[PATCH v2 05/27] arm: pci: add a align_resource hook

Thomas Petazzoni thomas.petazzoni at free-electrons.com
Wed Jan 30 06:47:21 EST 2013

Dear Arnd Bergmann,

On Wed, 30 Jan 2013 09:55:49 +0000, Arnd Bergmann wrote:

> I don't remember the kernel ever caring about whether hardware complies
> to a standard or not. The kernel's job is to make hardware work, based
> on the actual implementation of that hardware. In a lot of cases that
> means taking the standard document as a reference, and adding quirks
> for the devices that are different.
> In the end, it comes down to the impact on the code complexity, and
> the run-time overhead for whatever hardware is most common when adding
> those quirks.

This is not only about standards, it is also about re-using the PCI
resource allocation code.

In my RFCv1, sent December, 7th, I wasn't using any emulated PCI-to-PCI
bridge. So it *can* perfectly work without it.

However, one major drawback of my RFCv1 version is that since I didn't
know how much I/O space and memory space was needed for each PCIe
device, I had to oversize the address decoding windows. And also, I had
to have a special allocator (certainly simple, but still) to find an
available physical address to set up each address decoding window.

Emulating a PCI-to-PCI bridge very nicely allows to re-use the PCI core
resource allocation code. I think it's really the main reason for
emulated those PCI-to-PCI bridges, rather than willing to comply to
some standards.

So what I'm going to do now is rework my patch series by removing the
emulated host bridge (which is normally mandatory by PCIe standard, but
Linux doesn't need it, so we don't care), but I'll keep the emulated
PCI-to-PCI bridges in order to benefit for the PCI core resource
allocation mechanisms.

Is this ok for you?

I'd like to settle on the strategy to follow, because we're really
going a funny road here: on December 7th, I submit a series that
doesn't use any PCI-to-PCI bridge, and I'm told that I should emulate
some. I spent a long time working on an implementation that uses
emumlated PCI-to-PCI bridges, which I submitted on Monday, now to be
told that I should work really hard not to use PCI-to-PCI bridges. I
hope you can feel my little embarrassment here...


Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.

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