linux-kernel - Re: [PATCH] PCI: tegra: Update comment about config space

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Date:   Wed, 2 Nov 2022 00:29:58 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-tegra@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: tegra: Update comment about config space

On Thursday 06 October 2022 14:50:25 Thierry Reding wrote:
> On Wed, Oct 05, 2022 at 09:43:36PM +0200, Pali Rohár wrote:
> > On Wednesday 28 September 2022 16:38:27 Thierry Reding wrote:
> > > On Sun, Sep 11, 2022 at 01:32:16PM +0200, Pali Rohár wrote:
> > > > Like many other ARM PCIe controllers, it uses old PCI Configuration
> > > > Mechanism #1 from PCI Local Bus for accessing PCI config space.
> > > > It is not PCIe ECAM in any case.
> > > > 
> > > > Signed-off-by: Pali Rohár <pali@...nel.org>
> > > > ---
> > > >  drivers/pci/controller/pci-tegra.c | 8 +++++---
> > > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > > 
> > > Perhaps this should be rolled into the PCI_CONF1_EXT_ADDRESS patch?
> > 
> > Well, I split documentation change and PCI_CONF1_EXT_ADDRESS usage into
> > two patches as those are two different / separate things. Documentation
> > change is a fix (because documentation is wrong) and PCI_CONF1_EXT_ADDRESS
> > is an improvement - code cleanup. And in case if there is a issue with
> > "cleanup" patch it can be reverted without need to revert also "fix"
> > part. This is just information how I looked at these changes and why I
> > decided to split them.
> > 
> > > On
> > > the other hand there's really no use in keeping this comment around
> > > after that other patch because the documentation for the new macro lays
> > > out the details already.
> > > 
> > > Thierry
> > 
> > Ok, whether documentation is needed or not - it is your maintainer
> > decision. Maybe really obvious things do not have to be documented.
> > Also another look at this problem can be that if somebody wrote wrong
> > documentation for it, maybe it is not too obvious? I do not have opinion
> > on this, so choose what is better :-)
> 
> I wrote that documentation back at the time and I fail to see what
> exactly is wrong about it. Granted, it doesn't mention the Intel PCI
> Configuration mechanism #1 from the PCI Local Bus Specification, but
> that's just because I didn't know about it. Back when I wrote this I
> was looking at the PCIe specifications (because, well, this supports
> PCIe) and I noticed that it was similar to ECAM. And that's exactly
> what the comment says and it points out what the differences are.
> 
> So just because the mapping is closer to PCI_CONF1_EXT_ADDRESS than
> ECAM, it doesn't automatically make the comment wrong. The mapping also
> isn't exactly PCI_CONF1_EXT_ADDRESS, so the new comment can be
> considered equally wrong. The mapping is neither ECAM nor PCI_CONF1, so
> describing it one way or the other doesn't make a difference.

PCI_CONF1_EXT_ADDRESS express indirect register access. If you look at
the address space of Intel PCI Configuration Mechanism #1 then it is
really what this ARM PCIe controller implements (plus uses additional
bits for larger 4kB space). This is really common what lot of non-ECAM
ARM SoC implements. It is really bad to mix this mapping with ECAM.