linux-kernel - RE: [PATCH] RISC-V: Add PCIe I/O BAR memory mapping

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Date:   Fri, 25 Oct 2019 06:35:16 +0000
From:   Yash Shah <yash.shah@...ive.com>
To:     "Paul Walmsley ( Sifive)" <paul.walmsley@...ifive.com>,
        Logan Gunthorpe <logang@...tatee.com>
CC:     "Paul Walmsley ( Sifive)" <paul.walmsley@...ifive.com>,
        "Palmer Dabbelt ( Sifive)" <palmer@...ifive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sorear2@...il.com" <sorear2@...il.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "alex@...ti.fr" <alex@...ti.fr>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "Anup.Patel@....com" <Anup.Patel@....com>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        Sachin Ghadi <sachin.ghadi@...ive.com>,
        Greentime Hu <greentime.hu@...ifive.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "will@...nel.org" <will@...nel.org>,
        "allison@...utok.net" <allison@...utok.net>
Subject: RE: [PATCH] RISC-V: Add PCIe I/O BAR memory mapping

> On Thu, 24 Oct 2019, Logan Gunthorpe wrote:
> 
> > On 2019-10-24 3:14 a.m., Yash Shah wrote:
> > > For I/O BARs to work correctly on RISC-V Linux, we need to establish
> > > a reserved memory region for them, so that drivers that wish to use
> > > I/O BARs can issue reads and writes against a memory region that is
> > > mapped to the host PCIe controller's I/O BAR MMIO mapping.
> >
> > I don't think other arches do this.
> 
> $ git grep 'define PCI_IOBASE' arch/
> arch/arm/include/asm/io.h:#define PCI_IOBASE            ((void __iomem
> *)PCI_IO_VIRT_BASE)
> arch/arm64/include/asm/io.h:#define PCI_IOBASE          ((void __iomem
> *)PCI_IO_START)
> arch/m68k/include/asm/io_no.h:#define PCI_IOBASE        ((void __iomem *)
> PCI_IO_PA)
> arch/microblaze/include/asm/io.h:#define PCI_IOBASE     ((void __iomem
> *)_IO_BASE)
> arch/unicore32/include/asm/io.h:#define PCI_IOBASE
> PKUNITY_PCILIO_BASE
> arch/xtensa/include/asm/io.h:#define PCI_IOBASE         ((void __iomem
> *)XCHAL_KIO_BYPASS_VADDR)
> $
> 
> This is for the old x86-style, non-memory mapped I/O address space the
> legacy PCI stuff that one would use in{b,w,l}()/out{b,w,l}() for.
> 
> Yash, you might consider updating your patch description to note that this is
> for "legacy I/O BARs (i.e., non-MMIO BARs)" or something similar.  That
> might make things clearer.

Sure, will update the description and send v2.

- Yash

> 
> > ioremap() typically just uses virtual address space in the VMALLOC
> > region, PCI doesn't need it's own range. As far as I know the
> > ioremap() implementation in riscv already does this.
> >
> > In any case, 16MB for PCI bar space seems woefully inadequate.
> 
> The modern MMIO PCI resources wind up in jost controller apertures, which
> as you note, are usually much larger.  They don't go in this legacy space.
> 
> Regarding sizing - I haven't seen any PCIe cards with more than 64KiB of
> legacy I/O resources.  (16MiB / 64KiB) = 256, so 16MiB sounds reasonable
> from that point of view?  ARM64 is using that.
> 
> 
> - Paul