linux-kernel - Re: [PATCH v3 2/2] asm-generic: Add new pci.h and use it

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Date:   Fri, 22 Jul 2022 23:28:05 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Rob Herring <robh@...nel.org>
cc:     Palmer Dabbelt <palmer@...belt.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Stafford Horne <shorne@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-csky@...r.kernel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-um@...ts.infradead.org, PCI <linux-pci@...r.kernel.org>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] asm-generic: Add new pci.h and use it

On Fri, 22 Jul 2022, Rob Herring wrote:

> > > So again, how does one get a 0 address handed out when that's not even
> > > a valid region according to DT? Is there some legacy stuff that
> > > ignores the bridge windows?
> >
> >  It doesn't matter as <asm/pci.h> just sets it as a generic parameter for
> > the platform, reflecting the limitation of PCI core, which in the course
> > of the discussion referred was found rather infeasible to remove.  The
> > FU740 does not decode to PCI at 0, but another RISC-V device could.  And I
> > think that DT should faithfully describe hardware and not our software
> > limitations.
> 
> Let me ask this another way. When would a 0 memory or i/o address ever
> work? It doesn't seem this s/w limitation has anything specific to
> Risc-V. Given pci_iomap_range() rejects 0, I can't see how it could
> ever work. Maybe only for legacy ISA? So should the generic defaults
> just be what Risc-V is using instead of 0?

 Absolutely, cf.: 
<https://lore.kernel.org/lkml/alpine.DEB.2.21.2202260044180.25061@angie.orcam.me.uk/>.

  Maciej