linux-kernel - Re: [PATCH v3 1/3] PCI: Introduce pcibios_ignore_alignment

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Message-ID: <20190529140058.GB28250@google.com>
Date:   Wed, 29 May 2019 09:00:58 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Oliver <oohall@...il.com>
Cc:     Shawn Anastasio <shawn@...stas.io>, linux-pci@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sam Bobroff <sbobroff@...ux.ibm.com>,
        xyjxie@...ux.vnet.ibm.com, rppt@...ux.ibm.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] PCI: Introduce pcibios_ignore_alignment_request

On Tue, May 28, 2019 at 03:36:34PM +1000, Oliver wrote:
> On Tue, May 28, 2019 at 2:03 PM Shawn Anastasio <shawn@...stas.io> wrote:
> >
> > Introduce a new pcibios function pcibios_ignore_alignment_request
> > which allows the PCI core to defer to platform-specific code to
> > determine whether or not to ignore alignment requests for PCI resources.
> >
> > The existing behavior is to simply ignore alignment requests when
> > PCI_PROBE_ONLY is set. This is behavior is maintained by the
> > default implementation of pcibios_ignore_alignment_request.
> >
> > Signed-off-by: Shawn Anastasio <shawn@...stas.io>
> > ---
> >  drivers/pci/pci.c   | 9 +++++++--
> >  include/linux/pci.h | 1 +
> >  2 files changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 8abc843b1615..8207a09085d1 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -5882,6 +5882,11 @@ resource_size_t __weak pcibios_default_alignment(void)
> >         return 0;
> >  }
> >
> > +int __weak pcibios_ignore_alignment_request(void)
> > +{
> > +       return pci_has_flag(PCI_PROBE_ONLY);
> > +}
> > +
> >  #define RESOURCE_ALIGNMENT_PARAM_SIZE COMMAND_LINE_SIZE
> >  static char resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
> >  static DEFINE_SPINLOCK(resource_alignment_lock);
> > @@ -5906,9 +5911,9 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
> >         p = resource_alignment_param;
> >         if (!*p && !align)
> >                 goto out;
> > -       if (pci_has_flag(PCI_PROBE_ONLY)) {
> > +       if (pcibios_ignore_alignment_request()) {
> >                 align = 0;
> > -               pr_info_once("PCI: Ignoring requested alignments (PCI_PROBE_ONLY)\n");
> > +               pr_info_once("PCI: Ignoring requested alignments\n");
> >                 goto out;
> >         }
> 
> I think the logic here is questionable to begin with. If the user has
> explicitly requested re-aligning a resource via the command line then
> we should probably do it even if PCI_PROBE_ONLY is set. When it breaks
> they get to keep the pieces.

I agree.  I don't like PCI_PROBE_ONLY in the first place.  It's a
sledgehammer approach that doesn't tell us which resource assignments
need to be preserved or why.  I'd rather use IORESOURCE_PCI_FIXED and
set it for the BARs where there's actually some sort of
hypervisor/firmware/OS dependency.

If there's a way to avoid another pciobios_*() weak function, that
would also be better.

Bjorn