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

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Message-ID: <20200224182730.GA29799@roeck-us.net>
Date:   Mon, 24 Feb 2020 10:27:30 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Martin Volf <martin.volf.42@...il.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>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>
Subject: Re: [regression] nct6775 does not load in 5.4 and 5.5, bisected to
 b84398d6d7f90080

On Mon, Feb 24, 2020 at 01:27:40PM +0200, Mika Westerberg wrote:
> On Mon, Feb 24, 2020 at 12:51:25PM +0200, Mika Westerberg wrote:
> > > I'm wondering if
> > > 
> > > 		pci_dev_is_present(...);
> > > 
> > > returns false here.
> > 
> > Well that might also be the case since lspci shows this:
> > 
> > 00:1f.0 ISA bridge: Intel Corporation Z390 Chipset LPC/eSPI Controller (rev 10)
> > 00:1f.3 Audio device: Intel Corporation Cannon Lake PCH cAVS (rev 10)
> > 00:1f.4 SMBus: Intel Corporation Cannon Lake PCH SMBus Controller (rev 10)
> > 
> > PMC is 1f.2 and not present here. However, it may be that the PMC is
> > still there it just does not "enumerate" because its devid/vendorid are
> > set to 0xffff. Similar hiding was done for the P2SB bridge.
> 
> Actually I think this is the case here.
> 
> I don't know the iTCO_wdt well enough to say if it could live without
> the ICH_RES_IO_SMI. It looks like this register is used to disable SMI
> generation but not sure how well it works if it is left to BIOS to
> configure. I suppose these systems should use WDAT instead.
> 

In practice the region is only used if
	if (turn_SMI_watchdog_clear_off >= p->iTCO_version) {
where turn_SMI_watchdog_clear_off is 1 by default. It is also
passed to vendor specific code, but that is only relevant for
really old systems. In short, it isn't really needed for any
recent-generation systems, and could be made optional with
a few lines of code in iTCO_wdt.c.

Guenter