linux-kernel - Re: [PATCH 3/3] PCI: Set pref for mem64 resource of pcie device

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Date:	Wed, 8 Apr 2015 23:26:50 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	David Miller <davem@...emloft.net>,
	David Ahern <david.ahern@...cle.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 3/3] PCI: Set pref for mem64 resource of pcie device

On Wed, Apr 8, 2015 at 10:17 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Wed, 2015-04-08 at 17:06 -0700, Yinghai Lu wrote:
>> On Wed, Apr 8, 2015 at 2:12 PM, Benjamin Herrenschmidt
>> <benh@...nel.crashing.org> wrote:
>>
>> > Thanks Bjorn. We can fix Yinghai patch for 4.2, it would be indeed handy
>> > even for us to be able to support putting 64-bit NP BARs in prefetch
>> > windows (For some SR-IOV adapters for example) too, but we need to do it
>> > right.
>>
>> Please check if you are ok with attached.
>
> I'll let Bjorn be the final judge here but I am not fan of the way you
> set/clear/set/clear the IORESOURCE_PREFETCH bit with
> pci_set_pref_under_pref(). It's error prone and confusing, the code is
> already barely readable as it is ...
>
> I would rather you replace those various masks compares with a helper
> that does something like pci_resource_compatible(parent_res, child_res),
> which has the logic to test. That or a helper that does something like
> pci_calc_compatible_res_flags() which returns a "flags" that has
> PREFETCH set, which you use in place of res->flags in the various
> allocation path.

I'm not planning to review this until after the merge window opens,
but I took a quick glance, and I agree with Ben.  I don't want to add
a new IORESOURCE_ flag.  I think a pci_resource_compatible() helper is
a great idea.

I am absolutely not in favor of "minimally intrusive" as a goal.
"Minimally intrusive" sounds good but it is often used to justify
clever hacks which end up being an anti-maintainer strategy in the
long term.  "Maximum readability" is what I'm looking for.

Bjorn
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