linux-kernel - Re: [PATCH] PCI: decline to resize resources if boot config must be preserved

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Date:   Wed, 13 Jan 2021 10:55:57 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH] PCI: decline to resize resources if boot config must be preserved

On Tue, 12 Jan 2021 at 23:49, Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Sat, Jan 09, 2021 at 10:53:53AM +0100, Ard Biesheuvel wrote:
> > The _DSM #5 method in the ACPI host bridge object tells us whether the
> > OS is permitted to deviate from the resource assignment configured by
> > the firmware. If this is not the case, we should not permit drivers to
> > resize BARs on the fly. So make pci_resize_resource() take this into
> > account.
> >
> > Cc: <stable@...r.kernel.org> # v5.4+
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>
> Capitalized subject to match convention and applied to pci/resource
> for v5.11, thanks!
>
> Is there an email, bug report, etc that prompted this change?
>

No, I was just reviewing the recent Tianocore changes to perform BAR
resizing before resource assignment even takes places, which is
obviously a more appropriate time to do it, as it does not require the
OS to modify the firmware configuration at all. This reminded me of
_DSM #5 and the fact that the OS may not even be permitted to make any
changes.


> > ---
> >  drivers/pci/setup-res.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/pci/setup-res.c b/drivers/pci/setup-res.c
> > index 43eda101fcf4..3b38be081e93 100644
> > --- a/drivers/pci/setup-res.c
> > +++ b/drivers/pci/setup-res.c
> > @@ -410,10 +410,16 @@ EXPORT_SYMBOL(pci_release_resource);
> >  int pci_resize_resource(struct pci_dev *dev, int resno, int size)
> >  {
> >       struct resource *res = dev->resource + resno;
> > +     struct pci_host_bridge *host;
> >       int old, ret;
> >       u32 sizes;
> >       u16 cmd;
> >
> > +     /* Check if we must preserve the firmware's resource assignment */
> > +     host = pci_find_host_bridge(dev->bus);
> > +     if (host->preserve_config)
> > +             return -ENOTSUPP;
> > +
> >       /* Make sure the resource isn't assigned before resizing it. */
> >       if (!(res->flags & IORESOURCE_UNSET))
> >               return -EBUSY;
> > --
> > 2.17.1
> >