linux-kernel - Re: [PATCH v2 2/2] asm-generic: Add new pci.h and use it

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Date:   Mon, 18 Jul 2022 08:56:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Stafford Horne <shorne@...il.com>,
        LKML <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>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-csky@...r.kernel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-um <linux-um@...ts.infradead.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] asm-generic: Add new pci.h and use it

On Mon, Jul 18, 2022 at 6:37 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Sun, Jul 17, 2022 at 12:34:53PM +0900, Stafford Horne wrote:
> > Two things to note are:
> >
> >  - isa_dma_bridge_buggy, traditionally this is defined in asm/dma.h but
> >    these architectures avoid creating that file and add the definition
> >    to asm/pci.h.
>
> This doesn't have anyting to do with PCI support.  I think adding a
> separate header just for this that always stubs it out unless a config
> option is set (which x86 then selects) is the besy idea here.  I also
> think the isa_dma_bridge_buggy needs to move out of the PCI code as
> well.

Most architectures have it in asm/dma.h, which is probably the right place
(if we end up keeping it), since this is for the ISA DMA API.

I would copy this declaration from x86

#ifdef CONFIG_PCI
extern int isa_dma_bridge_buggy;
#else
#define isa_dma_bridge_buggy (0)
#endif

to asm-generic/dma.h and remove it from arch/sh to avoid the
one duplicate definition. The architectures that have the declaration
in asm/pci.h (arm64, csky, riscv) already get the asm-generic version
of asm/dma.h.

As mentioned before, it would be even better to just remove it
entirely from everything except x86, and enclose the four
references in an explicit "#ifdef X86_32". The variable declaration
only exists because drivers/pci/quirks.c is compiled on all
architecture, but the individual quirk is only active  based on
the PCI device ID of certain early PCI-ISA bridges.

      Arnd