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Date: Mon, 13 Mar 2017 16:40:24 -0500
From: Bjorn Helgaas <helgaas@kernel.org>
To: Mason <slash.tmp@free.fr>
Cc: linux-pci <linux-pci@vger.kernel.org>,
        linux-usb <linux-usb@vger.kernel.org>,
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        Kevin Hilman <khilman@kernel.org>
Subject: Re: Neophyte questions about PCIe
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On Sat, Mar 11, 2017 at 11:57:56AM +0100, Mason wrote:
> On 10/03/2017 18:49, Mason wrote:

> > static void tango_pcie_bar_quirk(struct pci_dev *dev)
> > {
> > 	struct pci_bus *bus = dev->bus;
> > 
> > 	printk("%s: bus=%d devfn=%d\n", __func__, bus->number, dev->devfn);
> > 
> >         pci_write_config_dword(dev, PCI_BASE_ADDRESS_0, 0x80000004);
> > }
> > DECLARE_PCI_FIXUP_FINAL(0x1105, PCI_ANY_ID, tango_pcie_bar_quirk);
> 
> And this is where the elusive "black magic" happens.
> 
> Is it "safe" to configure a BAR behind Linux's back?

No.  Linux maintains a struct resource for every BAR.  This quirk
makes the BAR out of sync with the resource, so Linux no longer has an
accurate idea of what bus address space is consumed and what is
available.

Normally a BAR is for mapping device registers into PCI bus address
space.  If this BAR controls how the RC forwards PCI DMA transactions
to RAM, then it's not really a BAR and you should prevent Linux from
seeing it as a BAR.  You could do this by special-casing it in the
config accessor so reads return 0 and writes are dropped.  Then you
could write the register in your host bridge driver safely because the
PCI core would think the BAR is not implemented.

Bjorn