linux-kernel - Re: [PATCH v2] PCI hotplug Eq v2

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Message-ID: <1537218051.11525.4.camel@intel.com>
Date:   Mon, 17 Sep 2018 21:00:53 +0000
From:   "Derrick, Jonathan" <jonathan.derrick@...el.com>
To:     "helgaas@...nel.org" <helgaas@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "okaya@...nel.org" <okaya@...nel.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "hch@....de" <hch@....de>,
        "sbates@...thlin.com" <sbates@...thlin.com>,
        "logang@...tatee.com" <logang@...tatee.com>,
        "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Busch, Keith" <keith.busch@...el.com>
Subject: Re: [PATCH v2] PCI hotplug Eq v2

On Mon, 2018-09-17 at 15:53 -0500, Bjorn Helgaas wrote:
> On Thu, Aug 30, 2018 at 04:11:59PM -0600, Jon Derrick wrote:
> > Hi Bjorn,
> > 
> > Sorry for the delay on this one and pushing it after RC1.
> > Feel free to queue it up for 4.20 if it looks fine.
> > 
> > I've added comments to the git log and source explaining why
> > calculate_iosize was left unchanged. Basically I could not
> > synthesize a condition where it would have affected the topology.
> 
> In other words, the only reason you didn't change the
> calculate_iosize() path was because you couldn't test it?
> 
I did unsuccessfully try to synthesize it in hardware and qemu. The
firmwares didn't provide the neccessary topology to hit the flexible IO
provisioning conditions


> I appreciate your desire to avoid untested changes, but I think it's
> very important to preserve and even improve the symmetry between
> calculate_memsize() and calculate_iosize().  For example, it's not
> obvious why the order is different here:
> 
>   calculate_iosize():
>     size = ALIGN(size + size1, align);
>     if (size < old_size)
>       size = old_size;
> 
I agree this part didn't make that much sense to me, which was another
reason I left it as-is. Looking at it again, I think its a harmless
calculation that bounds IO size tightly, but could also be reordered as
below to provide for the additional IO (assuming this code ever runs).

>   calculate_memsize():
>     if (size < old_size)
>       size = old_size;
>     size = ALIGN(size + size1, align);
> 
> So I don't want to diverge them further unless there's a real
> functional reason why we need to handle I/O port space differently
> than MMIO space.
> 
> You've tested the MMIO path, and I'm willing to take the risk of
> doing the same thing in the I/O port path.
> 
> Bjorn
Great! I'll follow-up with a patch as soon as I can

Jon

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