Date:	Sun, 30 Nov 2014 08:04:19 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Pali Rohár <pali.rohar@...il.com>
CC:	Gabriele Mazzotta <gabriele.mzt@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steven Honeyman <stevenhoneyman@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i8k: Add support for temperature sensor labels

On 11/30/2014 02:11 AM, Pali Rohár wrote:
> On Sunday 30 November 2014 02:25:09 Guenter Roeck wrote:
>> On 11/29/2014 11:07 AM, Pali Rohár wrote:
>> [ ... ]
>>
>>> Original Dell DOS executable ignores all temperature sensors
>>> if type SMM function fails (if I decoded and understand
>>> that DOS assembler code correctly). So maybe we should do
>>> same...
>>
>> Pali,
>>
>> Makes me wonder - does the assembler code tell you what to do
>> if the reported temperature is invalid, and does it
>> distinguish between error codes ?
>
> I do not see anything like that. But there are lot of indirect
> calls (offset to pointer to function is stored in some global at
> init zero data), so it is hard to understand what that DOS binary
> is doing. I'm happy that I decoded loop which trying to call that
> type function and if it does not fail then it call read
> temperature function. And in that section I do not see any error
> handling of invalid values (but it could be somewhere else).
>
> Anyway DOS binary is quite old (7 years maybe?). It is not even
> available for my last E6440 model. Now all new Dell laptops have
> EFI system and ePSA application (new version of diagnostic tool
> which reports info about fan, temperature, ...). That tool looks
> like is burned directly into machine (I can start it with empty
> HDD from Setup screen) or into BIOS image.
>
> And what is interesting about this ePSA:
>
> * it show more temperature sensors (battery temperature)
> * it show correct RPM of fan and *can* control fan speed
>
> I think that DOS binary has no idea about Optimus or PowerExpress
> cards so for that error handling we need to understand what is
> doing new EFI ePSA application...
>
> And because function for turning card on/off is controlled via
> ACPI I bet that DOS or EFI application does not touch it, so
> assume that card is always on and does not need any error
> handling.
>
> Another info about DOS binary: After SMM code for reading fan RPM
> is finished, then function divide returned RPM value by some
> number stored in local data. So now I think that magic fan
> multiplier is not constant, but runtime value. I will try to look
> at it, if we can fix this problem in linux i8k.c.
>
It might be system specific. After all, it is known that old laptops
need a different multiplier.

>> So far we have
>> 	0x99 - presumably a spurious error
>> 	0xc1 - GPU temperature sensor, GPU turned off
>>
>> It would be nice if we could find a better solution for error
>> handling.
>>
>
> Yes, but now we can only guess... My idea is that Dell SMM
> handler does not check GPU presence at runtime and just try to
> read info from PCI bus. And because turned off card is not there
> just random (or non random) garbage is returned...
>
Well, it was worth a try. It might be that, or SMM does handle it,
but that the DOS application is too old to understand it.

Thanks,
Guenter

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