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

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Message-ID: <CA+-6iNw2i=4KVr+VYOxacWpP7Pw0E5mDwKY1i9_6V_hjxu9neQ@mail.gmail.com>
Date: Tue, 4 Mar 2025 12:37:26 -0500
From: Jim Quinlan <james.quinlan@...adcom.com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
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 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.

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

Regards,
Jim Quiinlan
Broadcom STB/CM

>
> - Mani
>
> --
> மணிவண்ணன் சதாசிவம்

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