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From:   Rob Herring <robh@kernel.org>
Date:   Fri, 22 Jul 2022 13:42:27 -0600
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Subject: Re: [PATCH v3 2/2] asm-generic: Add new pci.h and use it
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc:     Palmer Dabbelt <palmer@dabbelt.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Stafford Horne <shorne@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Guo Ren <guoren@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Richard Weinberger <richard@nod.at>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-csky@vger.kernel.org,
        linux-riscv <linux-riscv@lists.infradead.org>,
        linux-um@lists.infradead.org, PCI <linux-pci@vger.kernel.org>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@vger.kernel.org>
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On Fri, Jul 22, 2022 at 1:23 PM Maciej W. Rozycki <macro@orcam.me.uk> wrote:
>
> On Fri, 22 Jul 2022, Rob Herring wrote:
>
> > > Maybe the right thing to do here is actually to make the default
> > > definitions of these macros non-zero, or to add some sort of ARCH_
> > > flavor of them and move that non-zero requirement closer to where it
> > > comes from?  From the look of it any port that uses the generic port I/O
> > > functions and has 0 for these will be broken in the same way.
> > >
> > > That said, I'm not really a PCI guy so maybe Bjorn or Maciej has a
> > > better idea?
> >
> > >From fu740:
> >                        ranges = <0x81000000  0x0 0x60080000  0x0
> > 0x60080000 0x0 0x10000>,      /* I/O */
> >                                  <0x82000000  0x0 0x60090000  0x0
> > 0x60090000 0x0 0xff70000>,    /* mem */
> >                                  <0x82000000  0x0 0x70000000  0x0
> > 0x70000000 0x0 0x1000000>,    /* mem */
> >                                  <0xc3000000 0x20 0x00000000 0x20
> > 0x00000000 0x20 0x00000000>;  /* mem prefetchable */
> >
> > 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?

Rob