linux-kernel - Re: [PATCH 3/4] x86, pci: Add interface to force mmconfig

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Date:   Tue, 14 Mar 2017 21:55:49 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>, bhelgaas@...gle.com,
        x86@...nel.org, linux-pci@...r.kernel.org, eranian@...gle.com,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86, pci: Add interface to force mmconfig

On Tue, Mar 14, 2017 at 07:24:14PM -0700, Andi Kleen wrote:
> > I agree that it should be fairly safe to do ECAM/MMCONFIG without
> > locking.  Can we handle the decision part by adding a "lockless" bit
> > to struct pci_ops?  Old ops don't mention that bit, so it will be
> > initialized to zero and we'll do locking as today.  ECAM/MMCONFIG ops
> > can set it and we can skip the locking.
> 
> That's what my other patch already did. 

Yes, your 1/4 patch does add the "ll_allowed" bit in struct pci_ops.

What I was wondering, but didn't explain very well, was whether
instead of setting that bit at run-time in pci_mmcfg_arch_init(), we
could set it statically in the pci_ops definition, e.g.,

  static struct pci_ops ecam_ops = {
    .lockless = 1,
    .read = ecam_read,
    .write = ecam_write,
  };

I think it would be easier to read if the lockless-ness were declared
right next to the accessors that need it (or don't need it).

But it is a little confusing with all the different paths, at least on
x86, so maybe it wouldn't be quite that simple.

Bjorn