linux-kernel - Re: [PATCH v7 1/6] pci: Introduce pci_register_io

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Date:	Thu, 26 Jun 2014 09:59:26 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Liviu Dudau <Liviu.Dudau@....com>,
	linux-pci <linux-pci@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linaro-kernel <linaro-kernel@...ts.linaro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	LAKML <linux-arm-kernel@...ts.infradead.org>,
	Tanmay Inamdar <tinamdar@....com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Jon Masters <jonathan@...masters.org>
Subject: Re: [PATCH v7 1/6] pci: Introduce pci_register_io_range() helper
 function.

(sorry for replying to a months old thread)

On Mon, Apr 07, 2014 at 06:58:24PM +0100, Bjorn Helgaas wrote:
> On Mon, Apr 7, 2014 at 5:36 AM, Arnd Bergmann <arnd@...db.de> wrote:
> 
> > I think migrating other architectures to use the same code should be
> > a separate effort from adding a generic implementation that can be
> > used by arm64. It's probably a good idea to have patches to convert
> > arm32 and/or microblaze.
> 
> Let me reiterate that I am 100% in favor of replacing arch-specific
> code with more generic implementations.
> 
> However, I am not 100% in favor of doing it as separate efforts
> (although maybe I could be convinced).  The reasons I hesitate are
> that (1) if only one architecture uses a new "generic" implementation,
> we really don't know whether it is generic enough, (2) until I see the
> patches to convert other architectures, I have to assume that I'm the
> one who will write them, and (3) as soon as we add the code to
> drivers/pci, it becomes partly my headache to maintain it, even if
> only one arch benefits from it.

I agree and understand your point.

> Please don't think I'm questioning anyone's intent or good will.  It's
> just that I understand the business pressures, and I know how hard it
> can be to justify this sort of work to one's management, especially
> after the immediate problem has been solved.

But, unfortunately, that's something we failed to address in reasonable
time (even though I was one of the proponents of the generic PCIe
implementation). This work is very likely to slip further into the late
part of this year and I am aware that several ARM partners are blocked
on the (upstream) availability of PCIe support for the arm64 kernel.

Although a bit late, I'm raising this now and hopefully we'll come to a
conclusion soon. Delaying arm64 PCIe support even further is not a real
option, which leaves us with:

1. Someone else (with enough PCIe knowledge) volunteering to take over
   soon or
2. Dropping Liviu's work and going for an arm64-specific implementation
   (most likely based on the arm32 implementation, see below)

First option is ideal but there is work to do as laid out by Arnd here:

http://article.gmane.org/gmane.linux.kernel/1679304

The latest patches from Liviu are here (they only target arm64 and there
are additional comments to be addressed from the above thread):

http://linux-arm.org/git?p=linux-ld.git;a=shortlog;h=refs/heads/for-upstream/pci-next

The main reason for the second option is timing. We could temporarily
move Liviu's code under arch/arm64 with the hope that we generalise it
later. However, the risk is even higher that once the code is in
mainline, the generic implementation won't happen. In which case, I
don't see much point in departing from the arm32 PCI API, making bios32
clone the best second option.

For the alternative implementation above, we already have a heavily cut
down version of the arm32 PCI support but only tested in a virtual
environment so far:

https://git.kernel.org/cgit/linux/kernel/git/will/linux.git/log/?h=pci/bios32

In conclusion, unless someone volunteers for the first option fairly
soon, we'll post the alternative patches for review and take it from
there.

Thanks.

-- 
Catalin
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