linux-kernel - Re: [PATCH v2 6/8] PCI: brcmstb: Use same constant table for config space access

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Message-ID: <20250305054729.5jkxen5vahoxvlrr@thinkpad>
Date: Wed, 5 Mar 2025 11:17:29 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Jim Quinlan <james.quinlan@...adcom.com>
Cc: linux-pci@...r.kernel.org, Nicolas Saenz Julienne <nsaenz@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Cyril Brulebois <kibi@...ian.org>,
	Stanimir Varbanov <svarbanov@...e.de>,
	bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>,
	"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" <linux-rpi-kernel@...ts.infradead.org>,
	"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" <linux-arm-kernel@...ts.infradead.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 6/8] PCI: brcmstb: Use same constant table for config
 space access

On Tue, Mar 04, 2025 at 12:37:26PM -0500, Jim Quinlan wrote:
> On Tue, Mar 4, 2025 at 11:58 AM Manivannan Sadhasivam
> <manivannan.sadhasivam@...aro.org> wrote:
> >
> > On Tue, Mar 04, 2025 at 11:37:14AM -0500, Jim Quinlan wrote:
> > > On Tue, Mar 4, 2025 at 10:08 AM Manivannan Sadhasivam
> > > <manivannan.sadhasivam@...aro.org> wrote:
> > > >
> > > > On Fri, Feb 14, 2025 at 12:39:34PM -0500, Jim Quinlan wrote:
> > > > > The constants EXT_CFG_DATA and EXT_CFG_INDEX vary by SOC. One of the
> > > > > map_bus methods used these constants, the other used different constants.
> > > > > Fortunately there was no problem because the SoCs that used the latter
> > > > > map_bus method all had the same register constants.
> > > > >
> > > > > Remove the redundant constants and adjust the code to use them.  In
> > > > > addition, update EXT_CFG_DATA to use the 4k-page based config space access
> > > > > system, which is what the second map_bus method was already using.
> > > > >
> > > >
> > > > What is the effect of this change? Why is it required? Sounds like it got
> > > > sneaked in.
> > >
> > > Hello,
> > > There is no functional difference with this commit -- the code will
> > > behave the same.  A previous commit set up the "EXT_CFG_DATA" and
> > > "EXT_CFG_INDEX" constants in the offset table but one of the map_bus()
> > > methods did not use them, instead it relied on old generic #define
> > > constants.  This commit uses them and gets rid of the old #defines.
> > >
> >
> > My comment was about the change that modified the offset of EXT_CFG_DATA. This
> > was not justified properly.
> 
> Okay, got it.  You are referring to (for example)
> -      [EXT_CFG_DATA]          = 0x9004,
> +       [EXT_CFG_DATA]          = 0x8000,
> 
> We have two ways of accessing the config space: (1) by writing a full
> index and reading a  designated register (0x9004) and (2) by writing
> the index and then reading from a 4k register region (0x8000 +
> offset).  We previously used (1).  An update was made to use (2) but
> instead of updating EXT_CFG_DATA from 0x9004 to 0x8000,
> PCIE_EXT_CFG_DATA (0x8000) was used by the code of one of the map_bus
> methods.
> 
> This commit changes the code in the offending map_bus method to use
> the offset table for (2) and updates the offset table EXT_CFG_DATA to
> its proper value.
> 

Ok, this clarifies.

> If you want me to expand the commit message with the above text I can do that.
> 

No need. Krzysztof should be able to ammend the commit message for you.

- Mani

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