Re: PCI trouble on mvebu (Turris Omnia)

["Pali Rohár" <pali@kernel.org>]

On Friday 30 October 2020 00:15:57 Toke Høiland-Jørgensen wrote:
> Thomas Petazzoni <thomas.petazzoni@bootlin.com> writes:
> 
> > Hello,
> >
> > On Thu, 29 Oct 2020 14:30:22 -0500
> > Bjorn Helgaas <helgaas@kernel.org> wrote:
> >
> >> We could quirk these NICs to avoid the retrain, but since aardvark and
> >> mvebu have no obvious connection and WLE200/WLE900 and MT76 have no
> >> obvious connection, I doubt there's a simple hardware defect that
> >> explains all these.  
> >
> > aardvark and mvebu have one very strong connection: they are the only
> > two drivers making use of the PCI Bridge emulation logic in
> > drivers/pci/pci-bridge-emul.c:
> >
> > drivers/pci$ git grep pci-bridge-emul
> > akefile:obj-$(CONFIG_PCI_BRIDGE_EMUL)  += pci-bridge-emul.o
> > controller/pci-aardvark.c:#include "../pci-bridge-emul.h"
> > controller/pci-mvebu.c:#include "../pci-bridge-emul.h"
> > pci-bridge-emul.c:#include "pci-bridge-emul.h"
> >
> > I haven't read the whole thread, but it is important to keep in mind
> > that on those two platforms, the PCI Bridge seen by Linux is *not* a
> > real HW bridge. It is faked by the the pci-bridge-emul code. So if this
> > code has defects/bugs in how it emulates a PCI Bridge behavior, you
> > might see weird things.
> 
> Ohh, that's interesting. Why does it need to emulate it?

I could speculate, they wanted to decrease cost of hw, so they did not
include bridge into hw and let user to emulate it (if is needed).

> And could this cause things weird interactions like what I'm seeing,
> where a somewhat buggy device in slot 2 affects the ability to retrain
> the link also in slot 1, but only if there's no device in slot 3?

I doubt, slots and registers are independent. Every slot/card has own
(emulated) bridge.