linux-kernel - Re: [PATCH v1 3/7] PCI: New Primary to Sideband (P2SB) bridge support library

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Date:   Thu, 1 Apr 2021 18:45:02 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Henning Schild <henning.schild@...mens.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Jean Delvare <jdelvare@...e.de>,
        Lee Jones <lee.jones@...aro.org>,
        Tan Jui Nee <jui.nee.tan@...el.com>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        Jonathan Yong <jonathan.yong@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-pci@...r.kernel.org, Jean Delvare <jdelvare@...e.com>,
        Peter Tyser <ptyser@...-inc.com>, hdegoede@...hat.com
Subject: Re: [PATCH v1 3/7] PCI: New Primary to Sideband (P2SB) bridge
 support library

On Tue, Mar 09, 2021 at 09:42:52AM +0100, Henning Schild wrote:
> Am Mon, 8 Mar 2021 19:42:21 -0600
> schrieb Bjorn Helgaas <helgaas@...nel.org>:
> > On Mon, Mar 08, 2021 at 09:16:50PM +0200, Andy Shevchenko wrote:
> > > On Mon, Mar 08, 2021 at 12:52:12PM -0600, Bjorn Helgaas wrote:  
> > > > On Mon, Mar 08, 2021 at 02:20:16PM +0200, Andy Shevchenko wrote:  

...

> > > > > +	/* Read the first BAR of the device in question */
> > > > > +	__pci_bus_read_base(bus, devfn, pci_bar_unknown, mem,
> > > > > PCI_BASE_ADDRESS_0, true);  
> > > > 
> > > > I don't get this.  Apparently this normally hidden device is
> > > > consuming PCI address space.  The PCI core needs to know about
> > > > this.  If it doesn't, the PCI core may assign this space to
> > > > another device.  
> > > 
> > > Right, it returns all 1:s to any request so PCI core *thinks* it's
> > > plugged off (like D3cold or so).  
> > 
> > I'm asking about the MMIO address space.  The BAR is a register in
> > config space.  AFAICT, clearing P2SBC_HIDE_BYTE makes that BAR
> > visible.  The BAR describes a region of PCI address space.  It looks
> > like setting P2SBC_HIDE_BIT makes the BAR disappear from config space,
> > but it sounds like the PCI address space *described* by the BAR is
> > still claimed by the device.  If the device didn't respond to that
> > MMIO space, you would have no reason to read the BAR at all.
> > 
> > So what keeps the PCI core from assigning that MMIO space to another
> > device?
> 
> The device will respond to MMIO while being hidden. I am afraid nothing
> stops a collision, except for the assumption that the BIOS is always
> right and PCI devices never get remapped. But just guessing here.
> 
> I have seen devices with coreboot having the P2SB visible, and most
> likely relocatable. Making it visible in Linux and not hiding it again
> might work, but probably only as long as Linux will not relocate it.
> Which i am afraid might seriously upset the BIOS, depending on what a
> device does with those GPIOs and which parts are implemented in the
> BIOS.

So the question is, do we have knobs in PCI core to mark device fixes in terms
of BARs, no relocation must be applied, no other devices must have the region?

-- 
With Best Regards,
Andy Shevchenko