Subject: Re: multi-domain PCI and sysfs
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "David S. Miller" <davem@davemloft.net>
Cc: Jon Smirl <jonsmirl@gmail.com>, jbarnes@engr.sgi.com, willy@debian.org,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mer, 2004-09-08 at 19:21, David S. Miller wrote:
> On ppc, sparc, and other non-x86 platforms, when you perform load/store
> instructions within the port I/O space window, the PCI controller emits
> the IN/OUT transactions exactly as if an x86 processor had executed
> an in{bwl}/out{bwl} instruction.

Some of them are not quite that pretty. In certain cases outb gets 
translated into code that does horrors vaguely of the form

	spin_lock_irqsave
	lane = (addr & 3) << 3;
	writel(1<<(addr&3), somecontroller->lanes);
	writel(addr, somecontroller->offset);
	writel(val << lane, somecontroller->somewhere);
	spin_unlock_irqrestore

Other code has extra magic reads in to work around fpga pci bridges that
forgot out is synchronous and writel is posted.

A better summary from the higher levels of the kernel would be "don't
look behind the sofa, there might be a monster lurking".

The only way I can see VGA routing working is to have some kind of arch
code that can tell you which devices are on the same VGA legacy tree.
That then allows a vga layer to walk VGA devices and ask arch code the
typically simple question

		pci_vga_shared_router(pdev1, pdev2)

Alan

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