Re: [PATCH] pci: Add pci quirk to turnoff Nosnoop and Relaxed Ordering

[Keith Busch <kbusch@kernel.org>]

On Wed, Sep 01, 2021 at 05:23:53PM -0500, Bjorn Helgaas wrote:
> On Sun, Oct 18, 2015 at 07:55:04PM +0530, Hariprasad Shenai wrote:
> > Some devices violate the PCI Specification regarding echoing the Root
> > Port Transaction Layer Packet Request (TLP) No Snoop and Relaxed
> > Ordering Attributes into the TLP Response. The PCI Specification
> > "encourages" compliant Root Port implementation to drop such
> > malformed TLP Responses leading to device access timeouts. Many Root Port
> > implementations accept such malformed TLP Responses and a few more
> > strict implementations do drop them.
> > 
> > For devices which fail this part of the PCI Specification, we need to
> > traverse up the PCI Chain to the Root Port and turn off the Enable No
> > Snoop and Enable Relaxed Ordering bits in the Root Port's PCI-Express
> > Device Control register. This does affect all other devices which
> > are downstream of that Root Port.
> 
> While researching another thread about RO [1], I got concerned about
> setting RO for root ports.
> 
> Setting RO for *endpoints* makes sense: that allows (but does not
> require) the endpoint to issue writes that don't require strong
> ordering.
> 
> Setting RO for *root ports* seems more problematic.  It allows the
> root port to issue PCIe writes that don't require strong ordering.
> These would be CPU MMIO writes to devices.  But Linux currently does
> not have a way for drivers to indicate that some MMIO writes need to
> be ordered while others do not, and I think drivers assume that all
> MMIO writes are performed in order.

Is that not what writel_relaxed() is for? While it appears that most
archs just have that call the generic writel(), it does let drivers
indicate which writes are not strongly ordered.