linux-kernel - Re: [regression] nct6775 does not load in 5.4 and 5.5, bisected to b84398d6d7f90080

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Message-ID: <bec1f81c-09a8-ba48-c6c4-5d9b340f7c0b@roeck-us.net>
Date:   Sat, 22 Feb 2020 13:26:48 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Martin Volf <martin.volf.42@...il.com>
Cc:     Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jean Delvare <jdelvare@...e.com>,
        Wolfram Sang <wsa@...-dreams.de>, linux-i2c@...r.kernel.org,
        linux-hwmon@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [regression] nct6775 does not load in 5.4 and 5.5, bisected to
 b84398d6d7f90080

On 2/22/20 12:49 PM, Martin Volf wrote:
> Hello,
> 
> On Sat, Feb 22, 2020 at 8:05 PM Guenter Roeck <linux@...ck-us.net> wrote:
>> On 2/22/20 9:55 AM, Martin Volf wrote:
>>> On Sat, Feb 22, 2020 at 4:41 PM Guenter Roeck <linux@...ck-us.net> wrote:
>>>> On 2/22/20 3:13 AM, Martin Volf wrote:
>>>>> hardware monitoring sensors NCT6796D on my Asus PRIME Z390M-PLUS
>>>>> motherboard with Intel i7-9700 CPU don't work with 5.4 and newer linux
>>>>> kernels, the driver nct6775 does not load.
>>>>>
>>>>> It is working OK in version 5.3. I have used almost all released stable
>>>>> versions from 5.3.8 to 5.3.16; I didn't try older kernels.
>>> ...
>>>> My wild guess would be that the i801 driver is a bit aggressive with
>>>> reserving memory spaces, but I don't immediately see what it does
>>>> differently in that regard after the offending patch. Does it work
>>>> if you unload the i2c_i801 driver first ?
>>>
>>> Yes, after unloading i2c_i801, the nct6775 works.
> ...
>>> This is diff of /proc/ioports in 5.3.18 with loaded nct6775 and in
>>> 5.4.21 without:
>>>
>>> --- ioports-5.3.18
>>> +++ ioports-5.4.21
>>> @@ -2,6 +2,7 @@
>>>      0000-001f : dma1
>>>      0020-0021 : pic1
>>>      002e-0031 : iTCO_wdt
>>> +    002e-0031 : iTCO_wdt
>>>      0040-0043 : timer0
>>>      0050-0053 : timer1
> ...
>>> So 0x2e is the resource the two drivers are fighting for.
> ...
>> Yes, and it should not do that, since the range can be used to access
>> different segments of the same chip from multiple drivers. This region
>> should only be reserved temporarily, using request_muxed_region() when
>> needed and release_region() after the access is complete. Either case,
>> I don't immediately see why that region would be interesting for the
>> iTCO watchdog driver.
>>
>> Can you add some debugging into the i801 driver to see what memory regions
>> it reserves, and how it gets to reserve 0x2e..0x31 ? That range really
>> doesn't make any sense to me.
> 
> in the function i801_add_tco() in drivers/i2c/busses/i2c-i801.c
> (line 1601 in 5.4.21), there is this code:
> 
>          /*
>           * Power Management registers.
>           */
>          devfn = PCI_DEVFN(PCI_SLOT(pci_dev->devfn), 2);
>          pci_bus_read_config_dword(pci_dev->bus, devfn, ACPIBASE, &base_addr);
> 
>          res = &tco_res[ICH_RES_IO_SMI];
>          res->start = (base_addr & ~1) + ACPIBASE_SMI_OFF;
>          res->end = res->start + 3;
>          res->flags = IORESOURCE_IO;
> 
> base_addr is 0xffffffff after pci_bus_read_config_dword() call.
> ACPIBASE_SMI_OFF is 0x030, therefore res->start is 0x2e.
> Not that I understand even a bit of this...
> 

Outch. This means that the code is broken. ACPIBASE is not configured,
or disabled, or the code reads from the wrong PCI configuration register.
What I don't understand is why this works with v5.3 kernels; the code
looks just as bad there for me. I must be missing something. Either case,
the only thing you can really do at this point is to blacklist the
iTCO_wdt driver.

Other than that, we can only hope that someone who understands above
code can provide a fix. Maybe Wolfram has an idea.


Guenter