Date:	Wed, 22 Oct 2014 10:10:05 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Pali Rohár <pali.rohar@...il.com>
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 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
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