linux-kernel - Re: [RFC PATCH] PCI/portdrv: Support for subtractive decode bridge

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Message-ID: <20190207195230.GJ7268@google.com>
Date:   Thu, 7 Feb 2019 13:52:30 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     honghui.zhang@...iatek.com
Cc:     youlin.pei@...iatek.com, lorenzo.pieralisi@....com,
        poza@...eaurora.org, fred@...dlawl.com, linux-pci@...r.kernel.org,
        rafael.j.wysocki@...el.com, linux-kernel@...r.kernel.org,
        jianjun.wang@...iatek.com, ryder.lee@...iatek.com,
        linux-mediatek@...ts.infradead.org, matthias.bgg@...il.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] PCI/portdrv: Support for subtractive decode bridge

On Thu, Feb 07, 2019 at 09:18:16AM -0600, Bjorn Helgaas wrote:
> On Fri, Dec 14, 2018 at 11:40:29AM +0800, honghui.zhang@...iatek.com wrote:
> > From: Honghui Zhang <honghui.zhang@...iatek.com>
> > 
> > The Class Code for subtractive decode PCI-to-PCI bridge is 060401h,
> > change the class_mask values to make portdrv support this type bridge.
> 
> I assume you have a Root Port or Switch Port that supports subtractive
> decode?  I'm trying to understand how such a device would work.
> 
> Out of curiosity, can you show the "lspci -vv" output for the device
> and the downstream devices of interest?

Actually, since subtractive decode has to do with how the bridge
interacts with its *peers*, what would be interesting is the host
bridge window information from ACPI _CRS or DT and the lspci info for
everything under that host bridge.

Assuming we're talking about a Root Port, I guess that would mean
anything inside the host bridge windows but outside the positive
decode windows (the normal PCI-PCI bridge apertures in the Root Ports)
would be claimed by the subtractive decode Root Port?

I guess you would want this because this path ultimately leads to an
ISA or similar bus where you don't know what resources the device
actually consumes?

Bjorn