linux-kernel - Re: [RFC PATCH 00/17] Drop uses of pci_read_config

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Date:   Sun, 2 Aug 2020 14:14:06 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Saheed Bolarinwa <refactormyself@...il.com>, trix@...hat.com,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Joerg Roedel <joro@...tes.org>, bjorn@...gaas.com,
        skhan@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-mtd@...ts.infradead.org, iommu@...ts.linux-foundation.org,
        linux-rdma@...r.kernel.org, linux-ide@...r.kernel.org,
        linux-i2c@...r.kernel.org, linux-hwmon@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-gpio@...r.kernel.org, linux-fpga@...r.kernel.org,
        linux-edac@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-crypto@...r.kernel.org,
        linux-atm-general@...ts.sourceforge.net
Subject: Re: [RFC PATCH 00/17] Drop uses of pci_read_config_*() return value

On Sun, Aug 02, 2020 at 08:46:48PM +0200, Borislav Petkov wrote:
> On Sun, Aug 02, 2020 at 07:28:00PM +0200, Saheed Bolarinwa wrote:
> > Because the value ~0 has a meaning to some drivers and only
> 
> No, ~0 means that the PCI read failed. For *every* PCI device I know.

Wait, I'm not convinced yet.  I know that if a PCI read fails, you
normally get ~0 data because the host bridge fabricates it to complete
the CPU load.

But what guarantees that a PCI config register cannot contain ~0?
If there's something about that in the spec I'd love to know where it
is because it would simplify a lot of things.

I don't think we should merge any of these patches as-is.  If we *do*
want to go this direction, we at least need some kind of macro or
function that tests for ~0 so we have a clue about what's happening
and can grep for it.

Bjorn