linux-kernel - Re: [PATCH -next 1/2] i2c: designware: Switch from using MMIO access to SMN access

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Message-ID: <Y8WPUyOT1t5ALYth@smile.fi.intel.com>
Date:   Mon, 16 Jan 2023 19:54:27 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Limonciello, Mario" <Mario.Limonciello@....com>
Cc:     Jan Dąbroś <jsd@...ihalf.com>,
        Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...e.de>,
        Hans de Goede <hdegoede@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "jarkko.nikula@...ux.intel.com" <jarkko.nikula@...ux.intel.com>,
        "wsa@...nel.org" <wsa@...nel.org>,
        "rrangel@...omium.org" <rrangel@...omium.org>,
        "upstream@...ihalf.com" <upstream@...ihalf.com>,
        "M K, Muralidhara" <Muralidhara.MK@....com>,
        "Chatradhi, Naveen Krishna" <NaveenKrishna.Chatradhi@....com>,
        "Ghannam, Yazen" <Yazen.Ghannam@....com>
Subject: Re: [PATCH -next 1/2] i2c: designware: Switch from using MMIO access
 to SMN access

On Mon, Jan 16, 2023 at 07:48:10PM +0200, Andy Shevchenko wrote:
> On Mon, Jan 16, 2023 at 04:22:09PM +0000, Limonciello, Mario wrote:
> > > From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > > Sent: Monday, January 16, 2023 06:39
> > > To: Jan Dąbroś <jsd@...ihalf.com>
> > > On Mon, Jan 16, 2023 at 11:19:00AM +0100, Jan Dąbroś wrote:
> 
> > > > > Make init_amd_nbs() arch_initcall_sync() so that it executes after PCI
> > > init.
> > > >
> > > > I described earlier in this thread why such option is not working -
> > > > let me quote myself:
> > > >
> > > > It's not enough for running init_amd_nbs() to have only
> > > > pci_arch_init() done. We need the pci bus to be created and registered
> > > > with all devices found on the bus. We are traversing through them and
> > > > trying to find northbridge VID/DID. Due to the above, we need to run
> > > > init_amd_nbs() only after acpi_scan_init() that is invoked from
> > > > acpi_init() which is registered as subsys_initcall. That's why the
> > > > trick with switching init_amd_nbs() to arch_initcall_sync will not
> > > > work.
> > > >
> > > > We have a kind of chicken-and-egg problem here. Or is there something I
> > > missed?
> > > >
> > > > I wonder if there is upstreamable option to control order of the
> > > > drivers' init by forcing link order?
> > > 
> > > But what exactly do you need from North Bridge? Is it only its existence or
> > > do you need to have fully instantiated PCI device (if so, why?)?
> > 
> > There is a need to be able to write and read PCI config space.
> 
> So, it's available even on early stages, are there some specifics why it can't
> be done using the respective APIs?

I think I understood the problem with the above. You probably don't know where
NB can be located in the topology and that's why you can't simply access a
_fixed_ BDF. So you want to have the topology being scanned beforehand.

-- 
With Best Regards,
Andy Shevchenko