linux-kernel - Re: [PATCH] arch: powerpc: pci-common: fix wrong return value check on phd

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Date:   Thu, 21 Jun 2018 10:38:46 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Daniel Walker <danielwa@...co.com>,
        "Guilherme G. Piccoli" <guilherme@...ccoli.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        xe-kernel@...ernal.cisco.com, linux-kernel@...r.kernel.org,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Mauro Rodrigues <maurosr@...ux.vnet.ibm.com>,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH] arch: powerpc: pci-common: fix wrong return value check
 on phd_id

On Thu, 2018-06-21 at 10:28 +1000, Michael Ellerman wrote:
> 
> That's true, though I think yours is the first report we've had of
> problems.
> 
> The old behaviour relied on device tree ordering in nearly all cases, so
> you basically get whatever order your firmware happened to flatten the
> device tree in.
> 
> That tends to be consistent on a single system or with a single firmware
> version, but it's not stable in general. If your firmware changes, or
> you kexec then the ordering can change.
> 
> So I'd definitely prefer we didn't go back to that behaviour, because
> it's basically "random order".
> 
> If there's anything you can do on your end to cope with the ne

I think the numbering change has to be coped with. However:

The main issue I see is that it somewhat hard wires that "reg"
is a 64-bit property with the "interesting" bits in the bottom,
and that "interesting" part somewhat happens to fit in 16-bits.

It would have been better to get the full address out of reg (using the
appropriate size specified in the parent #address-cells) and hash it.

Cheers,
Ben.