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

[Thierry Reding <thierry.reding@gmail.com>]

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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@kernel.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.

> In any case, wrong documentation (which is the current state) should be
> fixed (and removal in most case is also proper fix).

Again, I don't see that this fixes anything because there was no bug.
The documentation change makes the most sense when combined with the
change that actually implements this in terms of the new macro.

The existing documentation exists to give further background information
about the mapping. If we remove the comment out of context we loose that
extra information. However, if at the same time we change the code to
use another (documented) macro, then we replace the information without
loosing anything.

Thierry

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