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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 23 Apr 2020 11:22:20 -0700
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Subject: Re: [GIT PULL] PCI fixes for v5.7
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     linux-pci@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Rob Herring <robh@kernel.org>,
        Andrew Murray <amurray@thegoodpenguin.co.uk>,
        =?UTF-8?B?THXDrXMgTWVuZGVz?= <luis.p.mendes@gmail.com>,
        Todd Poynor <toddpoynor@google.com>
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On Thu, Apr 23, 2020 at 10:40 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
>   - Workaround Apex TPU class code issue that prevents resource
>     assignment (Bjorn Helgaas)

Hmm.

I have no objections to that patch, but I do wonder if it might not be
better to try to actually assign the resource at enable_resource time?

Put another way: if I read the situation correctly, what happened is
that the hardware is broken and doesn't have the proper class code,
and so the resource is not initially assigned at all. But then the
driver matches on the device ID, and tries to use the device, and then
we get into trouble at pci_enable_resources().

But is there any reason we don't just at least try to do
pci_assign_resource() at that point? Yeah, because we didn't do it at
bus scanning, maybe there's no room for it, but that's what we do for
the PCI ROM resources (which I think we also don't claim by default)
when drivers ask to map them.

The pci/rom.c code does

        /* assign the ROM an address if it doesn't have one */
        if (res->parent == NULL && pci_assign_resource(pdev, PCI_ROM_RESOURCE))
                return NULL;

could we perhaps do the same in enable_resource?

Your patch is obviously much better for an -rc kernel, so this is more
of a longer-term "wouldn't it be less fragile to ..." query.

Alternatively, maybe we should do resource assignment even for
PCI_CLASS_NOT_DEFINED?

                     Linus