linux-kernel - Re: [PATCH 02/22] PCI: Unify PCI error response checking

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Date:   Wed, 13 Oct 2021 17:12:53 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Naveen Naidu <naveennaidu479@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        PCI <linux-pci@...r.kernel.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Pali Rohár <pali@...nel.org>
Subject: Re: [PATCH 02/22] PCI: Unify PCI error response checking

On Wed, Oct 13, 2021 at 04:47:43PM -0500, Rob Herring wrote:

> Presumably, there could be some register somewhere where all 1s is
> valid? So I think we need the error values.

We have to assume ~0 is a valid value for any config registers except
the few defined by the spec that have bits required to be 0.  There
can be all kinds of vendor-defined stuff in config space that can be
anything.

> Also, I seem to recall only the vendor/device IDs are defined to be
> all 1s for non-existent devices. Other errors are undefined?

I think this case is actually an instance of the PCI controller
fabricating ~0 because a PCI/PCIe error occurred (I think on PCI it's
a Master Abort when nothing responds; on PCIe the read terminates as
an Unsupported Request (PCIe r5.0, sec 2.3.2)).