. I looked into NBSVC.MDM and there 
is something related for type of temperature sensors...

-- 
Pali Rohár
pali.rohar@...il.com

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